Williams Obstetrics, 24th Edition

CHAPTER 5. Implantation and Placental Development









All obstetricians should be aware of the basic reproductive biological processes required for women to successfully achieve pregnancy. Several abnormalities can affect each of these and lead to infertility or pregnancy loss. In most women, spontaneous, cyclical ovulation at 25- to 35-day intervals continues during almost 40 years between menarche and menopause. Without contraception, there are approximately 400 opportunities for pregnancy, which may occur with intercourse on any of 1200 days—the day of ovulation and its two preceding days. This narrow window for fertilization is controlled by tightly regulated production of ovarian steroids. Moreover, these hormones promote optimal endometrial regeneration after menstruation in preparation for the next implantation window.

Should fertilization occur, events that begin after initial blastocyst implantation onto the endometrium and continue through to parturition result from a unique interaction between fetal trophoblasts and maternal endometrium-decidua. The ability of mother and fetus to coexist as two distinct immunological systems results from endocrine, paracrine, and immunological modification of fetal and maternal tissues in a manner not seen elsewhere. The placenta mediates a unique fetal–maternal communication system, which creates a hormonal environment that initially maintains pregnancy and eventually initiates events leading to parturition. The following sections address the physiology of the ovarian-endometrial cycle, implantation, placenta, and fetal membranes, as well as specialized endocrine arrangements between fetus and mother.


Predictable, regular, cyclical, and spontaneous ovulatory menstrual cycles are regulated by complex interactions of the hypothalamic-pituitary axis, ovaries, and genital tract (Fig. 5-1). The average cycle duration is approximately 28 days, with a range of 25 to 32 days. The hormonal sequence leading to ovulation directs this cycle. Concurrently, cyclical changes in endometrial histology are faithfully reproduced. Rock and Bartlett (1937) first suggested that endometrial histological features were sufficiently characteristic to permit cycle “dating.” In this scheme, the follicular-proliferative phase and the postovulatory luteal-secretory phase are customarily divided into early and late stages. These changes are detailed in Chapter 15 of Williams Gynecology, 2nd edition (Halvorson, 2012).


FIGURE 5-1 Gonadotropin control of the ovarian and endometrial cycles. The ovarian-endometrial cycle has been structured as a 28-day cycle. The follicular phase (days 1 to 14) is characterized by rising estrogen levels, endometrial thickening, and selection of the dominant “ovulatory” follicle. During the luteal phase (days 14 to 21), the corpus luteum (CL) produces estrogen and progesterone, which prepare the endometrium for implantation. If implantation occurs, the developing blastocyst begins to produce human chorionic gonadotropin (hCG) and rescues the corpus luteum, thus maintaining progesterone production. FSH = follicle-stimulating hormone; LH = luteinizing hormone.

image The Ovarian Cycle

Follicular or Preovulatory Ovarian Phase

The human ovary contains 2 million oocytes at birth, and approximately 400,000 follicles are present at puberty onset (Baker, 1963). The remaining follicles are depleted at a rate of approximately 1000 follicles per month until age 35, when this rate accelerates (Faddy, 1992). Only 400 follicles are normally released during female reproductive life. Therefore, more than 99.9 percent of follicles undergo atresia through a process of cell death termed apoptosis (Gougeon, 1996; Kaipia, 1997).

Follicular development consists of several stages, which include the gonadotropin-independent recruitment of primordial follicles from the resting pool and their growth to the antral stage. This appears to be controlled by locally produced growth factors. Two members of the transforming growth factor-β family—growth differentiation factor 9 (GDF9) and bone morphogenetic protein 15 (BMP-15)—regulate granulosa cell proliferation and differentiation as primary follicles grow (Trombly, 2009; Yan, 2001). They also stabilize and expand the cumulus oocyte complex in the oviduct (Hreinsson, 2002). These factors are produced by oocytes, suggesting that the early steps in follicular development are, in part, oocyte controlled. As antral follicles develop, surrounding stromal cells are recruited, by a yet-to-be-defined mechanism, to become thecal cells.

Although not required for early follicular maturation, follicle-stimulating hormone (FSH) is required for further development of large antral follicles (Hillier, 2001). During each ovarian cycle, a group of antral follicles, known as a cohort, begins a phase of semisynchronous growth based on their maturation state during the FSH rise in the late luteal phase of the previous cycle. This FSH rise leading to follicle development is called the selection window of the ovarian cycle (Macklon, 2001). Only the follicles progressing to this stage develop the capacity to produce estrogen.

During the follicular phase, estrogen levels rise in parallel to growth of a dominant follicle and to the increase in its number of granulosa cells (see Fig. 5-1). These cells are the exclusive site of FSH receptor expression. The rise of circulating FSH levels during the late luteal phase of the previous cycle stimulates an increase in FSH receptors and subsequently, the ability of cytochrome P450 aromatase within granulosa cells to convert androstenedione into estradiol. The requirement for thecal cells, which respond to luteinizing hormone (LH), and granulosa cells, which respond to FSH, represents the two-gonadotropin, two-cell hypothesis for estrogen biosynthesis (Short, 1962). As shown in Figure 5-2, FSH induces aromatase and expansion of the antrum of growing follicles. The follicle within the cohort that is most responsive to FSH is likely to be the first to produce estradiol and initiate expression of LH receptors.


FIGURE 5-2 The two-cell, two-gonadotropin principle of ovarian steroid hormone production. During the follicular phase (left panel), luteinizing hormone (LH) controls theca cell production of androstenedione, which diffuses into the adjacent granulosa cells and acts as precursor for estradiol biosynthesis. The granulosa cell capacity to convert androstenedione to estradiol is controlled by follicle-stimulating hormone (FSH). After ovulation (right panel), the corpus luteum forms and both theca-lutein and granulosa-lutein cells respond to LH. The theca-lutein cells continue to produce androstenedione, whereas granulosa-lutein cells greatly increase their capacity to produce progesterone and to convert androstenedione to estradiol. LH and hCG bind to the same LH-hCG receptor. If pregnancy occurs (right panel), human chorionic gonadotropin (hCG) rescues the corpus luteum through their shared LH-hCG receptor. Low-density lipoprotein (LDL) is an important source of cholesterol for steroidogenesis. cAMP = cyclic adenosine monophosphate.

After the appearance of LH receptors, the preovulatory granulosa cells begin to secrete small quantities of progesterone. The preovulatory progesterone secretion, although somewhat limited, is believed to exert positive feedback on the estrogen-primed pituitary to either cause or augment LH release. In addition, during the late follicular phase, LH stimulates thecal cell production of androgens, particularly androstenedione, which are then transferred to the adjacent follicles where they are aromatized to estradiol (see Fig. 5-2). During the early follicular phase, granulosa cells also produce inhibin B, which can feed back on the pituitary to inhibit FSH release (Groome, 1996). As the dominant follicle begins to grow, production of estradiol and the inhibins increases and results in a decline of follicular-phase FSH. This drop in FSH levels is responsible for the failure of other follicles to reach preovulatory status—the Graafian follicle stage—during any one cycle. Thus, 95 percent of plasma estradiol produced at this time is secreted by the dominant follicle—the one destined to ovulate. Concurrently, the contralateral ovary is relatively inactive.


The onset of the gonadotropin surge resulting from increasing estrogen secretion by preovulatory follicles is a relatively precise predictor of ovulation. It occurs 34 to 36 hours before ovum release from the follicle (see Fig. 5-1). LH secretion peaks 10 to 12 hours before ovulation and stimulates resumption of meiosis in the ovum and release of the first polar body. Current studies suggest that in response to LH, increased progesterone and prostaglandin production by the cumulus cells, as well as GDF9 and BMP-15 by the oocyte, activates expression of genes critical to formation of a hyaluronan-rich extracellular matrix by the cumulus complex (Richards, 2007). As seen in Figure 5-3, during synthesis of this matrix, cumulus cells lose contact with one another and move outward from the oocyte along the hyaluronan polymer—this process is called expansion. This results in a 20-fold increase in the complex volume along with an LH-induced remodeling of the ovarian extracellular matrix to allow release of the mature oocyte and its surrounding cumulus cells through the surface epithelium. Activation of proteases likely plays a pivotal role in weakening of the follicular basement membrane and ovulation (Curry, 2006; Ny, 2002).


FIGURE 5-3 An ovulated cumulus-oocyte complex. An oocyte is at the center of the complex. Cumulus cells are widely separated from each other in the cumulus layer by the hyaluronan-rich extracellular matrix. (Photograph contributed by Dr. Kevin J. Doody.)

Luteal or Postovulatory Ovarian Phase

Following ovulation, the corpus luteum develops from the dominant or Graafian follicle remains in a process referred to as luteinization. The basement membrane separating the granulosa-lutein and theca-lutein cells breaks down, and by day 2 postovulation, blood vessels and capillaries invade the granulosa cell layer. The rapid neovascularization of the once-avascular granulosa may be due to angiogenic factors that include vascular endothelial growth factor (VEGF) and others produced in response to LH by theca-lutein and granulosa-lutein cells (Albrecht, 2003; Fraser, 2001). During luteinization, these cells undergo hypertrophy and increase their capacity to synthesize hormones.

LH is the primary luteotropic factor responsible for corpus luteum maintenance (Vande Wiele, 1970). Indeed, LH injections can extend the corpus luteum life span in normal women by 2 weeks (Segaloff, 1951). In normal cycling women, the corpus luteum is maintained by low-frequency, high-amplitude LH pulses secreted by gonadotropes in the anterior pituitary (Filicori, 1986).

The hormone secretion pattern of the corpus luteum differs from that of the follicle (see Fig. 5-1). The increased capacity of granulosa-lutein cells to produce progesterone is the result of increased access to considerably more steroidogenic precursors through blood-borne low-density lipoprotein (LDL)-derived cholesterol as depicted in Figure 5-2 (Carr, 1981a). The important role for LDL in progesterone biosynthesis is supported by the observation that women with extremely low LDL cholesterol levels exhibit minimal progesterone secretion during the luteal phase (Illingworth, 1982). In addition, high-density lipoprotein (HDL) may contribute to progesterone production in granulosa-lutein cells (Ragoobir, 2002).

Estrogen levels follow a more complex pattern of secretion. Specifically, just after ovulation, estrogen levels decrease followed by a secondary rise that reaches a peak production of 0.25 mg/day of 17β-estradiol at the midluteal phase. Toward the end of the luteal phase, there is a secondary decline in estradiol production.

Ovarian progesterone production peaks at 25 to 50 mg/day during the midluteal phase. With pregnancy, the corpus luteum continues progesterone production in response to embryonic human chorionic gonadotropin (hCG), which binds to the same receptor as LH (see Fig. 5-2).

The human corpus luteum is a transient endocrine organ that, in the absence of pregnancy, will rapidly regress 9 to 11 days after ovulation via apoptotic cell death (Vaskivuo, 2002). The mechanisms that control luteolysis remain unclear. However, in part, it results from decreased levels of circulating LH in the late luteal phase and decreased LH sensitivity of luteal cells (Duncan, 1996; Filicori, 1986). The role of other factors is less clear, however, prostaglandin F (PGF) appears to be luteolytic in nonhuman primates (Auletta, 1987; Wentz, 1973). The endocrine effects, consisting of a dramatic drop in circulating estradiol and progesterone levels, are critical for follicular development and ovulation during the next ovarian cycle. In addition, corpus luteum regression and decline in circulating steroid concentrations signal the endometrium to initiate molecular events that lead to menstruation.

Estrogen and Progesterone Action

The fluctuating levels of ovarian steroids are the direct cause of the endometrial cycle. Recent advances in the molecular biology of estrogen and progesterone receptors have greatly improved understanding of their function. The most biologically potent naturally occurring estrogen—17β-estradiol—is secreted by granulosa cells of the dominant follicle and luteinized granulosa cells of the corpus luteum (see Fig. 5-2). Estrogen is the essential hormonal signal on which most events in the normal menstrual cycle depend. Estradiol action is complex and appears to involve two classic nuclear hormone receptors designated estrogen receptor α (ERα) and β (ERβ) (Katzenellenbogen, 2001). These isoforms are the products of separate genes and can exhibit distinct tissue expression. Both estradiol-receptor complexes act as transcriptional factors that become associated with the estrogen response element of specific genes. They share a robust activation by estradiol. However, differences in their binding affinities to other estrogens and their cell-specific expression patterns suggest that ERα and ERβ receptors may have both distinct and overlapping function (Saunders, 2005). Both receptors are expressed in the uterine endometrium (Bombail, 2008; Lecce, 2001). Estrogens function in many cell types to regulate follicular development, uterine receptivity, or blood flow.

Most progesterone actions on the female reproductive tract are mediated through the nuclear hormone receptors, progesterone receptor type A (PR-A) and B (PR-B). Progesterone enters cells by diffusion and in responsive tissues becomes associated with progesterone receptors (Conneely, 2002). Both progesterone receptor isoforms arise from a single gene, are members of the steroid receptor superfamily of transcription factors, and regulate transcription of target genes. These receptors have unique actions. When PR-A and PR-B receptors are coexpressed, it appears that PR-A can inhibit PR-B gene regulation. The endometrial glands and stroma appear to have different expression patterns for progesterone receptors that vary during the menstrual cycle (Mote, 1999). In addition, progesterone can evoke rapid responses such as changes in intracellular free calcium levels that cannot be explained by genomic mechanisms. G-protein-coupled membrane receptors for progesterone have been identified, but their role in the ovarian-endometrium cycle remains to be elucidated (Peluso, 2007).

image The Endometrial Cycle

Proliferative or Preovulatory Endometrial Phase

Fluctuations in estrogen and progesterone levels produce striking effects on the reproductive tract, particularly the endometrium. Epithelial—glandular cells; stromal—mesenchymal cells; and blood vessels of the endometrium replicate cyclically in reproductive-aged women at a rapid rate. The endometrium is regenerated during each ovarian–endometrial cycle. The superficial endometrium, termed the functionalis layer, is shed and regenerated from the deeper basalis layer almost 400 times during the reproductive lifetime of most women (Fig. 5-4). There is no other example in humans of such cyclical shedding and regrowth of an entire tissue.


FIGURE 5-4 The endometrium consists of two layers, the functionalis layer and basalis layer. These layers are supplied by the spiral and basal arteries, respectively. Numerous glands also span these layers. As the menstrual cycle progresses, greater coiling of the spiral arteries and increased gland folding can be seen. Near the end of the menstrual cycle (day 27), the coiled arteries constrict, depriving the functionalis layer of its blood supply and leading to necrosis and sloughing of this layer.

Follicular-phase estradiol production is the most important factor in endometrial recovery following menstruation. Although up to two thirds of the functionalis endometrium is fragmented and shed during menstruation, reepithelialization begins even before menstrual bleeding has ceased. By the fifth day of the endometrial cycle—fifth day of menses, the epithelial surface of the endometrium has been restored, and revascularization is in progress. The preovulatory endometrium is characterized by proliferation of glandular, stromal, and vascular endothelial cells. During the early part of the proliferative phase, the endometrium is usually less than 2 mm thick. The glands are narrow, tubular structures that pursue almost a straight and parallel course from the basalis layer toward the endometrial cavity. Mitotic figures, especially in the glandular epithelium, are identified by the fifth cycle day. Mitotic activity in both epithelium and stroma persists until day 16 to 17, or 2 to 3 days after ovulation. Although blood vessels are numerous and prominent, there is no extravascular blood or leukocyte infiltration in the endometrium at this stage.

Clearly, reepithelialization and angiogenesis are important to endometrial bleeding cessation (Chennazhi, 2009; Rogers, 2009). These are dependent on tissue regrowth, which is estrogen regulated. Epithelial cell growth also is regulated in part by epidermal growth factor (EGF) and transforming growth factor α (TGFα). Stromal cell proliferation appears to increase through paracrine and autocrine actions of estrogen and increased local levels of fibroblast growth factor-9 (Tsai, 2002). Estrogens also increase local production of VEGF, which causes angiogenesis through vessel elongation in the basalis (Gargett, 2001; Sugino, 2002).

By the late proliferative phase, the endometrium thickens from both glandular hyperplasia and increased stromal ground substance, which is edema and proteinaceous material. The loose stroma is especially prominent, and the glands in the functionalis layer are widely separated. This is compared with those of the basalis layer, in which the glands are more crowded and the stroma is denser. At midcycle, as ovulation nears, glandular epithelium becomes taller and pseudostratified. The surface epithelial cells acquire numerous microvilli, which increase epithelial surface area, and cilia, which aid in the movement of endometrial secretions during the secretory phase (Ferenczy, 1976).

Dating the menstrual cycle day by endometrial histological criteria is difficult during the proliferative phase because of considerable phase-length variation among women. Specifically, the follicular phase normally may be as short as 5 to 7 days or as long as 21 to 30 days. In contrast, the luteal or secretory postovulatory phase of the cycle is remarkably constant at 12 to 14 days.

Secretory or Postovulatory Endometrial Phase

During the early secretory phase, endometrial dating is based on glandular epithelium histology. After ovulation, the estrogen-primed endometrium responds to rising progesterone levels in a highly predictable manner. By day 17, glycogen accumulates in the basal portion of glandular epithelium, creating subnuclear vacuoles and pseudostratification. This is the first sign of ovulation that is histologically evident. It is likely the result of direct progesterone action through receptors expressed in glandular cells (Mote, 2000). On day 18, vacuoles move to the apical portion of the secretory nonciliated cells. By day 19, these cells begin to secrete glycoprotein and mucopolysaccharide contents into the lumen (Hafez, 1975). Glandular cell mitosis ceases with secretory activity on day 19 due to rising progesterone levels, which antagonize the mitotic effects of estrogen. Estradiol action is also decreased because of glandular expression of the type 2 isoform of 17β-hydroxysteroid dehydrogenase. This converts estradiol to the less active estrone (Casey, 1996).

Dating in the mid- to late-secretory phase relies on endometrial stromal changes. On days 21 to 24, the stroma becomes edematous. On days 22 to 25, stromal cells surrounding the spiral arterioles begin to enlarge, and stromal mitosis becomes apparent. Days 23 to 28 are characterized by predecidual cells, which surround spiral arterioles.

An important feature of secretory-phase endometrium between days 22 and 25 is striking changes associated with predecidual transformation of the upper two thirds of the functionalis layer. The glands exhibit extensive coiling, and luminal secretions become visible. Changes within the endometrium also can mark the so-called window of implantation seen on days 20 to 24. Epithelial surface cells show decreased microvilli and cilia but appearance of luminal protrusions on the apical cell surface (Nikas, 2003). These pinopodes are important in preparation for blastocyst implantation. They also coincide with changes in the surface glycocalyx that allow acceptance of a blastocyst (Aplin, 2003).

The secretory phase is also highlighted by the continuing growth and development of the spiral arteries. Boyd and Hamilton (1970) emphasized the extraordinary importance of the endometrial spiral or coiled arteries. They arise from the radial arteries, which are myometrial branches of the arcuate and ultimately, uterine vessels (see Fig. 5-4). The morphological and functional properties of spiral arteries are unique and essential for establishing blood flow changes to permit either menstruation or implantation. During endometrial growth, spiral arteries lengthen at a rate appreciably greater than the rate of endometrial tissue thickening (Fig. 5-5). This growth discordance obliges even greater coiling of the already spiraling vessels. Spiral artery development reflects a marked induction of angiogenesis, consisting of widespread vessel sprouting and extension. Perrot-Applanat and associates (1988) described progesterone and estrogen receptors in the smooth muscle cells of the uterus and spiral arteries. Such rapid angiogenesis is regulated, in part, through estrogen- and progesterone-regulated synthesis of VEGF (Ancelin, 2002; Chennazhi, 2009).


FIGURE 5-5 The spiral arteries of human endometrium are modified during the ovulatory cycle. Initially, blood flow changes through these vessels aid endometrial growth. Excessive coiling and blood flow stasis coincide with regression of corpus luteum function and lead to a decline in endometrial tissue volume. Finally, spiral artery coiling leads to endometrial hypoxia and necrosis. Before endometrial bleeding, intense spiral artery vasospasm serves to limit blood loss during menses.


The midluteal–secretory phase of the endometrial cycle is a critical branch point in endometrial development and differentiation. With corpus luteum rescue and continued progesterone secretion, the decidualization process continues. If luteal progesterone production decreases with luteolysis, events leading to menstruation are initiated (Critchley, 2006; Thiruchelvam, 2013).

A notable histological characteristic of late premenstrual-phase endometrium is stromal infiltration by neutrophils, giving a pseudoinflammatory appearance to the tissue. These cells infiltrate primarily on the day or two immediately preceding menses onset. The endometrial stromal and epithelial cells produce interleukin-8 (IL-8), a chemotactic–activating factor for neutrophils (Arici, 1993). Similarly, monocyte chemotactic protein-1 (MCP-1) is synthesized by endometrium and promotes monocyte recruitment (Arici, 1995).

Leukocyte infiltration is considered key to both endometrial extracellular matrix breakdown and repair of the functionalis layer. The term “inflammatory tightrope” refers to the ability of macrophages to assume phenotypes that vary from proinflammatory and phagocytic to immunosuppressive and reparative. These are likely relevant to menstruation, in which tissue breakdown and restoration occur simultaneously (Evans, 2012). Invading leukocytes secrete enzymes that are members of the matrix metalloprotease (MMP) family. These add to the proteases already produced by endometrial stromal cells and effectively initiate matrix degradation. This phenomenon has been proposed to initiate the events leading to menstruation (Dong, 2002). As tissue shedding is completed, microenvironment-regulated changes in macrophage phenotype promote repair and resolution (Evans, 2012; Thiruchelvam, 2013).

Anatomical Events During Menstruation. The classic study by Markee (1940) described tissue and vascular changes in endometrium before menstruation. First, there are marked changes in endometrial blood flow essential for menstruation. With endometrial regression, spiral artery coiling becomes sufficiently severe that resistance to blood flow increases strikingly, causing endometrial hypoxia. Resultant stasis is the primary cause of endometrial ischemia and tissue degeneration (see Fig. 5-5). Vasoconstriction precedes menstruation and is the most striking and constant event observed in the cycle. Intense spiral artery vasoconstriction also serves to limit menstrual blood loss. Blood flow appears to be regulated in an endocrine manner by sex steroid hormone–induced modifications of a paracrine-mediated vasoactive peptide system as described subsequently.

Prostaglandins and Menstruation. Progesterone withdrawal increases expression of cyclooxygenase 2 (COX-2), also called prostaglandin synthase 2, to synthesize prostaglandins. Withdrawal also decreases expression of 15-hydroxyprostaglandin dehydrogenase (PGDH), which degrades prostaglandins (Casey, 1980, 1989). The net result is increased prostaglandin production by endometrial stromal cells and increased prostaglandin-receptor density on blood vessels and surrounding cells.

A role for prostaglandins—especially vasoconstricting PGF—in menstruation initiation has been suggested (Abel, 2002). Large amounts of prostaglandins are present in menstrual blood. PGFadministration to nonpregnant women prompts menstruation and symptoms that mimic dysmenorrhea. Painful menstruation is common and likely caused by myometrial contractions and uterine ischemia. This response is believed to be mediated by PGF-induced spiral artery vasoconstriction that causes the uppermost endometrial zones to become hypoxic. The hypoxic environment is a potent inducer of angiogenesis and vascular permeability factors such as VEGF. Prostaglandins serve an important function in the event cascade leading to menstruation that includes vasoconstriction, myometrial contractions, and upregulation of proinflammatory responses.

Activation of Lytic Mechanisms. Following vasoconstriction and endometrial cytokine changes, protease activation within stromal cells and leukocyte invasion is required to degrade the endometrial interstitial matrix. Matrix metalloproteases—MMP-1 and MMP-3—are released from stromal cells and may activate other neutrophilic proteases such as MMP-8 and MMP-9.

Origin of Menstrual Blood. Menstrual bleeding is appreciably arterial rather than venous bleeding. Endometrial bleeding appears to follow rupture of a spiral arteriole and consequent hematoma formation. With a hematoma, the superficial endometrium is distended and ruptures. Subsequently, fissures develop in the adjacent functionalis layer, and blood and tissue fragments are sloughed. Hemorrhage stops with arteriolar constriction. Changes that accompany partial tissue necrosis also serve to seal vessel tips.

The endometrial surface is restored by growth of flanges, or collars, that form the everted free ends of the endometrial glands (Markee, 1940). These flanges increase in diameter very rapidly, and epithelial continuity is reestablished by fusion of the edges of these sheets of migrating cells.

Interval between Menses. The modal interval of menstruation is considered to be 28 days, but variation is considerable among women, as well as in the cycle lengths of a given woman. Marked differences in the intervals between menstrual cycles are not necessarily indicative of infertility. Arey (1939) analyzed 12 studies comprising approximately 20,000 calendar records from 1500 women. He concluded that there is no evidence of perfect menstrual cycle regularity. Among average adult women, a third of cycles departed by more than 2 days from the mean of all cycle lengths. In his analysis of 5322 cycles in 485 normal women, an average interval of 28.4 days was estimated.


This is a specialized, highly modified endometrium of pregnancy. It is essential for hemochorial placentation, that is, one in which maternal blood contacts trophoblast. This relationship requires trophoblast invasion, and considerable research has focused on the interaction between decidual cells and invading trophoblasts. Decidualization, that is, transformation of secretory endometrium to decidua, is dependent on estrogen and progesterone and factors secreted by the implanting blastocyst. The special relationship that exists between the decidua and the invading trophoblast seemingly defies the laws of transplantation immunology. The success of this unique semiallograft not only is of great scientific interest but may involve processes that harbor insights leading to more successful transplantation surgery and perhaps even immunological treatment of neoplasia (Billingham, 1986; Lala, 2002).

image Decidual Structure

The decidua is classified into three parts based on anatomical location. Decidua directly beneath blastocyst implantation is modified by trophoblast invasion and becomes the decidua basalis. The decidua capsularis overlies the enlarging blastocyst and initially separates the conceptus from the rest of the uterine cavity (Fig. 5-6). This portion is most prominent during the second month of pregnancy and consists of decidual cells covered by a single layer of flattened epithelial cells. Internally, it contacts the avascular, extraembryonic fetal membrane—the chorion laeve. The remainder of the uterus is lined by decidua parietalis. During early pregnancy, there is a space between the decidua capsularis and parietalis because the gestational sac does not fill the entire uterine cavity. By 14 to 16 weeks’ gestation, the expanding sac has enlarged to completely fill the uterine cavity. The resulting apposition of the decidua capsularis and parietalis creates the decidua vera, and the uterine cavity is functionally obliterated.


FIGURE 5-6 Three portions of the decidua—the basalis, capsularis, and parietalis—are illustrated.

In early pregnancy, the decidua begins to thicken, eventually attaining a depth of 5 to 10 mm. With magnification, furrows and numerous small openings, representing the mouths of uterine glands, can be detected. Later in pregnancy, the decidua becomes thinner, presumably because of pressure exerted by the expanding uterine contents.

The decidua parietalis and basalis are composed of three layers. There is a surface or compact zone—zona compacta; a middle portion or spongy zone—zona spongiosa—with remnants of glands and numerous small blood vessels; and a basal zone—zona basalis. The zona compacta and spongiosa together form the zona functionalis. The basal zone remains after delivery and gives rise to new endometrium.

image Decidual Reaction

In human pregnancy, the decidual reaction is completed only with blastocyst implantation. Predecidual changes, however, commence first during the midluteal phase in endometrial stromal cells adjacent to the spiral arteries and arterioles. Thereafter, they spread in waves throughout the uterine endometrium and then from the implantation site. The endometrial stromal cells enlarge to form polygonal or round decidual cells. The nuclei become round and vesicular, and the cytoplasm becomes clear, slightly basophilic, and surrounded by a translucent membrane. Each mature decidual cell becomes surrounded by a pericellular membrane. Thus, the human decidual cells clearly build walls around themselves and possibly around the fetus. The pericellular matrix surrounding the decidual cells may allow attachment of cytotrophoblasts through cellular adhesion molecules. The cell membrane also may provide decidual cell protection against selected cytotrophoblastic proteases.

image Decidual Blood Supply

As a consequence of implantation, the blood supply to the decidua capsularis is lost as the embryo-fetus grows. Blood supply to the decidua parietalis through spiral arteries persists. These arteries retain a smooth-muscle wall and endothelium and thereby remain responsive to vasoactive agents.

In contrast, the spiral arterial system supplying the decidua basalis directly beneath the implanting blastocyst, and ultimately the intervillous space, is altered remarkably. These spiral arterioles and arteries are invaded by cytotrophoblasts. During this process, the vessel walls in the basalis are destroyed. Only a shell without smooth muscle or endothelial cells remains. Importantly, as a result, these vascular conduits of maternal blood—which become the uteroplacental vessels—are not responsive to vasoactive agents. Conversely, the fetal chorionic vessels, which transport blood between the placenta and the fetus, contain smooth muscle and thus do respond to vasoactive agents.

image Decidual Histology

Early in pregnancy, the zona spongiosa of the decidua consists of large distended glands, often exhibiting marked hyperplasia and separated by minimal stroma. At first, the glands are lined by typical cylindrical uterine epithelium with abundant secretory activity that contributes to blastocyst nourishment. As pregnancy progresses, the epithelium gradually becomes cuboidal or even flattened and later degenerates and sloughs to a greater extent into the gland lumens. With advanced pregnancy, the glandular elements largely disappear. In comparing the decidua parietalis at 16 weeks’ gestation with the early proliferative endometrium of a nonpregnant woman, there is marked hypertrophy but only slight hyperplasia of the endometrial stroma during decidual transformation.

The decidua basalis contributes to formation of the placental basal plate (Fig. 5-7). It differs histologically from the decidua parietalis in two important respects. First, the spongy zone of the decidua basalis consists mainly of arteries and widely dilated veins, and by term, glands have virtually disappeared. Second, the decidua basalis is invaded by many interstitial trophoblast cells and trophoblastic giant cells. Although most abundant in the decidua, the giant cells commonly penetrate the upper myometrium. Their number and invasiveness can be so extensive as to resemble choriocarcinoma.


FIGURE 5-7 Section through a junction of chorion, villi, and decidua basalis in early first-trimester pregnancy. (Photograph contributed by Dr. Kurt Benirschke.)

The Nitabuch layer is a zone of fibrinoid degeneration in which invading trophoblasts meet the decidua basalis. If the decidua is defective, as in placenta accreta, the Nitabuch layer is usually absent (Chap. 41p. 804). There is also a more superficial, but inconsistent, deposition of fibrin—Rohr stria—at the bottom of the intervillous space and surrounding the anchoring villi. McCombs and Craig (1964) found that decidual necrosis is a normal phenomenon in the first and probably second trimesters. Thus, necrotic decidua obtained through curettage after spontaneous abortion in the first trimester should not necessarily be interpreted as either a cause or an effect of the pregnancy loss.

Both deciduas contain numerous cell types whose composition varies with gestational stage (Loke, 1995). The primary cellular components are the true decidual cells, which differentiated from the endometrial stromal cells, and numerous maternal bone marrow–derived cells.

Early in pregnancy, a striking abundance of large, granular lymphocytes termed decidual natural killer (NK) cells are present in the decidua. In peripheral blood, there are two subsets of NK cells. Approximately 90 percent are highly cytolytic. Ten percent show less cytolytic ability but increased cytokine secretion. In contrast to peripheral blood, 95 percent of NK cells in decidua secrete cytokines, and about half of these unique cells also express angiogenic factors. These decidua NK cells likely play an important role in trophoblast invasion and vasculogenesis.

image Decidual Prolactin

In addition to placental development, the decidua potentially provides other functions. The decidua is the source of prolactin that is present in enormous amounts in amnionic fluid (Golander, 1978; Riddick, 1979). Decidual prolactinis not to be confused with placental lactogen (hPL), which is produced only by syncytiotrophoblast. Rather, decidual prolactin is a product of the same gene that encodes for anterior pituitary prolactin. And although the amino-acid sequence of prolactin in both tissues is identical, an alternative promoter is used within the prolactin gene to initiate transcription in decidua (Telgmann, 1998). This may explain the different mechanisms that regulate expression in the decidua versus pituitary (Christian, 2002a,b).

Prolactin preferentially enters amnionic fluid, and little enters maternal blood. Consequently, prolactin levels in amnionic fluid are extraordinarily high and may reach 10,000 ng/mL at 20 to 24 weeks’ gestation (Tyson, 1972). This compares with fetal serum levels of 350 ng/mL and maternal serum levels of 150 to 200 ng/mL. As a result, decidual prolactin is a classic example of paracrine function between maternal and fetal tissues.

The exact physiological roles of decidual prolactin are still unknown. Its action is mediated by the relative expression of two unique prolactin receptors and by the amount of intact or full-length prolactin protein compared with the truncated 16-kDa form (Jabbour, 2001). Receptor expression has been demonstrated in decidua, chorionic cytotrophoblasts, amnionic epithelium, and syncytiotrophoblast (Maaskant, 1996). There are several possible roles for decidual prolactin. First, most or all of this protein hormone enters amnionic fluid. Thus, it may serve in transmembrane solute and water transport and in amnionic fluid volume maintenance. Second, there are prolactin receptors in several bone marrow-derived immune cells, and prolactin may stimulate T cells in an autocrine or paracrine manner (Pellegrini, 1992). This raises the possibility that decidual prolactin may act in regulating immunological functions during pregnancy. Prolactin may play a role in angiogenesis regulation during implantation. Last, decidual prolactin has been shown in the mouse to have a protective function by repressing expression of genes detrimental to pregnancy maintenance (Bao, 2007).

Regulation of decidual prolactin is not clearly defined. Most agents known to inhibit or stimulate pituitary prolactin secretion—including dopamine, dopamine agonists, and thyrotropin-releasing hormone—do not alter decidual prolactin secretion either in vivo or in vitro. Brosens and colleagues (2000) demonstrated that progestins act synergistically with cyclic adenosine monophosphate on endometrial stromal cells in culture to increase prolactin expression. This suggests that the level of progesterone receptor expression may determine the decidualization process, at least as marked by prolactin production. Conversely, various cytokines and growth factors—endothelin-1, IL-1, IL-2, and epidermal growth factor—decrease decidual prolactin secretion (Chao, 1994; Frank, 1995). Studies in decidualized human endometrial cells in culture have led to identification of several transcription factors that regulate decidual prolactin (Jiang, 2011; Lynch, 2009).


The fetus is dependent on the placenta for pulmonary, hepatic, and renal functions. These are accomplished through the unique anatomical relationship of the placenta and its uterine interface. Maternal blood spurts from uteroplacental vessels into the placental intervillous space and bathes the outer syncytiotrophoblast. This allows exchange of gases, nutrients, and other substances with fetal capillary blood within the villous core. Thus, fetal and maternal blood are not normally mixed in this hemochorial placenta. There is also a paracrine system that links mother and fetus through the anatomical and biochemical juxtaposition of the maternal decidua parietalis and the extraembryonic chorion laeve, which is fetal. This is an extraordinarily important arrangement for communication between fetus and mother and for maternal immunological acceptance of the conceptus (Guzeloglu-Kayisli, 2009).

image Fertilization and Implantation

With ovulation, the secondary oocyte and adhered cells of the cumulus-oocyte complex are freed from the ovary. Although technically this mass of cells is released into the peritoneal cavity, the oocyte is quickly engulfed by the fallopian tube infundibulum. Further transport through the tube is accomplished by directional movement of cilia and tubal peristalsis. Fertilization, which normally occurs in the oviduct, must take place within a few hours, and no more than a day after ovulation. Because of this narrow opportunity window, spermatozoa must be present in the fallopian tube at the time of oocyte arrival. Almost all pregnancies result when intercourse occurs during the 2 days preceding or on the day of ovulation. Thus, postovulatory and postfertilization developmental ages are similar.

Steps of fertilization are highly complex. Molecular mechanisms allow spermatozoa to pass between follicular cells; through the zona pellucida, which is a thick glycoprotein layer surrounding the oocyte cell membrane; and into the oocyte cytoplasm. Fusion of the two nuclei and intermingling of maternal and paternal chromosomes creates the zygote. These steps are reviewed by Primakoff and Myles (2002).

Early human development is described by days or weeks postfertilization, that is, post-conceptional. By contrast, in most chapters of this book, clinical pregnancy dating is calculated from the start of the last menses. As discussed earlier, the follicular phase length is more variable than the luteal phase. Thus, 1 week postfertilization corresponds to approximately 3 weeks from the last menstrual period in women with regular 28-day cycles.

The Zygote

After fertilization, the zygote—a diploid cell with 46 chromosomes—undergoes cleavage, and zygote cells produced by this division are called blastomeres (Fig. 5-8). In the two-cell zygote, the blastomeres and polar body continue to be surrounded by the zona pellucida. The zygote undergoes slow cleavage for 3 days while still within the fallopian tube. As the blastomeres continue to divide, a solid mulberry-like ball of cells—the morula—is produced. The morula enters the uterine cavity about 3 days after fertilization. Gradual accumulation of fluid between the morula cells leads to formation of the early blastocyst.


FIGURE 5-8 Zygote cleavage and blastocyst formation. The morula period begins at the 12- to 16-cell stage and ends when the blastocyst forms, which occurs when there are 50 to 60 blastomeres present. The polar bodies, shown in the 2-cell stage, are small nonfunctional cells that soon degenerate.

The Blastocyst

As early as 4 to 5 days after fertilization, the 58-cell blastula differentiates into five embryo-producing cells—the inner cell mass, and 53 cells destined to form trophoblast (Hertig, 1962). In a 58-cell blastocyst, the outer cells, called the trophectoderm, can be distinguished from the inner cell mass that forms the embryo (see Fig. 5-8).

Interestingly, the 107-cell blastocyst is found to be no larger than the earlier cleavage stages, despite the accumulated fluid. It measures approximately 0.155 mm in diameter, which is similar to the size of the initial postfertilization zygote. At this stage, the eight formative, embryo-producing cells are surrounded by 99 trophoblastic cells. And, the blastocyst is released from the zona pellucida secondary to secretion of specific proteases from the secretory-phase endometrial glands (O’Sullivan, 2002).

Release from the zona pellucida allows blastocyst-produced cytokines and hormones to directly influence endometrial receptivity (Lindhard, 2002). IL-1α and IL-1β are secreted by the blastocyst, and these cytokines likely directly influence the endometrium. Embryos also have been shown to secrete hCG, which may influence endometrial receptivity (Licht, 2001; Lobo, 2001). The receptive endometrium is thought to respond by producing leukemia inhibitory factor (LIF) and colony-stimulating factor-1 (CSF-1). These serve to increase trophoblast protease production. This degrades selected endometrial extracellular matrix proteins and allows trophoblast invasion. Thus, embryo “hatching” is a critical step toward successful pregnancy as it allows association of trophoblasts with endometrial epithelial cells and permits release of trophoblast-produced hormones into the uterine cavity.

image Blastocyst Implantation

Six or 7 days after fertilization, the embryo implants the uterine wall. This process can be divided into three phases: (1) apposition—initial contact of the blastocyst to the uterine wall; (2) adhesion—increased physical contact between the blastocyst and uterine epithelium; and (3) invasion—penetration and invasion of syncytiotrophoblast and cytotrophoblasts into the endometrium, inner third of the myometrium, and uterine vasculature.

Successful implantation requires a receptive endometrium appropriately primed with estrogen and progesterone by the corpus luteum. Such uterine receptivity is limited to days 20 to 24 of the cycle. Adherence is mediated by cell-surface receptors at the implantation site that interact with blastocyst receptors (Carson, 2002; Lessey, 2002; Lindhard, 2002; Paria, 2002). If the blastocyst approaches the endometrium after cycle day 24, the potential for adhesion is diminished because antiadhesive glycoprotein synthesis prevents receptor interactions (Navot, 1991).

At the time of its interaction with the endometrium, the blastocyst is composed of 100 to 250 cells. The blastocyst loosely adheres to the endometrial epithelium by apposition. This most commonly occurs on the upper posterior uterine wall. Attachment of the blastocyst trophectoderm to the endometrial surface by apposition and adherence appears to be closely regulated by paracrine interactions between these two tissues.

Successful endometrial blastocyst adhesion involves modification in expression of cellular adhesion molecules (CAMs). The integrins—one of four families of CAMs—are cell-surface receptors that mediate cell adhesion to extracellular matrix proteins (Lessey, 2002). Great diversity of cell binding to several different extracellular matrix proteins is possible by differential regulation of the integrin receptors. Endometrial integrins are hormonally regulated, and a specific set of integrins is expressed at implantation (Lessey, 1996). Specifically, αVβ3 and α4β1 integrins expressed on endometrial epithelium are considered a receptivity marker for blastocyst attachment. Aberrant expression of αVβ3 has been associated with infertility (Lessey, 1995). Recognition-site blockade on integrins for binding to extracellular matrix molecules such as fibronectin will prevent blastocyst attachment (Kaneko, 2013).

image The Trophoblast

Human placental formation begins with the trophectoderm, which appears at the morula stage. It gives rise to a trophoblast cell layer encircling the blastocyst. From then until term, the trophoblast plays a critical part at the fetal-maternal interface. Trophoblast exhibits the most variable structure, function, and developmental pattern of all placental components. Its invasiveness promotes implantation, its nutritional role for the conceptus is reflected in its name, and its endocrine organ function is essential to maternal physiological adaptations and to pregnancy maintenance.

Trophoblast Differentiation

By the eighth day postfertilization, after initial implantation, the trophoblast has differentiated into an outer multinucleated syncytium—primitive syncytiotrophoblast, and an inner layer of primitive mononuclear cells—cytotrophoblasts (Fig. 5-9). The latter are germinal cells for the syncytium. Each cytotrophoblast has a well-demarcated cell border, a single nucleus, and ability to undergo DNA synthesis and mitosis (Arnholdt, 1991). These are lacking in the syncytiotrophoblast. It is so named because instead of individual cells, it has an amorphous cytoplasm without cell borders, nuclei that are multiple and diverse in size and shape, and a continuous syncytial lining. This configuration aids transport.


FIGURE 5-9 Drawing of sections through implanted blastocysts. A. At 10 days. B. At 12 days after fertilization. This stage is characterized by the intercommunication of the lacunae filled with maternal blood. Note in (B) that large cavities have appeared in the extraembryonic mesoderm, forming the beginning of the extraembryonic coelom. Also note that extraembryonic endodermal cells have begun to form on the inside of the primitive yolk sac. (Adapted from Moore, 1988.)

After implantation is complete, trophoblast further differentiate along two main pathways, giving rise to villous and extravillous trophoblast. As shown in Figure 5-10, both pathways create populations of trophoblast cells that have distinct functions (Loke, 1995). The villous trophoblast gives rise to the chorionic villi, which primarily transport oxygen, nutrients, and other compounds between the fetus and mother. Extravillous trophoblasts migrate into the decidua and myometrium and also penetrate maternal vasculature, thus coming into contact with various maternal cell types (Pijnenborg, 1994). Extravillous trophoblasts are thus further classified as interstitial trophoblasts and endovascular trophoblasts. The interstitial trophoblasts invade the decidua and eventually penetrate the myometrium to form placental bed giant cells. These trophoblasts also surround spiral arteries. The endovascular trophoblasts penetrate the spiral artery lumens (Pijnenborg, 1983). These are both discussed in greater detail in subsequent sections.


FIGURE 5-10 Extravillous trophoblasts are found outside the villus and can be subdivided into endovascular and interstitial categories. Endovascular trophoblasts invade and transform spiral arteries during pregnancy to create low-resistance blood flow that is characteristic of the placenta. Interstitial trophoblasts invade the decidua and surround spiral arteries.

image Early Trophoblast Invasion

After gentle erosion between epithelial cells of the surface endometrium, invading trophoblasts burrow deeper. By the 10th day, the blastocyst becomes totally encased within the endometrium. The mechanisms leading to trophoblast invasion are similar to those of metastasizing malignant cells and are discussed further on page 93.

At 9 days of development, the blastocyst wall facing the uterine lumen is a single layer of flattened cells (see Fig. 5-9A). The opposite, thicker wall comprises two zones—the trophoblasts and the embryo-forming inner cell mass. As early as 7½ days after fertilization, the inner cell mass or embryonic disc is differentiated into a thick plate of primitive ectoderm and an underlying layer of endoderm. Some small cells appear between the embryonic disc and the trophoblast and enclose a space that will become the amnionic cavity.

Embryonic mesenchyme first appears as isolated cells within the blastocyst cavity. When the cavity is completely lined with this mesoderm, it is termed the chorionic vesicle, and its membrane, now called the chorion, is composed of trophoblasts and mesenchyme. Some mesenchymal cells eventually will condense to form the body stalk. This stalk joins the embryo to the nutrient chorion and later develops into the umbilical cord. The body stalk can be recognized at an early stage at the caudal end of the embryonic disc (Fig. 7-4, 129).

Lacunae Formation within the Syncytiotrophoblast

As the embryo enlarges, more maternal decidua basalis is invaded by syncytiotrophoblast. Beginning approximately 12 days after conception, the syncytiotrophoblast is permeated by a system of intercommunicating channels called trophoblastic lacunae. After invasion of superficial decidual capillary walls, lacunae become filled with maternal blood (see Figs. 5-9B and 5-11). At the same time, the decidual reaction intensifies in the surrounding stroma. This is characterized by decidual stromal cell enlargement and glycogen storage.


FIGURE 5-11 Early implantation of a conceptus. (Photograph contributed by Dr. Kurt Benirschke.)

image Placental Organization

Chorionic Villi

With deeper blastocyst invasion into the decidua, the extravillous cytotrophoblasts give rise to solid primary villi composed of a cytotrophoblast core covered by syncytiotrophoblast. These arise from buds of cytotrophoblast that protrude into the primitive syncytium before 12 days postfertilization. As the lacunae join, a complicated labyrinth is formed that is partitioned by these solid cytotrophoblastic columns. The trophoblast-lined labyrinthine channels form the intervillous space, and the solid cellular columns form the primary villous stalks. The villi initially are located over the entire blastocyst surface. They later disappear except over the most deeply implanted portion, which is destined to form the placenta.

Beginning on approximately the 12th day after fertilization, mesenchymal cords derived from extraembryonic mesoderm invade the solid trophoblast columns. These form secondary villi. Once angiogenesis begins in the mesenchymal cores, tertiary villi are formed. Although maternal venous sinuses are tapped early in implantation, maternal arterial blood does not enter the intervillous space until around day 15. By approximately the 17th day, however, fetal blood vessels are functional, and a placental circulation is established. The fetal–placental circulation is completed when the blood vessels of the embryo are connected with chorionic vessels. In some villi, angiogenesis fails from lack of circulation. They can be seen normally, but the most striking exaggeration of this process is seen with hydatidiform mole (Fig. 20-1p. 397).

Villi are covered by an outer layer of syncytium and an inner layer of cytotrophoblasts, which are also known as Langhans cells. Cytotrophoblast proliferation at the villous tips produces the trophoblastic cell columns that form anchoring villi. They are not invaded by fetal mesenchyme, and they are anchored to the decidua at the basal plate. Thus, the base of the intervillous space faces the maternal side and consists of cytotrophoblasts from cell columns, the covering shell of syncytiotrophoblast, and maternal decidua of the basal plate. The base of the chorionic plate forms the roof of the intervillous space. It consists of two layers of trophoblasts externally and fibrous mesoderm internally. The “definitive” chorionic plate is formed by 8 to 10 weeks as the amnionic and primary chorionic plate mesenchyme fuse together. This formation is accomplished by expansion of the amnionic sac, which also surrounds the connective stalk and the allantois and joins these structures to form the umbilical cord (Kaufmann, 1992).

Villus Ultrastructure

Interpretation of the fine structure of the placenta came from electron microscopic studies of Wislocki and Dempsey (1955). There are prominent microvilli on the syncytial surface that correspond to the so-called brush border described by light microscopy. Associated pinocytotic vacuoles and vesicles are related to absorptive and secretory placental functions. Microvilli act to increase surface area in direct contact with maternal blood. This contact between the trophoblast and maternal blood is the defining characteristic of a hemochorial placenta.

The human hemochorial placenta can be subdivided into hemodichorial or hemomonochorial (Enders, 1965). The dichorial type is more prominent during the first trimester of gestation. It consists of the inner layer of the cytotrophoblasts and associated basal lamina, covered by a syncytiotrophoblast layer. Later in gestation the inner layer of cytotrophoblasts is no longer continuous, and by term, there are only scattered cells present (Fig. 5-12). These create a narrower hemomonochorial barrier that aids nutrient and oxygen transport to the fetus.


FIGURE 5-12 Electron micrograph of term human placenta villus. A villus capillary filled with red blood cells (asterisks) is seen in close proximity to the microvilli border. (From Boyd, 1970, with permission.)


image Chorion and Decidua Development

In early pregnancy, the villi are distributed over the entire periphery of the chorionic membrane (Fig. 5-13). As the blastocyst with its surrounding trophoblasts grows and expands into the decidua, one pole faces the endometrial cavity. The opposite pole will form the placenta from villous trophoblasts and anchoring cytotrophoblasts. Chorionic villi in contact with the decidua basalis proliferate to form the chorion frondosum—or leafy chorion—which is the fetal component of the placenta. As growth of embryonic and extraembryonic tissues continues, the blood supply to the chorion facing the endometrial cavity is restricted. Because of this, villi in contact with the decidua capsularis cease to grow and then degenerate. This portion of the chorion becomes the avascular fetal membrane that abuts the decidua parietalis, that is, the chorion laeve—or smooth chorion. This smooth chorion is composed of cytotrophoblasts and fetal mesodermal mesenchyme that survives in a relatively low-oxygen atmosphere.


FIGURE 5-13 Photograph of an opened chorionic sac. An early embryo and yolk sac are seen. Note the prominent fringe of chorionic villi. (From Boyd, 1970, with permission.)

Until near the end of the third month, the chorion laeve is separated from the amnion by the exocoelomic cavity. Thereafter, they are in intimate contact to form an avascular amniochorion. The chorion laeve is generally more translucent than the amnion and rarely exceeds 1-mm thickness. These two structures are important sites of molecular transfer and metabolic activity. Moreover, they constitute an important paracrine arm of the fetal–maternal communication system.

Maternal Regulation of Trophoblast Invasion and Vascular Growth

During the first half of pregnancy, decidual natural killer cells (dNK) accumulate in the decidua and are found in direct contact with trophoblasts. As described on page 88, these cells lack cytotoxic functions and are able to dampen inflammatory T(H)17 cells. These along with other unique properties distinguish dNK cells from circulating natural killer cells and from natural killer cells in the endometrium before pregnancy (Fu, 2013; Winger, 2013). Recent studies suggest that decidual macrophages play a regulatory role in inhibiting NK cell killing during pregnancy (Co, 2013). This importantly prevents them from recognizing and destroying fetal cells as “foreign.” Hanna and colleagues (2006) have elucidated the ability of dNK cells to attract and promote trophoblast invasion into the decidua and promote vascular growth. Decidual NK cells express both IL-8 and interferon-inducible protein-10, which bind to receptors on invasive trophoblast cells to promote their decidual invasion toward the spiral arteries. Decidual NK cells also produce proangiogenic factors, including VEGF and placental growth factor (PlGF), which promote vascular growth in the decidua. In addition, trophoblasts secrete specific chemokines that attract the dNK cells to the maternal-fetal interface. Thus, both cell types simultaneously attract each other.

Trophoblast Invasion of the Endometrium

Extravillous trophoblasts of the first-trimester placenta are highly invasive. They form cell columns that extend from the endometrium to the inner third of the myometrium. Recall that hemochorial placental development requires invasion of endometrium and spiral arteries. This process occurs under low-oxygen conditions, and regulatory factors that are induced under hypoxic conditions contribute in part to invasive trophoblast activation (Soares, 2012). Invasive trophoblasts secrete numerous proteolytic enzymes that digest extracellular matrix and activate proteinases already present in the endometrium. Trophoblasts produce urokinase-type plasminogen activator, which converts plasminogen into the broadly acting serine protease, plasmin. This in turn both degrades matrix proteins and activates matrix metalloproteases. One member of the MMP family, MMP-9, appears to be critical for human trophoblast invasion. The timing and extent of trophoblast invasion is regulated by a balanced interplay between pro- and antiinvasive factors.

The relative ability to invade maternal tissue in early pregnancy compared with limited invasiveness in late pregnancy is controlled by autocrine and paracrine trophoblastic and endometrial factors. Trophoblasts secrete insulin-like growth factor II, which acts in an autocrine manner. It promotes invasion into the endometrium, whereas decidual cells secrete insulin-like growth factor binding-protein type 4, which blocks this autocrine loop. Thus, the degree of trophoblast invasion is controlled by matrix degradation regulation and by factors that cause trophoblast migration.

Low estradiol levels in the first trimester are critical for trophoblast invasion and remodeling of the spiral arteries. Recent studies in nonhuman primates suggest that the increase in second-trimester estradiol levels suppresses and limits vessel remodeling by reducing trophoblast expression of VEGF and specific integrin receptors (Bonagura, 2012). As the extravillous trophoblast differentiates, it gains expression of integrin receptors that recognize the extracellular matrix proteins collagen IV, laminin, and fibronectin. Binding of these extracellular matrix proteins to specific integrin receptors initiates signals that promote trophoblast cell migration and differentiation. As pregnancy advances, increasing estradiol levels repress and thus control the extent of uterine vessel transformation via downregulation of VEGF and integrin receptor expression.

image Invasion of Spiral Arteries

One of the most remarkable features of human placental development is the extensive modification of maternal vasculature by trophoblasts, which are by definition of fetal origin. These events occur in the first half of pregnancy and are considered in detail because of their importance to uteroplacental blood flow. They are also integral to some pathological conditions such as preeclampsia, fetal-growth restriction, and preterm birth. Spiral artery modifications are carried out by two populations of extravillous trophoblast—interstitial trophoblasts, which surround the arteries, and endovascular trophoblasts, which penetrate the spiral artery lumen (see Fig. 5-10). Although earlier work has focused on the role of the endovascular trophoblast, interstitial trophoblast function has more recently been investigated (Benirschke, 2012; Pijnenborg, 1983). These interstitial cells constitute a major portion of the placental bed and penetrate the decidua and adjacent myometrium. They aggregate around spiral arteries, and their functions may include vessel preparation for endovascular trophoblast invasion.

Hamilton and Boyd (1966) report that Friedlander in 1870 first described structural changes in spiral arteries. Endovascular trophoblasts first enter the spiral-artery lumens and initially form cellular plugs. They then destroy vascular endothelium via an apoptosis mechanism and invade and modify the vascular media. Thus, fibrinoid material replaces smooth muscle and connective tissue of the vessel media. Spiral arteries later regenerate endothelium. Invading endovascular trophoblasts can extend several centimeters along the vessel lumen, and they must migrate against arterial flow. These vascular changes are not observed in the decidua parietalis, that is, in decidual sites removed from the invading cytotrophoblasts. Of note, invasion by trophoblasts involves only the decidual spiral arteries and not decidual veins.

In their summary of uteroplacental vasculature, Ramsey and Donner (1980) described uteroplacental vessel development as proceeding in two waves or stages. The first wave occurs before 12 weeks’ postfertilization and consists of invasion and modification of spiral arteries up to the border between the decidua and myometrium. The second wave is between 12 and 16 weeks and involves some invasion of the intramyometrial segments of spiral arteries. Remodeling converts narrow-lumen, muscular spiral arteries into dilated, low-resistance uteroplacental vessels. Molecular mechanisms of these crucial events, and their significance in the pathogenesis of preeclampsia and fetal-growth restriction, have been reviewed by Kaufmann (2003) and Red-Horse (2006).

Establishment of Maternal Blood Flow

Approximately 1 month after conception, maternal blood enters the intervillous space in fountain-like bursts from the spiral arteries. Blood is propelled outside of the maternal vessels and sweeps over and directly bathes the syncytiotrophoblast. The apical surface of the syncytiotrophoblast consists of a complex microvillous structure that undergoes continual shedding and reformation during pregnancy.

image Villus Branching

Although certain villi of the chorion frondosum extend from the chorionic plate to the decidua to serve as anchoring villi, most villi arborize and end freely within the intervillous space. As gestation proceeds, the short, thick, early stem villi branch to form progressively finer subdivisions and greater numbers of increasingly smaller villi (Fig. 5-14). Each of the truncal or main stem villi and their ramifications (rami) constitutes a placental lobule, or cotyledon. Each lobule is supplied with a single truncal branch of the chorionic artery. And each lobule has a single vein so that lobules constitute functional units of placental architecture.


FIGURE 5-14 Electron micrographs (A, C) and photomicrographs (B, D) of early and late human placentas. A and B. Limited branching of villi is seen in this early placenta. C and D. With placenta maturation, increasing villous arborization is seen, and villous capillaries lie closer to the surface of each villus. (Photomicrographs contributed by Dr. Kurt Benirschke. Electron micrographs from King, 1975, with permission.)

image Placental Growth and Maturation

Placental Growth

In the first trimester, placental growth is more rapid than that of the fetus. But by approximately 17 postmenstrual weeks, placental and fetal weights are approximately equal. By term, placental weight is approximately one sixth of fetal weight.

The mature placenta and its variant forms are discussed in detail in Chapter 6 (p. 116). Briefly, viewed from the maternal surface, the number of slightly elevated convex areas, called lobes, varies from 10 to 38 (Fig. 5-15). Lobes are incompletely separated by grooves of variable depth that overlie placental septa, which arise from folding of the basal plate. The total number of placental lobes remains the same throughout gestation, and individual lobes continue to grow—although less actively in the final weeks (Crawford, 1959). Although grossly visible lobes are commonly referred to as cotyledons, this is not accurate. Correctly used, lobules or cotyledons are the functional units supplied by each main stem villus.


FIGURE 5-15 Photograph of the maternal surface of a placenta. Placental lobes are formed by clefts on the surface that originate from placental septa. (Photograph contributed by Dr. Judith J. Head.)

Placental Maturation

As villi continue to branch and the terminal ramifications become more numerous and smaller, the volume and prominence of cytotrophoblasts decrease. As the syncytium thins, the fetal vessels become more prominent and lie closer to the surface. The villous stroma also exhibits changes as gestation progresses. In early pregnancy, the branching connective-tissue cells are separated by an abundant loose intercellular matrix. Later, the villous stroma becomes denser, and the cells more spindly and more closely packed.

Another change in the stroma involves the infiltration of Hofbauer cells, which are fetal macrophages. These are nearly round with vesicular, often eccentric nuclei and very granular or vacuolated cytoplasm. Hofbauer cells are characterized histochemically by intracytoplasmic lipid and by phenotypic markers specific for macrophages. They increase in numbers and maturational state throughout pregnancy and appear to be important mediators of protection at the maternal-fetal interface (Johnson, 2012). These macrophages are phagocytic, have an immunosuppressive phenotype, can produce various cytokines, and are capable of paracrine regulation of trophoblast functions (Cervar, 1999; Vince, 1996).

Some of the histological changes that accompany placental growth and maturation provide an increased efficiency of transport and exchange to meet increasing fetal metabolic requirements. Among these changes are decreased syncytiotrophoblast thickness, significantly reduced cytotrophoblast number, decreased stroma, and increased number of capillaries with close approximation to the syncytial surface. By 16 weeks, the apparent continuity of the cytotrophoblasts is lost. At term, villi may be focally reduced to a thin layer of syncytium covering minimal villous connective tissue in which thin-walled fetal capillaries abut the trophoblast and dominate the villi (see Fig. 5-14D).

There are some changes in placental architecture that can cause decreased placental exchange efficiency if they are substantive. As described in Chapter 6 (p. 119), these include thickening of the basal lamina of trophoblast or capillaries, obliteration of certain fetal vessels, increased villous stroma, and fibrin deposition on the villous surface.

image Fetal and Maternal Blood Circulation in the Mature Placenta

Because the placenta is functionally an intimate approximation of the fetal capillary bed to maternal blood, its gross anatomy primarily concerns vascular relations. The fetal surface is covered by the transparent amnion, beneath which chorionic vessels course. A section through the placenta includes amnion, chorion, chorionic villi and intervillous space, decidual (basal) plate, and myometrium (Figs. 5-16and 5-17).


FIGURE 5-16 Uterus of pregnant woman showing a normal placenta in situ. A. Photomicrograph of a histologic section through amnion, chorion, and decidua vera that is depicted in (B) (green slice). (Photograph contributed by Dr. Kurt Benirschke.)


FIGURE 5-17 Photomicrograph of early implanted blastocyst. Trophoblasts are seen invading the decidua basalis. (Photograph contributed by Dr. Kurt Benirschke.)

Fetal Circulation

Deoxygenated venous-like fetal blood flows to the placenta through the two umbilical arteries. As the cord joins the placenta, these umbilical vessels branch repeatedly beneath the amnion and again within the villi, finally forming capillary networks in the terminal villous branches. Blood with significantly higher oxygen content returns from the placenta via a single umbilical vein to the fetus.

The branches of the umbilical vessels that traverse along the fetal surface of the placenta in the chorionic plate are referred to as the placental surface or chorionic vessels. These vessels are responsive to vasoactive substances, but anatomically, morphologically, histologically, and functionally, they are unique. Chorionic arteries always cross over chorionic veins. Vessels are most readily recognized by this interesting relationship, but they are difficult to distinguish by histological criteria. In 65 percent of placentas, chorionic arteries form a fine network supplying the cotyledons—a pattern of disperse-type branching. In the remaining 35 percent, the arteries radiate to the edge of the placenta without narrowing. Both types are end arteries that supply one cotyledon as each branch turns downward to pierce the chorionic plate.

Truncal arteries are perforating branches of the surface arteries that pass through the chorionic plate. Each truncal artery supplies one main stem villus and thus one cotyledon. The amount of vessel wall smooth muscle decreases, and the vessel caliber increases as it penetrates the chorionic plate. The loss in muscle continues as the truncal arteries and veins branch into their smaller rami.

Before 10 weeks, there is no end-diastolic flow pattern within the umbilical artery at the end of the fetal cardiac cycle (Cole, 1991; Fisk, 1988; Loquet, 1988). After 10 weeks, end-diastolic flow appears and is maintained throughout normal pregnancy (Maulik, 1997). Clinically, these are studied with Doppler sonography to assess fetal well-being (Chap. 10p. 219).

Maternal Circulation

Because an efficient maternal–placental circulation is requisite, many investigators have sought to define factors that regulate blood flow into and from the intervillous space. An adequate mechanism must explain how blood can: (1) leave maternal circulation; (2) flow into an amorphous space lined by syncytiotrophoblast, rather than endothelium; and (3) return through maternal veins without producing arteriovenous-like shunts that would prevent maternal blood from remaining in contact with villi long enough for adequate exchange. Early studies of Ramsey and Davis (1963) and Ramsey and Harris (1966) provide a physiological explanation of placental circulation. These researchers demonstrated, by careful, low-pressure injections of radiocontrast material, that arterial entrances and venous exits are scattered randomly over the entire placental base.

The physiology of maternal-placental circulation is depicted in Figure 5-18. Maternal blood enters through the basal plate and is driven high up toward the chorionic plate by arterial pressure before laterally dispersing. After bathing the external microvillous surface of chorionic villi, maternal blood drains back through venous orifices in the basal plate and enters uterine veins. Thus, maternal blood traverses the placenta randomly without preformed channels. The previously described trophoblast invasion of the spiral arteries creates low-resistance vessels that can accommodate massive increase in uterine perfusion during gestation. Generally, spiral arteries are perpendicular to, but veins are parallel to, the uterine wall. This arrangement aids closure of veins during a uterine contraction and prevents the exit of maternal blood from the intervillous space. The number of arterial openings into the intervillous space becomes gradually reduced by cytotrophoblast invasion. According to Brosens and Dixon (1963), there are about 120 spiral arterial entries into the intervillous space at term. These discharge blood in spurts that bathes the adjacent villi (Borell, 1958). After the 30th week, a prominent venous plexus separates the decidua basalis from the myometrium and thus participates in providing a cleavage plane for placental separation.


FIGURE 5-18 Schematic drawing of a section through a full-term placenta. Maternal blood flows into the intervillous spaces in funnel-shaped spurts. Exchanges occur with fetal blood as maternal blood flows around the villi. In-flowing arterial blood pushes venous blood into the endometrial veins, which are scattered over the entire surface of the decidua basalis. Note also that the umbilical arteries carry deoxygenated fetal blood to the placenta and that the umbilical vein carries oxygenated blood to the fetus. Placental lobes are separated from each other by placental (decidual) septa.

As discussed, both inflow and outflow are curtailed during uterine contractions. Bleker and associates (1975) used serial sonography during normal labor and found that placental length, thickness, and surface area increased during contractions. They attributed this to distention of the intervillous space by impairment of venous outflow compared with arterial inflow. During contractions, therefore, a somewhat larger volume of blood is available for exchange even though the rate of flow is decreased. Similarly, Doppler velocimetry has shown that diastolic flow velocity in spiral arteries is diminished during uterine contractions. Thus, principal factors regulating intervillous space blood flow are arterial blood pressure, intrauterine pressure, uterine contraction pattern, and factors that act specifically on arterial walls.

image Breaks in the Placental “Barrier”

The placenta does not maintain absolute integrity of the fetal and maternal circulations. There are numerous examples of trafficking cells between mother and fetus in both directions. This situation is best exemplified clinically by erythrocyte D-antigen alloimmunization and resulting erythroblastosis fetalis (Chap. 15p. 307). Although fetal cell admixtures likely are small in most cases, occasionally the fetus exsanguinates into the maternal circulation (Silver, 2007).

It is indisputable that fetal cells can become engrafted in the mother during pregnancy and can be identified decades later. Fetal lymphocytes, CD34+ mesenchymal stem cells, and endothelial colony-forming cells reside in maternal blood, bone marrow, or uterine vasculature (Nguyen, 2006; Piper, 2007; Sipos, 2013). Termed microchimerism, such residual stem cells may participate in maternal tissue regeneration and have been implicated in the disparate female:male ratio of autoimmune disorders (Gleicher, 2007; Greer, 2011; Stevens, 2006). As discussed in Chapter 59 (p. 1168), they are associated with the pathogenesis of lymphocytic thyroiditis, scleroderma, and systemic lupus erythematosus.

image Immunological Considerations of the Fetal–Maternal Interface

The lack of uterine transplantation immunity is unique compared with that of other tissues, and there have been many attempts to explain survival of the semiallogenic fetal graft. Some note the immunological peculiarity of cells involved in implantation and fetal-placental development. These include decidual natural killer cells with their inefficient cytotoxic abilities, decidual stromal cells, and invasive trophoblasts that populate the decidua (Hanna, 2006; Santoni, 2007; Staun-Ram, 2005). The trophoblasts are the only fetal-derived cells in direct contact with maternal tissues. Previous studies have suggested that maternal natural killer cells act to control the invasion of trophoblast, which have adapted to survive in an immunologically hostile environment (Thellin, 2000). Subsequently, Hanna and coworkers (2006) reported a “peaceful” model of trophoblast invasion and maternal vascular remodeling. In this scheme, decidual natural killer cells work in concert with stromal cells. They mediate angiogenesis through production of proangiogenic factors such as VEGF and control trophoblast chemoattraction toward spiral arteries by production of IL-8 and interferon inducible protein-10.

Immunogenicity of the Trophoblasts

More than 50 years ago, Sir Peter Medawar (1953) suggested that fetal semiallograft survival might be explained by immunological neutrality. The placenta was considered immunologically inert and therefore unable to create a maternal immune response. Subsequently, research was focused on defining expression of the major histocompatibility complex (MHC) antigens on trophoblasts. Human leukocyte antigens (HLAs) are the human analogue of the MHC. And indeed, MHC class I and II antigens are absent from villous trophoblasts, which appear to be immunologically inert at all gestational stages (Weetman, 1999). But invasive extravillous cytotrophoblasts do express MHC class I molecules, which have been the focus of considerable study.

Trophoblast HLA Class I Expression. The HLA genes are the products of multiple genetic loci of the major histocompatibility complex located within the short arm of chromosome 6 (Hunt, 1992). There are 17 HLA class I genes, including three classic genes, HLA-A, -B, and -C, that encode the major class I (class Ia) transplantation antigens. Three other class I genes, designated HLA-E, -F, and -G, encode class Ib HLA antigens.

Moffett-King (2002) reasoned that normal implantation depends on controlled trophoblastic invasion of maternal endometrium–decidua and spiral arteries. Such invasion must proceed far enough to provide for normal fetal growth and development, but there must be a mechanism for regulating its depth. She suggests that uterine decidual natural killer (uNK) cells combined with unique expression of three specific HLA class I genes in extravillous cytotrophoblasts act in concert to permit and subsequently limit trophoblast invasion.

Class I antigens in extravillous cytotrophoblasts are accounted for by the expression of classic HLA-C and nonclassic class Ib molecules of HLA-E and HLA-G. HLA-G antigen is expressed only in humans, with expression restricted to extravillous cytotrophoblasts contiguous with maternal tissues, that is, decidual and uNK cells. Indeed, HLA-G antigen expression is identified only in extravillous cytotrophoblasts in the decidua basalis and in the chorion laeve (McMaster, 1995). During pregnancy, a soluble major isoform—HLA-G2—has increased levels (Hunt, 2000a,b). Embryos used for in vitro fertilization do not implant if they do not express this soluble HLA-G isoform (Fuzzi, 2002). Thus, HLA-G may be immunologically permissive of the maternal-fetal antigen mismatch (LeBouteiller, 1999). HLA-G has a proposed role in protecting extravillous trophoblasts from immune rejection via modulation of uNK functions (Apps, 2011; Rajagopalan, 2012). Finally, Goldman-Wohl and associates (2000) have provided evidence for abnormal HLA-G expression in extravillous trophoblasts from women with preeclampsia.

Uterine Natural Killer Cells

These distinctive lymphocytes are believed to originate in bone marrow and belong to the natural killer cell lineage. They are the predominant population of leukocytes present in midluteal phase endometrium at the expected time of implantation (Johnson, 1999). These uNKs have a distinct phenotype characterized by a high surface density of CD56 or neural cell adhesion molecule (Manaster, 2008; Moffett-King, 2002). Their infiltration is increased by progesterone and by stromal cell production of IL-15 and decidual prolactin (Dunn, 2002; Gubbay, 2002).

Near the end of the luteal phase of nonfertile ovulatory cycles, uterine NK cell nuclei begin to disintegrate. But if implantation proceeds, they persist in large numbers in the decidua during early pregnancy. By term, however, there are relatively few uNK cells in the decidua. In first-trimester decidua, there are many uNK cells in close proximity to extravillous trophoblast, where it is speculated that they serve to regulate trophoblast invasion. These uNK cells secrete large amounts of granulocyte-macrophage–colony-stimulating factor (GM-CSF), which suggests that they are in an activated state. Jokhi and coworkers (1999) speculate that GM-CSF may function primarily to forestall trophoblast apoptosis and not to promote trophoblast replication. Expression of angiogenic factors by uNK cells also suggests a function in decidual vascular remodeling (Li, 2001). In this case, it is uNKs, rather than the T lymphocytes, that are primarily responsible for decidual immunosurveillance.


At term, the amnion is a tough and tenacious but pliable membrane. This innermost avascular fetal membrane is contiguous with amnionic fluid and occupies a role of incredible importance in human pregnancy. The amnion provides almost all tensile strength of the fetal membranes. Thus, its resilience to rupture is vitally important to successful pregnancy outcome. Indeed, preterm rupture of fetal membranes is a major cause of preterm delivery (Chap. 42p. 839).

Bourne (1962) described five separate amnion layers. The inner surface, which is bathed by amnionic fluid, is an uninterrupted, single layer of cuboidal epithelium (Fig. 5-19). This epithelium is attached firmly to a distinct basement membrane that is connected to an acellular compact layer composed primarily of interstitial collagens. On the outer side of the compact layer, there is a row of fibroblast-like mesenchymal cells, which are widely dispersed at term. There also are a few fetal macrophages in the amnion. The outermost amnion layer is the relatively acellular zona spongiosa, which is contiguous with the second fetal membrane, the chorion laeve. The human amnion lacks smooth muscle cells, nerves, lymphatics, and importantly, blood vessels.


FIGURE 5-19 Photomicrograph of fetal membranes. From left to right: AE = amnion epithelium; AM = amnion mesenchyme; S = zona spongiosa; CM = chorionic mesenchyme; TR = trophoblast; D = decidua. (Photograph contributed by Dr. Judith R. Head.)

image Amnion Development

Early during implantation, a space develops between the embryonic cell mass and adjacent trophoblasts (see Fig. 5-9). Small cells that line this inner surface of trophoblasts have been called amniogenic cells—precursors of amnionic epithelium. The amnion is first identifiable the 7th or 8th day of embryo development. It is initially a minute vesicle, which then develops into a small sac that covers the dorsal embryo surface. As the amnion enlarges, it gradually engulfs the growing embryo, which prolapses into its cavity (Benirschke, 2012).

Distention of the amnionic sac eventually brings it into contact with the interior surface of the chorion laeve. Apposition of the chorion laeve and amnion near the end of the first trimester then causes an obliteration of the extraembryonic coelom. The amnion and chorion laeve, although slightly adhered, are never intimately connected and can be separated easily. Placental amnion covers the placenta surface and thereby is in contact with the adventitial surface of chorionic vessels. Umbilical amnion covers the umbilical cord. With diamnionic-monochorionic placentas, there is no intervening tissue between the fused amnions. In the conjoined portion of membranes of diamnionic-dichorionic twin placentas, fused amnions are separated by fused chorion laeve.

Amnion Cell Histogenesis

Epithelial cells of the amnion are thought to be derived from fetal ectoderm of the embryonic disc. They do not arise by delamination from trophoblasts. This is an important consideration from both embryological and functional perspectives. For example, HLA class I gene expression in amnion is more akin to that in embryonic cells than that in trophoblasts.

In contrast, the fibroblast-like mesenchymal cell layer previously described is likely derived from embryonic mesoderm. Early in human embryogenesis, the amnionic mesenchymal cells lie immediately adjacent to the basal surface of the amnion epithelium. At this time, the amnion surface is a two-cell layer with approximately equal numbers of epithelial and mesenchymal cells. Simultaneously with growth and development, interstitial collagens are deposited between these two cell layers. This marks formation of the amnion compact layer, which separates the two layers of amnion cells.

As the amnionic sac expands to line the placenta and then the chorion frondosum at 10 to 14 weeks, the compactness of the mesenchymal cells is progressively reduced and become sparsely distributed. Early in pregnancy, amnionic epithelium replicates at a rate appreciably faster than mesenchymal cells. At term, these cells form a continuous uninterrupted epithelium on the fetal amnionic surface. Conversely, mesenchymal cells are widely dispersed, being connected by a fine lattice network of extracellular matrix with the appearance of long slender fibrils.

Amnion Epithelial Cells. The apical surface of the amnionic epithelium is replete with highly developed microvilli. This structure reflects it function as a major site of transfer between amnionic fluid and amnion. This epithelium is metabolically active, and its cells synthesize tissue inhibitor of MMP-1, prostaglandin E2 (PGE2), and fetal fibronectin (fFN) (Rowe, 1997). In term pregnancies, amnionic expression of prostaglandin endoperoxide H synthase correlates with elevated fFN levels (Mijovic, 2000). Although epithelia produce fFN, recent studies suggest that fibronectin functions in the underlying mesenchymal cells. Here, fFN promotes synthesis of MMPs that break down the strength-bearing collagens and increases prostaglandin synthesis to prompt uterine contractions and cervical ripening (Mogami, 2013). This pathway is upregulated in premature rupture of membranes induced by infection.

Epithelial cells may respond to signals derived from the fetus or the mother, and they are responsive to various endocrine or paracrine modulators. Examples include oxytocin and vasopressin, both of which increase PGE2production in vitro (Moore, 1988). They may also produce cytokines such as IL-8 during labor initiation (Elliott, 2001).

Amnionic epithelium also synthesizes vasoactive peptides, including endothelin and parathyroid hormone-related protein (Economos, 1992; Germain, 1992). The tissue produces brain natriuretic peptide (BNP) and corticotropin-releasing hormone (CRH), which are peptides that invoke smooth-muscle relaxation (Riley, 1991; Warren, 1995). BNP production is positively regulated by mechanical stretch in fetal membranes and is proposed to function in uterine quiescence. Epidermal growth factor, a negative regulator of BNP, is upregulated in the membranes at term and leads to a decline in BNP-regulated uterine quiescence (Carvajal, 2013). It seems reasonable that vasoactive peptides produced in amnion gain access to the adventitial surface of chorionic vessels. Thus, the amnion may be involved in modulating chorionic vessel tone and blood flow. Amnion-derived vasoactive peptides function in both maternal and fetal tissues in diverse physiological processes. After their secretion, these bioactive agents enter amnionic fluid and thereby are available to the fetus by swallowing and inhalation.

Amnion Mesenchymal Cells. Mesenchymal cells of the amnionic fibroblast layer are responsible for other major functions. Synthesis of interstitial collagens that compose the compact layer of the amnion—the major source of its tensile strength—takes place in mesenchymal cells (Casey, 1996). These cells also synthesize cytokines that include IL-6, IL-8, and monocyte chemoattractant protein-1. Cytokine synthesis increases in response to bacterial toxins and IL-1. This functional capacity of amnion mesenchymal cells is an important consideration in amnionic fluid study of labor-associated accumulation of inflammatory mediators (Garcia-Velasco, 1999). Finally, mesenchymal cells may be a greater source of PGE2 than epithelial cells, in particular in the case of premature membrane rupture (Mogami, 2013; Whittle, 2000).

image Amnion Tensile Strength

During tests of tensile strength, the decidua and then the chorion laeve give way long before the amnion ruptures. Indeed, the membranes are elastic and can expand to twice normal size during pregnancy (Benirschke, 2012). The amnion tensile strength resides almost exclusively in the compact layer, which is composed of cross-linked interstitial collagens I and III and lesser amounts of collagens V and VI.

Collagens are the primary macromolecules of most connective tissues and are the most abundant proteins in the body. Collagen I is the major interstitial collagen in tissues characterized by great tensile strength, such as bone and tendon. In other tissues, collagen III is believed to contribute to tissue integrity and provides both tissue extensibility and tensile strength. For example, the ratio of collagen III to collagen I in the walls of a number of highly extensible tissues—amnionic sac, blood vessels, urinary bladder, bile ducts, intestine, and gravid uterus—is greater than that in nonelastic tissues (Jeffrey, 1991). Although collagen III provides some of the amnion extensibility, elastin microfibrils have also been identified (Bryant-Greenwood, 1998).

Amnion tensile strength is regulated in part by fibrillar collagen interacting with proteoglycans such as decorin, which promote tissue strength. Compositional changes at the time of labor include a decline in decorin and increase in hyaluronan. This leads to a loss of tensile strength and is further discussed in Chapter 42 (p. 840) (Meinert, 2007). Fetal membranes overlying the cervix have a reported regional decline in expression of matrix proteins such as fibulins. This change may contribute to tissue remodeling and tensile strength loss (Moore, 2009).

image Amnion Metabolic Functions

From the foregoing, it is apparent that the amnion is more than a simple avascular membrane that contains amnionic fluid. It is metabolically active, is involved in solute and water transport for amnionic fluid homeostasis, and produces an impressive array of bioactive compounds. The amnion is responsive both acutely and chronically to mechanical stretch, which alters amnionic gene expression (Carvajal, 2013; Nemeth, 2000). This in turn may trigger both autocrine and paracrine responses to include production of MMPs, IL-8, and collagenase (Bryant-Greenwood, 1998; Maradny, 1996; Mogami, 2013). Such factors may modulate changes in membrane properties during labor.

image Amnionic Fluid

Until about 34 weeks’ gestation, the normally clear fluid that collects within the amnionic cavity increases as pregnancy progresses. After this, the volume declines. At term, the average volume is approximately 1000 mL, although this may vary widely in normal and especially abnormal conditions. The origin, composition, circulation, and function of amnionic fluid are discussed further in Chapter 11(p. 231).


image Cord Development

The yolk sac and the umbilical vesicle into which it develops are prominent early in pregnancy. At first, the embryo is a flattened disc interposed between amnion and yolk sac. Its dorsal surface grows faster than the ventral surface, in association with the elongation of its neural tube. Thus, the embryo bulges into the amnionic sac, and the dorsal part of the yolk sac is incorporated into the embryo body to form the gut. The allantois projects into the base of the body stalk from the caudal wall of the yolk sac and later, from the anterior wall of the hindgut.

As pregnancy advances, the yolk sac becomes smaller and its pedicle relatively longer. By the middle of the third month, the expanding amnion obliterates the exocoelom, fuses with the chorion laeve, and covers the bulging placental disc and the lateral surface of the body stalk. The latter is then called the umbilical cord—or funis. A greater description of this cord and potential abnormalities is found in Chapter 6 (p. 121).

The cord at term normally has two arteries and one vein (Fig. 5-20). The right umbilical vein usually disappears early during fetal development, leaving only the original left vein. In sections of any portion of the cord near the center, the small duct of the umbilical vesicle can usually be seen. The vesicle is lined by a single layer of flattened or cuboidal epithelium. In sections just beyond the umbilicus, another duct representing the allantoic remnant is occasionally found. The intraabdominal portion of the duct of the umbilical vesicle, which extends from umbilicus to intestine, usually atrophies and disappears, but occasionally it remains patent, forming the Meckel diverticulum. The most common vascular anomaly is the absence of one umbilical artery, which may be associated with fetal anomalies (Chap. 6p. 122).


FIGURE 5-20 Cross section of umbilical cord. The large umbilical vein carries oxygenated blood to the fetus (right). To its left are the two smaller umbilical arteries, carrying deoxygenated blood from the fetus to the placenta. (Photograph contributed by Dr. Mandolin S. Ziadie.)

image Cord Function

The umbilical cord extends from the fetal umbilicus to the fetal surface of the placenta, that is, the chorionic plate. Blood flows from the umbilical vein and takes a path of least resistance via two routes within the fetus. One is the ductus venosus, which empties directly into the inferior vena cava (Fig. 7–8p. 136). The other route consists of numerous smaller openings into the hepatic circulation. Blood from the liver flows into the inferior vena cava via the hepatic vein. Resistance in the ductus venosus is controlled by a sphincter that is situated at the origin of the ductus at the umbilical recess and is innervated by a vagus nerve branch.

Blood exits the fetus via the two umbilical arteries. These are anterior branches of the internal iliac artery and become obliterated after birth. Remnants can be seen as the medial umbilical ligaments.


The production of steroid and protein hormones by human trophoblasts is greater in amount and diversity than that of any single endocrine tissue in all of mammalian physiology. A compendium of average production rates for various steroid hormones in nonpregnant and in near-term pregnant women is given in Table 5-1. It is apparent that alterations in steroid hormone production that accompany normal human pregnancy are incredible. The human placenta also synthesizes an enormous amount of protein and peptide hormones as summarized in Table 5-2. It is understandable, therefore, that yet another remarkable feature of human pregnancy is the successful physiological adaptations of pregnant women to the unique endocrine milieu as discussed throughout Chapter 4 (p. 46).

TABLE 5-1. Steroid Production Rates in Nonpregnant and Near-Term Pregnant Women


TABLE 5-2. Protein Hormones Produced by the Human Placenta


image Human Chorionic Gonadotropin

This so-called pregnancy hormone is a glycoprotein with biological activity similar to luteinizing hormone. Both act via the same plasma membrane LH-hCG receptor. Although hCG is produced almost exclusively in the placenta, low levels are synthesized in the fetal kidney. Other fetal tissues produce either the β-subunit or intact hCG molecule (McGregor, 1981, 1983).

Various malignant tumors also produce hCG, sometimes in large amounts—especially trophoblastic neoplasms (Chap. 20p. 396). Chorionic gonadotropin is produced in very small amounts in tissues of men and nonpregnant women, perhaps primarily in the anterior pituitary gland. Nonetheless, the detection of hCG in blood or urine almost always indicates pregnancy (Chap. 9p. 169).

Chemical Characteristics

Chorionic gonadotropin is a glycoprotein with a molecular weight of 36,000 to 40,000 Da. It has the highest carbohydrate content of any human hormone—30 percent. The carbohydrate component, and especially the terminal sialic acid, protects the molecule from catabolism. The 36-hour plasma half-life of intact hCG is much longer than the 2 hours for LH. The hCG molecule is composed of two dissimilar subunits termed α and β subunits. These are noncovalently linked and are held together by electrostatic and hydrophobic forces. Isolated subunits are unable to bind the LH-hCG receptor and thus lack biological activity.

This hormone is structurally related to three other glycoprotein hormones—LH, FSH, and TSH. All four glycoproteins share a common α-subunit. The β-subunits, although sharing certain similarities, are characterized by distinctly different amino-acid sequences. Recombination of an α- and a β-subunit of the four glycoprotein hormones gives a molecule with biological activity characteristic of the hormone from which the β-subunit was derived.


Syntheses of the α- and β-chains of hCG are regulated separately. A single gene located on chromosome 6 encodes the α-subunit common to hCG, LH, FSH, and TSH. Seven genes on chromosome 19 encode for the β-hCG–β-LH family of subunits. Six genes code for β-hCG and one for β-LH (Miller-Lindholm, 1997). Both subunits are synthesized as larger precursors, which are then cleaved by endopeptidases. Intact hCG is then assembled and rapidly released by secretory granule exocytosis (Morrish, 1987). There are multiple forms of hCG in maternal plasma and urine that vary enormously in bioactivity and immunoreactivity. Some result from enzymatic degradation, and others from modifications during molecular synthesis and processing.

Before 5 weeks, hCG is expressed in both syncytiotrophoblast and cytotrophoblast (Maruo, 1992). Later, in the first trimester when maternal serum levels peak, hCG is produced almost solely in the syncytiotrophoblast (Beck, 1986; Kurman, 1984). At this time, mRNA concentrations of both α- and β-subunits in the syncytiotrophoblast are greater than at term (Hoshina, 1982). This may be an important consideration when hCG is used as a screening procedure to identify abnormal fetuses.

Circulating free β-subunit levels are low to undetectable throughout pregnancy. In part, this is the result of its rate-limiting synthesis. Free α-subunits that do not combine with the β-subunit are found in placental tissue and maternal plasma. These levels increase gradually and steadily until they plateau at about 36 weeks’ gestation. At this time, they account for 30 to 50 percent of hormone (Cole, 1997). Thus, α-hCG secretion roughly corresponds to placental mass, whereas secretion of complete hCG molecules is maximal at 8 to 10 weeks.

Concentrations of hCG in Serum and Urine

The combined hCG molecule is detectable in plasma of pregnant women 7 to 9 days after the midcycle surge of LH that precedes ovulation. Thus, hCG likely enters maternal blood at the time of blastocyst implantation. Plasma levels increase rapidly, doubling every 2 days in the first trimester (Fig. 5-21). Appreciable fluctuations in levels for a given patient are observed on the same day—evidence that trophoblast secretion of protein hormones is episodic.


FIGURE 5-21 Distinct profiles for the concentrations of human chorionic gonadotropin (hCG), human placental lactogen (hPL), and corticotropin-releasing hormone (CRH) in serum of women throughout normal pregnancy.

Intact hCG circulates as multiple highly related isoforms with variable cross-reactivity between commercial assays. Thus, there is considerable variation in calculated serum hCG levels among the more than a hundred available assays. Peak maternal plasma levels reach approximately 100,000 mIU/mL between the 60th and 80th days after menses. At 10 to 12 weeks, plasma levels begin to decline, and a nadir is reached by approximately 16 weeks. Plasma levels are maintained at this lower level for the remainder of pregnancy (see Fig. 5-21).

The pattern of hCG appearance in fetal blood is similar to that in the mother. Fetal plasma levels, however, are only about 3 percent of those in maternal plasma. Amnionic fluid hCG concentration early in pregnancy is similar to that in maternal plasma. As pregnancy progresses, hCG concentration in amnionic fluid declines, and near term the levels are approximately 20 percent of those in maternal plasma.

Maternal urine contains the same variety of hCG degradation products as maternal plasma. The principal urinary form is the terminal degradation hCG product—the β-core fragment. Its concentrations follow the same general pattern as that in maternal plasma, peaking at about 10 weeks. It is important to recognize that the so-called β-subunit antibody used in most pregnancy tests reacts with both intact hCG—the major form in the plasma, and with fragments of hCG—the major forms found in urine.

Regulation of hCG Synthesis and Clearance

Placental gonadotropin-releasing hormone (GnRH) is likely involved in the regulation of hCG formation. Both GnRH and its receptor are expressed by cytotrophoblasts and syncytiotrophoblast (Wolfahrt, 1998). GnRH administration elevates circulating hCG levels, and cultured trophoblast cells respond to GnRH treatment with increased hCG secretion (Iwashita, 1993; Siler-Khodr, 1981). Pituitary GnRH production also is regulated by inhibin and activin. In cultured placental cells, activin stimulates and inhibin inhibits GnRH and hCG production (Petraglia, 1989; Steele, 1993).

Renal clearance of hCG accounts for 30 percent of its metabolic clearance. The remainder is likely cleared by metabolism in the liver (Wehmann, 1980). Clearances of β- and α-subunits are approximately 10-fold and 30-fold, respectively, greater than that of intact hCG.

Biological Functions of hCG

Both hCG subunits are required for binding to the LH-hCG receptor in the corpus luteum and the fetal testis. LH-hCG receptors are present in various other tissues, but their role there is less defined. The best-known biological function of hCG is the so-called rescue and maintenance of corpus luteum function—that is, continued progesterone production. Bradbury and colleagues (1950) found that the progesterone-producing life span of a corpus luteum of menstruation could be prolonged perhaps for 2 weeks by hCG administration. This is only an incomplete explanation for the physiological function of hCG in pregnancy. For example, maximum plasma hCG concentrations are attained well after hCG-stimulated corpus luteum secretion of progesterone has ceased. Specifically, progesterone luteal synthesis begins to decline at about 6 weeks despite continued and increasing hCG production.

A second hCG role is stimulation of fetal testicular testosterone secretion, which is maximum approximately when hCG levels peak. Thus, at a critical time in male sexual differentiation, hCG enters fetal plasma from the syncytiotrophoblast. In the fetus, it acts as an LH surrogate to stimulate Leydig cell replication and testosterone synthesis to promote male sexual differentiation (Chap. 7p. 148). Before approximately 110 days, there is no vascularization of the fetal anterior pituitary from the hypothalamus. Thus, pituitary LH secretion is minimal, and hCG acts as LH before this time. Thereafter, as hCG levels fall, pituitary LH maintains modest testicular stimulation.

The maternal thyroid gland is also stimulated by large quantities of hCG. In some women with gestational trophoblastic disease, biochemical and clinical evidence of hyperthyroidism sometimes develops (Chap. 20p. 399). This once was attributed to formation of chorionic thyrotropins by neoplastic trophoblasts. It was subsequently shown, however, that some forms of hCG bind to TSH receptors on thyrocytes (Hershman, 1999). And treatment of men with exogenous hCG increases thyroid activity. The thyroid-stimulatory activity in plasma of first-trimester pregnant women varies appreciably from sample to sample. Modifications of hCG oligosaccharides likely are important in the capacity of hCG to stimulate thyroid function. For example, acidic isoforms stimulate thyroid activity, and some more basic isoforms stimulate iodine uptake (Kraiem, 1994; Tsuruta, 1995; Yoshimura, 1994). Finally, the LH-hCG receptor is expressed by thyrocytes, which suggests that hCG stimulates thyroid activity via the LH-hCG receptor as well as by the TSH receptor (Tomer, 1992).

Other hCG functions include promotion of relaxin secretion by the corpus luteum (Duffy, 1996). LH-hCG receptors are found in myometrium and in uterine vascular tissue. It has been hypothesized that hCG may promote uterine vascular vasodilatation and myometrial smooth-muscle relaxation (Kurtzman, 2001). Chorionic gonadotropin also regulates expansion of uterine natural killer cell numbers during early stages of placentation, thus ensuring appropriate establishment of pregnancy (Kane, 2009).

Abnormally High or Low hCG Levels

There are several clinical circumstances in which substantively higher maternal plasma hCG levels are found. Some examples are multifetal pregnancy, erythroblastosis fetalis associated with fetal hemolytic anemia, and gestational trophoblastic disease. Relatively higher hCG levels may be found at midtrimester in women carrying a fetus with Down syndrome—an observation used in biochemical screening tests (Chap. 14p. 290). The reason for this is not clear, but reduced placental maturity has been speculated. Relatively lower hCG plasma levels are found in women with early pregnancy wastage, including ectopic pregnancy (Chap. 19p. 381).

image Human Placental Lactogen

Prolactin-like activity in the human placenta was first described by Ehrhardt (1936). Because of its potent lactogenic and growth hormone-like bioactivity, as well as an immunochemical resemblance to human growth hormone (hGH), it was called human placental lactogen or chorionic growth hormone. Currently, human placental lactogen is used by most.

Grumbach and Kaplan (1964) showed that this hormone, like hCG, was concentrated in syncytiotrophoblast. It is detected as early as the second or third week after fertilization. Also similar to hCG, hPL is demonstrated in cytotrophoblasts before 6 weeks (Maruo, 1992).

Chemical Characteristics and Synthesis

Human placental lactogen is a single, nonglycosylated polypeptide chain with a molecular weight of 22,279 Da. It is derived from a 25,000-Da precursor. The sequence of hPL and hGH is strikingly similar, with 96-percent homology. Also, hPL is structurally similar to human prolactin (hPRL), with a 67-percent amino-acid sequence similarity. For these reasons, it has been suggested that the genes for hPL, hPRL, and hGH evolved from a common ancestral gene—probably that for prolactin—by repeated gene duplication (Ogren, 1994).

There are five genes in the growth hormone–placental lactogen gene cluster that are linked and located on chromosome 17. Two of these—hPL2 and hPL3—encode hPL, and the amount of mRNA in the term placenta is similar for each.

Within 5 to 10 days after conception, hPL is demonstrable in the placenta and can be detected in maternal serum as early as 3 weeks. Maternal plasma concentrations are linked to placental mass, and they rise steadily until 34 to 36 weeks’ gestation. The hPL production rate near term—approximately 1 g/day—is by far the greatest of any known hormone in humans. The half-life of hPL in maternal plasma is between 10 and 30 minutes (Walker, 1991). In late pregnancy, maternal serum concentrations reach levels of 5 to 15 μg/mL (see Fig. 5-21).

Very little hPL is detected in fetal blood or in the urine of the mother or newborn. Amnionic fluid levels are somewhat lower than in maternal plasma. Because hPL is secreted primarily into the maternal circulation, with only very small amounts in cord blood, it appears that its role in pregnancy is mediated through actions in maternal rather than in fetal tissues. Nonetheless, there is continuing interest in the possibility that hPL serves select functions in fetal growth.

Regulation of hPL Biosynthesis

Levels of mRNA for hPL in syncytiotrophoblast remain relatively constant throughout pregnancy. This finding supports the idea that the hPL secretion rate is proportional to placental mass. There are very high plasma levels of hCG in women with trophoblastic neoplasms, but only low levels of hPL in these same women.

Prolonged maternal starvation in the first half of pregnancy leads to increased hPL plasma concentrations. Short-term changes in plasma glucose or insulin, however, have relatively little effect on plasma hPL levels. In vitro studies of syncytiotrophoblast suggest that hPL synthesis is stimulated by insulin and insulin-like growth factor-1 and inhibited by PGE2 and PGF (Bhaumick, 1987; Genbacev, 1977).

Metabolic Actions

Placental lactogen has putative actions in several important metabolic processes. First, hPL promotes maternal lipolysis with increased circulating free fatty acid levels. This provides an energy source for maternal metabolism and fetal nutrition. In vitro studies suggest that hPL inhibits leptin secretion by term trophoblast (Coya, 2005).

Second, hPL may aid maternal adaptation to fetal energy requirements. For example, increased maternal insulin resistance ensures nutrient flow to the fetus. It also favors protein synthesis and provides a readily available amino-acid source to the fetus. To counterbalance the increased insulin resistance and prevent maternal hyperglycemia, maternal insulin levels are increased. Both hPL and prolactin signal through the prolactin receptor to increase maternal beta cell proliferation to augment insulin secretion (Georgia, 2010). Recent animal studies provide insight into the mechanism by which lactogenic hormones drive beta cell expansion. Specifically, prolactin and hPL upregulate serotonin synthesis via regulation of the rate-limiting enzyme, tryptophan hydroxylase-1, which in turn increases beta cell proliferation (Kim, 2010).

Last, hPL is a potent angiogenic hormone. It may serve an important function in fetal vasculature formation (Corbacho, 2002).

image Other Placental Protein Hormones

The placenta has a remarkable capacity to synthesize numerous peptide hormones, including some that are analogous or related to hypothalamic and pituitary hormones. In contrast to their counterparts, the placental peptide/protein hormones are not subject to feedback inhibition. Examples include pro-opiomelanocortin-derived peptides, growth hormone variant V, and gonadotropin-releasing hormone.

Chorionic Adrenocorticotropin

Adrenocorticotropic hormone (ACTH), lipotropin, and β-endorphin—all proteolytic products of pro-opiomelanocortin—are recovered from placental extracts (Genazzani, 1975; Odagiri, 1979). The physiological action of placental ACTH is unclear. Placental corticotropin-releasing hormone stimulates synthesis and release of chorionic ACTH. Placental CRH production is positively regulated by cortisol, producing a novel positive feedback loop. As discussed later, this system may be important for controlling fetal lung maturation and parturition timing.

Growth Hormone Variant

The placenta expresses a growth hormone variant (hGH-V) that is not expressed in the pituitary. The gene encoding hGH-V is located in the hGH–hPL gene cluster on chromosome 17. Sometimes referred to as placental growth hormone, hGH-V is a 191 amino-acid protein that differs in 15 amino-acid positions from the sequence for hGH. Although hGH-V retains growth-promoting and antilipogenic functions similar to hGH, it has reduced diabetogenic and lactogenic functions relative to hGH (Vickers, 2009). Placental hGH-V presumably is synthesized in the syncytium. However, its pattern of synthesis and secretion during gestation is not precisely known because antibodies against hGH-V cross-react with hGH. It is believed that hGH-V is present in maternal plasma by 21 to 26 weeks’ gestation, increases in concentration until approximately 36 weeks, and remains relatively constant thereafter. There is a correlation between the levels of hGH-V in maternal plasma and those of insulin-like growth factor-1. Also, hGH-V secretion by trophoblasts in vitro is inhibited by glucose in a dose-dependent manner (Patel, 1995). Overexpression of hGH-V in mice causes severe insulin resistance, and thus it is a likely candidate to mediate insulin resistance of pregnancy (Barbour, 2002).

Hypothalamic-Like Releasing Hormones

The known hypothalamic-releasing or -inhibiting hormones include GnRH, CRH, thyroid-releasing hormone (TRH), growth hormone-releasing hormone (GHRH), and somatostatin. For each, there is an analogous hormone produced in the human placenta (Petraglia, 1992; Siler-Khodr, 1988). Many investigators suggest this indicates a hierarchy of control in chorionic trophic-agent synthesis.

Gonadotropin-Releasing Hormone. There is a reasonably large amount of immunoreactive GnRH in the placenta (Siler-Khodr, 1978, 1988). Interestingly, it is found in cytotrophoblasts, but not syncytiotrophoblast. Gibbons and coworkers (1975) and Khodr and Siler-Khodr (1980) demonstrated that the human placenta could synthesize both GnRH and TRH in vitro. Placental-derived GnRH functions to regulate trophoblast hCG production, hence the observation that GnRH levels are higher early in pregnancy. Placental-derived GnRH is also the likely cause of elevated maternal GnRH levels in pregnancy (Siler-Khodr, 1984).

Corticotropin-Releasing Hormone. This hormone is a member of a larger family of CRH-related peptides that includes CRH, urocortin, urocortin II, and urocortin III (Dautzenberg, 2002). Maternal serum CRH levels increase from 5 to 10 pmol/L in the nonpregnant woman to approximately 100 pmol/L in the early third trimester of pregnancy and to almost 500 pmol/L abruptly during the last 5 to 6 weeks (see Fig. 5-21). Urocortin also is produced by the placenta and secreted into the maternal circulation, but at much lower levels than seen for CRH (Florio, 2002). After labor begins, maternal plasma CRH levels increase further by two- to threefold (Petraglia, 1989, 1990).

The biological function of CRH synthesized in the placenta, membranes, and decidua has been somewhat defined. CRH receptors are present in many tissues: placenta, adrenal gland, sympathetic ganglia, lymphocytes, gastrointestinal tract, pancreas, gonads, and myometrium. Some findings suggest that CRH can act through two major families—the type 1 and type 2 CRH receptors (CRH-R1 and CRH-R2). Trophoblast, amniochorion, and decidua express both CRH-R1 and CRH-R2 receptors and several variant receptors (Florio, 2000). Both CRH and urocortin increase trophoblast ACTH secretion, supporting an autocrine-paracrine role (Petraglia, 1999). Large amounts of trophoblast CRH enter maternal blood. That said, there also is a large concentration of a specific CRH-binding protein in maternal plasma, and the bound CRH seems to be biologically inactive.

Other proposed biological roles include induction of smooth-muscle relaxation in vascular and myometrial tissue and immunosuppression. The physiological reverse, however, induction of myometrial contractions, has been proposed for the rising CRH levels seen near term. One hypothesis suggests that CRH may be involved with parturition initiation (Wadhwa, 1998). Prostaglandin formation in the placenta, amnion, chorion laeve, and decidua is increased with CRH treatment (Jones, 1989b). This latter observation further supports a potential action in parturition timing.

Glucocorticoids act in the hypothalamus to inhibit CRH release, but in the trophoblast, glucocorticoids stimulate CRH gene expression (Jones, 1989a; Robinson, 1988). Thus, there may be a novel positive feedback loop in the placenta by which placental CRH stimulates placental ACTH to stimulate fetal and maternal adrenal glucocorticoid production with subsequent stimulation of placental CRH expression (Nicholson, 2001; Riley, 1991).

Growth Hormone-Releasing Hormone. The role of placental GHRH is not known (Berry, 1992). Ghrelin is another regulator of hGH secretion that is produced by placental tissue (Horvath, 2001). Trophoblast ghrelin expression peaks at midpregnancy and is a potential regulator of hGH-V production or a paracrine regulator of differentiation (Fuglsang, 2005; Gualillo, 2001).

image Other Placental Protein Hormones


Expression of relaxin has been demonstrated in human corpus luteum, decidua, and placenta (Bogic, 1995). This peptide is synthesized as a single, 105 amino-acid preprorelaxin molecule that is cleaved to A and B molecules. Relaxin is structurally similar to insulin and insulin-like growth factor. Two of the three relaxin genes—H2 and H3—are transcribed in the corpus luteum (Bathgate, 2002; Hudson, 1983, 1984). Other tissues, including decidua, placenta, and membranes, express H1 and H2 (Hansell, 1991).

The rise in maternal circulating relaxin levels seen in early pregnancy is attributed to corpus luteum secretion, and levels parallel those of hCG. Relaxin, along with rising progesterone levels, may act on myometrium to promote relaxation and the quiescence of early pregnancy (Chap. 21p. 421). In addition, the production of relaxin and relaxin-like factors within the placenta and fetal membranes may play an autocrine-paracrine role in postpartum regulation of extracellular matrix degradation (Qin, 1997a,b). One important relaxin function is enhancement of the glomerular filtration rate (Chap. 4p. 64).

Parathyroid Hormone–Related Protein

In pregnancy, circulating parathyroid hormone-related protein (PTH-rP) levels are significantly elevated within maternal but not fetal circulation (Bertelloni, 1994; Saxe, 1997). Many potential functions of this hormone have been proposed. PTH-rP synthesis is found in several normal adult tissues, especially in reproductive organs that include myometrium, endometrium, corpus luteum, and lactating mammary tissue. PTH-rP is not produced in the parathyroid glands of normal adults. Based on insights from parathyroid hormone-related protein null mice, placental-derived PTH-rP may have an important function to regulate genes involved in transfer of calcium and other solutes. It also contributes to fetal mineral homeostasis in bone, amnionic fluid, and the fetal circulation (Simmonds, 2010).


This hormone is normally secreted by adipocytes. It functions as an antiobesity hormone that decreases food intake through its hypothalamic receptor. It also regulates bone growth and immune function (Cock, 2003; La Cava, 2004). In the placenta, leptin also is synthesized by both cytotrophoblast and syncytiotrophoblast (Henson, 2002). Relative contributions of leptin from maternal adipose tissue versus placenta are currently not well defined. Maternal serum levels are significantly higher than those in nonpregnant women. Fetal leptin levels correlate positively with birthweight and likely play an important function in fetal development and growth. Studies suggest that leptin inhibits apoptosis and promotes trophoblast proliferation (Magarinos, 2007).

Neuropeptide Y

This 36 amino-acid peptide is widely distributed in brain. It also is found in sympathetic neurons innervating the cardiovascular, respiratory, gastrointestinal, and genitourinary systems. Neuropeptide Y has been isolated from the placenta and localized in cytotrophoblasts (Petraglia, 1989). There are receptors for neuropeptide Y on trophoblast, and treatment of placental cells with neuropeptide Y causes CRH release (Robidoux, 2000).

Inhibin and Activin

Inhibin is a glycoprotein hormone that acts preferentially to inhibit pituitary FSH release. It is produced by human testis and by ovarian granulosa cells, including the corpus luteum. Inhibin is a heterodimer made up of an α-subunit and one of two distinct β-subunits, βA or βB. All three are produced by trophoblast, and maternal serum levels peak at term (Petraglia, 1991). One function may be to act in concert with the large amounts of sex steroid hormones to inhibit FSH secretion and thereby inhibit ovulation during pregnancy. Inhibin may act via GnRH to regulate placental hCG synthesis (Petraglia, 1987).

Activin is closely related to inhibin and is formed by the combination of the two β-subunits. Its receptor is expressed in the placenta and amnion. Activin A is not detectable in fetal blood before labor but is present in umbilical cord blood after labor begins. Petraglia (1994) found that serum activin A levels decline rapidly after delivery. It is not clear if chorionic activin and inhibin are involved in placental metabolic processes other than GnRH synthesis.

image Placental Progesterone Production

After 6 to 7 weeks’ gestation, little progesterone is produced in the ovary (Diczfalusy, 1961). Surgical removal of the corpus luteum or even bilateral oophorectomy during the 7th to 10th week does not decrease excretion rates of urinary pregnanediol, the principal urinary metabolite of progesterone. Before this time, however, corpus luteum removal will result in spontaneous abortion unless an exogenous progestin is given (Chap. 63p. 1227). After approximately 8 weeks, the placenta assumes progesterone secretion, resulting in a gradual increase in maternal serum levels throughout pregnancy (Fig. 5-22). By term, these levels are 10 to 5000 times those found in nonpregnant women, depending on the stage of the ovarian cycle.


FIGURE 5-22 Plasma levels of progesterone, estradiol, estrone, estetrol, and estriol in women during the course of gestation. (Modified from Mesiano, 2009, with permission.)

Progesterone Production Rates

The daily production rate of progesterone in late, normal, singleton pregnancies is approximately 250 mg. In multifetal pregnancies, the daily production rate may exceed 600 mg/day. Progesterone is synthesized from cholesterol in a two-step enzymatic reaction. First, cholesterol is converted to pregnenolone within the mitochondria, in a reaction catalyzed by cytochrome P450 cholesterol side-chain cleavage enzyme. Pregnenolone leaves the mitochondria and is converted to progesterone in the endoplasmic reticulum by 3β-hydroxysteroid dehydrogenase. Progesterone is released immediately through a process of diffusion.

Although the placenta produces a prodigious amount of progesterone, there is limited capacity for trophoblast cholesterol biosynthesis. Radiolabeled acetate is incorporated into cholesterol by placental tissue at a slow rate. The rate-limiting enzyme in cholesterol biosynthesis is 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase. Because of this, the placenta must rely on exogenous cholesterol for progesterone formation. Bloch (1945) and Werbin and colleagues (1957) found that after intravenous administration of radiolabeled cholesterol to pregnant women, the amount of radioactivity of urinary pregnanediol was similar to that of plasma cholesterol. Hellig and associates (1970) also found that maternal plasma cholesterol was the principal precursor—as much as 90 percent—of progesterone biosynthesis. The trophoblast preferentially uses LDL cholesterol for progesterone biosynthesis (Simpson, 1979, 1980). In studies of pregnant baboons, when maternal serum LDL levels were reduced, there was a significant drop in placental progesterone production (Henson, 1997). Thus, placental progesterone is formed through the uptake and use of a maternal circulating precursor. This mechanism is unlike the placental production of estrogens, which relies principally on fetal adrenal precursors.

Progesterone Synthesis and Fetal Relationships

Although there is a relationship between fetal well-being and placental estrogen production, this is not the case for placental progesterone. Fetal demise, ligation of the umbilical cord with the fetus and placenta remaining in situ, and anencephaly are all conditions associated with very low maternal plasma levels and low urinary excretion of estrogens. In these circumstances, there is not a concomitant decrease in progesterone levels until some indeterminate time after fetal death. Thus, placental endocrine function, including the formation of protein hormones such as hCG and progesterone biosynthesis, may persist for long periods (weeks) after fetal demise.

Progesterone Metabolism During Pregnancy

The metabolic clearance rate of progesterone in pregnant women is similar to that found in men and nonpregnant women. During pregnancy, the plasma concentration of 5α-dihydroprogesterone disproportionately increases due to synthesis in syncytiotrophoblast from both placenta-produced progesterone and fetal-derived precursor (Dombroski, 1997). Thus, the concentration ratio of this progesterone metabolite to progesterone is increased in pregnancy. The mechanisms for this are not defined completely. Progesterone also is converted to the potent mineralocorticoid deoxycorticosterone in pregnant women and in the fetus. The concentration of deoxycorticosterone is increased strikingly in both maternal and fetal compartments (see Table 5-1). The extraadrenal formation of deoxycorticosterone from circulating progesterone accounts for most of its production in pregnancy (Casey, 1982a,b).

image Placental Estrogen Production

The placenta produces huge amounts of estrogens using blood-borne steroidal precursors from the maternal and fetal adrenal glands. Near term, normal human pregnancy is a hyperestrogenic state. The amount of estrogen produced each day by syncytiotrophoblast during the last few weeks of pregnancy is equivalent to that produced in 1 day by the ovaries of no fewer than 1000 ovulatory women. The hyperestrogenic state of human pregnancy is one of continually increasing magnitude as pregnancy progresses, terminating abruptly after delivery.

During the first 2 to 4 weeks of pregnancy, rising hCG levels maintain production of estradiol in the maternal corpus luteum. Production of both progesterone and estrogens in the maternal ovaries decreases significantly by the 7th week of pregnancy. At this time, there is a luteal-placental transition. By the 7th week, more than half of estrogen entering maternal circulation is produced in the placenta (MacDonald, 1965a; Siiteri, 1963, 1966). These studies support the transition of a steroid milieu dependent on the maternal corpus luteum to one dependent on the developing placenta.

Placental Estrogen Biosynthesis

The estrogen synthesis pathways in the placenta differ from those in the ovary of nonpregnant women. Estrogen is produced during the follicular and luteal phases through the interaction of theca and granulosa cells that surround the follicles. Specifically, androstenedione is synthesized in ovarian theca cells and then is transferred to adjacent granulosa cells for estradiol synthesis. Estradiol production within the corpus luteum of nonpregnant women and in early pregnancy continues to require interaction between the luteinized theca and granulosa cells. However, in human trophoblast, neither cholesterol nor progesterone can serve as precursor for estrogen biosynthesis. A crucial enzyme necessary for sex steroid synthesis—steroid 17α-hydroxylase/17, 20-lyase (CYP17)—is not expressed in the human placenta. Consequently, the conversion of C21-steroids to C19-steroids—the latter being the immediate and obligatory precursors of estrogens—is not possible.

Dehydroepiandrosterone (DHEA) and its sulfate (DHEA-S) are C19-steroids. Although these are often called adrenal androgens, these steroids can also serve as estrogen precursors (Fig. 5-23). Ryan (1959a) found that the placenta had an exceptionally high capacity to convert appropriate C19-steroids to estrone and estradiol. The conversion of DHEA-S to estradiol requires placental expression of four key enzymes that are located principally in syncytiotrophoblast (Bonenfant, 2000; Salido, 1990). First, the placenta expresses high levels of steroid sulfatase (STS), which converts the conjugated DHEA-S to DHEA. DHEA is then acted upon by 3β-hydroxysteroid dehydrogenase type 1 (3βHSD) to produce androstenedione. Cytochrome P450 aromatase (CYP19) then converts androstenedione to estrone, which is then converted to estradiol by 17β-hydroxysteroid dehydrogenase type 1 (17βHSD1).


FIGURE 5-23 Schematic presentation of estrogen biosynthesis in the human placenta. Dehydroepiandrosterone sulfate (DHEA-S), secreted in prodigious amounts by the fetal adrenal glands, is converted to 16α-hydroxydehydroepiandrosterone sulfate (16αOHDHEA-S) in the fetal liver. These steroids, DHEA-S and 16αOHDHEA-S, are converted in the placenta to estrogens, that is, 17β-estradiol (E2) and estriol (E3). Near term, half of E2 is derived from fetal adrenal DHEA-S and half from maternal DHEA-S. On the other hand, 90 percent of E3 in the placenta arises from fetal 16αOHDHEA-S and only 10 percent from all other sources.

Plasma C19-Steroids as Estrogen Precursors

Frandsen and Stakemann (1961) found that urinary estrogens levels in women pregnant with an anencephalic fetus were only about 10 percent of that found in normal pregnancy. The adrenal glands of anencephalic fetuses are atrophic because of absent hypothalamic-pituitary function, which precludes ACTH stimulation. Thus, it seemed reasonable that fetal adrenal glands might provide substance(s) used for placental estrogen formation.

In subsequent studies, DHEA-S was found to be a major precursor of estrogens in pregnancy (Baulieu, 1963; Siiteri, 1963). The large amounts of DHEA-S in plasma and its much longer half-life uniquely qualify it as the principal precursor for placental estradiol synthesis. There is a 10- to 20-fold increased metabolic clearance rate of plasma DHEA-S in women at term compared with that in men and nonpregnant women (Gant, 1971). This rapid use results in a progressive decrease in plasma DHEA-S concentration as pregnancy progresses (Milewich, 1978). However, maternal adrenal glands do not produce sufficient amounts of DHEA-S to account for more than a fraction of total placental estrogen biosynthesis. The fetal adrenal glands are quantitatively the most important source of placental estrogen precursors in human pregnancy. A schematic representation of the estrogen formation pathways in the placenta is presented in Figure 5-23. As shown, the estrogen products released from the placenta are dependent on the substrate available. Thus, estrogen production during pregnancy reflects the unique interactions among fetal adrenal glands, fetal liver, placenta, and maternal adrenal glands.

Directional Secretion of Steroids from Syncytiotrophoblast

More than 90 percent of estradiol and estriol formed in syncytiotrophoblast as shown in Table 5-1 enters maternal plasma (Gurpide, 1966). And 85 percent or more of placental progesterone enters maternal plasma, with little maternal progesterone crossing the placenta to the fetus (Gurpide, 1972).

The major reason for directional movement of newly formed steroid into the maternal circulation is the nature of hemochorioendothelial placentation. In this system, steroids secreted from syncytiotrophoblast can enter maternal blood directly. Steroids that leave the syncytium do not enter fetal blood directly. They must first traverse the cytotrophoblasts and then enter the stroma of the villous core and then fetal capillaries. From either of these spaces, steroids can reenter the syncytium. The net result of this hemochorial arrangement is that there is substantially greater entry of steroids into the maternal circulation compared with the amount that enters fetal blood.


Morphologically, functionally, and physiologically, the fetal adrenal glands are remarkable organs. At term, the fetal adrenal glands weigh the same as those of the adult. More than 85 percent of the fetal gland is composed of a unique fetal zone, which has a great capacity for steroid biosynthesis. Daily steroid production of fetal adrenal glands near term is 100 to 200 mg/day. This compares with resting adult steroid secretion of 30 to 40 mg/day.

The fetal zone is lost in the first year of life and is not present in the adult. In addition to ACTH, fetal adrenal gland growth is influenced by factors secreted by the placenta. This is exemplified by the continued growth of the fetal glands throughout gestation, but rapid involution immediately after birth when placenta-derived factors dissipate. A discussion of the fetal adrenal and liver is warranted in this chapter, given the dependence of normal placental function on the unique fetal adrenal and vice versa.

image Placental Estriol Synthesis

The estrogen products released from the placenta are dependent on the substrate available from the developing fetus. Estradiol is the primary placental estrogen secretory product at term. In addition, significant levels of estriol and estetrol are found in the maternal circulation, and they increase, particularly late in gestation (see Fig. 5-22). These hydroxylated forms of estrogen are produced in the placenta using substrates formed by the combined efforts of the fetal adrenal gland and liver.

There are important fetal-maternal interactions through the fetal liver (see Fig. 5-23). High levels of fetal hepatic 16α-hydroxylase act on adrenal-derived steroids. Ryan (1959b) and MacDonald and Siiteri (1965b) found that 16α-hydroxylated C19-steroids, particularly 16α-hydroxydehydroepiandrosterone (16-OHDHEA), were converted to estriol by placental tissue. Thus, the disproportionate increase in estriol formation during pregnancy is accounted for by placental synthesis of estriol principally from plasma-borne 16-OHDHEA-sulfate. Near term, the fetus is the source of 90 percent of placental estriol and estetrol precursor in normal human pregnancy.

Thus, the placenta secretes several estrogens, including estradiol, estrone, estriol, and estetrol. Because of its hemochorial nature, most placental estrogens are released into the maternal circulation. Maternal estriol and estetrol are produced almost solely by fetal steroid precursors. Thus, levels of these steroids were used in the past as an indicator of fetal well-being. However, low sensitivity and specificity of such tests have caused them to be discarded.

image Enzymatic Considerations

There is very low expression of the microsomal enzyme 3α-hydroxysteroid dehydrogenase, Δ5–4-isomerase (3βHSD) in adrenal fetal zone cells (Doody, 1990; Rainey, 2001). This limits the conversion of pregnenolone to progesterone and of 17α-hydroxypregnenolone to 17α-hydroxyprogesterone, an obligatory step in cortisol biosynthesis. There is, however, very active steroid sulfotransferase activity in the fetal adrenal glands. As a consequence, the principal secretory products of the fetal adrenal glands are pregnenolone sulfate and DHEA-S. Comparatively, cortisol, which likely arises primarily in the neocortex and transitional zone of the fetal adrenal glands and not in the fetal zone, is a minor secretory product until late in gestation.

image Fetal Adrenal Steroid Precursor

The precursor for fetal adrenal steroidogenesis is cholesterol. The steroid biosynthesis rate in the fetal gland is so great that its steroidogenesis alone is equivalent to a fourth of the total daily LDL cholesterol turnover in adults. Fetal adrenal glands synthesize cholesterol from acetate. All enzymes involved in cholesterol biosynthesis are elevated compared with that of the adult adrenal gland (Rainey, 2001). Thus, the de novo cholesterol synthesis rate by fetal adrenal tissue is extremely high. Even so, it is insufficient to account for the steroids produced by these glands. Therefore, cholesterol must be assimilated from the fetal circulation. Plasma cholesterol and its esters are present in the form of very-low-density lipoprotein (VLDL), LDL, and HDL.

Simpson and colleagues (1979) found that fetal glands take up lipoproteins as a source of cholesterol for steroidogenesis. LDL was most effective, HDL was much less, and VLDL was devoid of stimulatory activity. They also evaluated relative contributions of cholesterol synthesized de novo and that of cholesterol derived from LDL uptake. These authors confirmed that fetal adrenal glands are highly dependent on circulating LDL as a source of cholesterol for optimum steroidogenesis (Carr, 1980, 1981b, 1982).

Most fetal plasma cholesterol arises by de novo synthesis in the fetal liver (Carr, 1984). The low LDL cholesterol level in fetal plasma is not the consequence of impaired fetal LDL synthesis, but instead, results from the rapid use of LDL by the fetal adrenal glands for steroidogenesis (Parker, 1980, 1983). As expected, in the anencephalic newborn with atrophic adrenal glands, the LDL cholesterol levels in umbilical cord plasma are high.

image Fetal Conditions That Affect Estrogen Production

Several fetal disorders alter the availability of substrate for placental steroid synthesis and thus highlight the interdependence of fetal development and placental function.

Fetal Demise

Fetal death is followed by a striking reduction in urinary estrogen levels. Similarly, after ligation of the umbilical cord with the fetus and placenta left in situ, placental estrogens production decline markedly (Cassmer, 1959). However, placental progesterone production was maintained. It was concluded that an important source of precursors of placental estrogen—but not progesterone—biosynthesis was eliminated upon fetal death.

Fetal Anencephaly

With absence of the adrenal cortex fetal zone, as seen with anencephaly, the placental estrogen formation rate—especially estriol—is severely limited because of diminished availability of C19-steroid precursors. Therefore, almost all estrogens produced in women pregnant with an anencephalic fetus arise from placental use of maternal plasma DHEA-S. Furthermore, in such pregnancies, estrogen production can be increased by maternal administration of ACTH, which stimulates the DHEA-S secretion rate by the maternal adrenal gland. Because ACTH does not cross the placenta, there is no fetal adrenal stimulation. Finally, placental estrogen production is decreased in women pregnant with an anencephalic fetus when a potent glucocorticoid is given to the mother. This suppresses ACTH secretion and thus decreases the DHEA-S secretion rate from the maternal adrenal cortex (MacDonald, 1965a).

Fetal Adrenal Hypoplasia

Congenital adrenal cortical hypoplasia occurs in perhaps 1 in 12,500 births (McCabe, 2001). Estrogen production in these pregnancies is limited, which suggests the absence of C19-precursors.

Fetal-Placental Sulfatase Deficiency

Placental estrogen formation is generally regulated by the availability of C19-steroid prohormones in fetal and maternal plasma. Specifically, there is no rate-limiting enzymatic reaction in the placental pathway from C19-steroids to estrogen biosynthesis. An exception to this generalization is placental sulfatase deficiency, which is associated with very low estrogen levels in otherwise normal pregnancies (France, 1969). Sulfatase deficiency precludes the hydrolysis of C19-steroid sulfates, the first enzymatic step in the placental use of these circulating prehormones for estrogen biosynthesis. This deficiency is an X-linked disorder, and all affected fetuses are male. Its estimated frequency is 1 in 2000 to 5000 births and is associated with delayed onset of labor. It also is associated with the development of ichthyosis in affected males later in life (Bradshaw, 1986).

Fetal-Placental Aromatase Deficiency

There are a few well-documented examples of aromatase deficiency (Grumbach, 2011; Simpson, 2000). Fetal adrenal DHEA-S, which is produced in large quantities, is converted in the placenta to androstenedione, but in cases of placental aromatase deficiency, androstenedione cannot be converted to estradiol. Rather, androgen metabolites of DHEA produced in the placenta, including androstenedione and some testosterone, are secreted into the maternal or fetal circulation, or both, causing virilization of the mother and the female fetus (Belgorosky, 2009; Harada, 1992; Shozu, 1991).

Trisomy 21—Down Syndrome

Second-trimester maternal serum screening for abnormal levels of hCG, alpha-fetoprotein, and other analytes has become universal (Chap. 14p. 290). As a result, it was discovered that serum unconjugated estriol levels were low in women with Down syndrome fetuses (Benn, 2002). The likely reason for this is inadequate formation of C19-steroids in the adrenal glands of these trisomic fetuses.

Deficiency in Fetal LDL Cholesterol Biosynthesis

A successful pregnancy in a woman with β-lipoprotein deficiency has been described (Parker, 1986). The absence of LDL in the maternal serum restricted progesterone formation in both the corpus luteum and placenta. In addition, estriol levels were lower than normal. Presumably, the diminished estrogen production was the result of decreased fetal LDL formation, which limited fetal adrenal production of estrogen precursors.

Fetal Erythroblastosis

In some cases of severe fetal D-antigen alloimmunization, maternal plasma estrogen levels are elevated. A likely cause is increased placental mass from hypertrophy, which can be seen with such fetal hemolytic anemia (Chap. 15p. 315).

image Maternal Conditions That Affect Placental Estrogen Production

Glucocorticoid Treatment

The administration of glucocorticoids to pregnant women causes a striking reduction in placental estrogen formation. Glucocorticoids inhibit ACTH secretion from the maternal and fetal pituitary glands. This leads to decreased maternal and fetal adrenal secretion of the placental estrogen precursor DHEA-S.

Maternal Adrenal Dysfunction

In pregnant women with Addison disease, maternal urinary estrogen levels are decreased (Baulieu, 1956). The decrease principally affects estrone and estradiol. The fetal adrenal contribution to estriol synthesis, particularly in later pregnancy, is quantitatively much more important.

Maternal Ovarian Androgen-Producing Tumors

The extraordinary efficiency of the placenta in the aromatization of C19-steroids may be exemplified by two considerations. First, Edman and associates (1981) found that virtually all androstenedione entering the intervillous space is taken up by syncytiotrophoblast and converted to estradiol. None of this C19-steroid enters the fetus. Second, a female fetus is rarely virilized if there is a maternal androgen-secreting ovarian tumor. The placenta efficiently converts aromatizable C19-steroids, including testosterone, to estrogens, thus precluding transplacental passage. Indeed, virilized female fetuses of women with an androgen-producing tumor may be cases in which a nonaromatizable C19-steroid androgen is produced by the tumor—for example, 5α-dihydrotestosterone. Another explanation is that testosterone is produced very early in pregnancy in amounts that exceed the placental aromatase capacity at that time.

Gestational Trophoblastic Disease

With complete hydatidiform mole or choriocarcinoma, there is no fetal adrenal source of C19-steroid precursor for trophoblast estrogen biosynthesis. Consequently, placental estrogen formation is limited to the use of C19-steroids in the maternal plasma, and therefore the estrogen produced is principally estradiol (MacDonald, 1964, 1966). Great variation is observed in the rates of both estradiol and progesterone formation in molar pregnancies.


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