Medical Physiology A Cellular and Molecular Approach, Updated 2nd Ed.

THE FEMALE REPRODUCTIVE SYSTEM

Ervin E. Jones

REPRODUCTIVE FUNCTION IN THE FEMALE HUMAN

Reproductive function in female humans is controlled by hormones that emanate from the hypothalamic-pituitary-gonadal axis (see Chapter 47). The release of a mature ovum from an ovary, known as ovulation, is the dominant event of the menstrual cycle. Whereas ovulation in some mammals is triggered by mating, ovulation in the female human is spontaneous and is regulated by cyclic functional interactions among signals coming from the hypothalamus, the anterior pituitary, and the ovaries. Although many aspects of female reproduction are cyclic, maturation and demise (i.e., atresia) of the functional units of the ovaries—the ovarian follicles—are continuous processes that occur throughout reproductive life.

The ovaries are not the only female organs that undergo rhythmic changes. Alterations in cervical and uterine function are controlled by changes in the circulating concentrations of ovarian hormones, that is, the estrogens and progestins. For example, the uterine lining or endometrium thickens under the influence of ovarian hormones and deteriorates and sloughs at the end of the cycle when ovarian estrogen and progestin secretion diminishes. Menstruation reflects this periodic shedding of the endometrium. Menstrual cycles are generally repetitive unless they are interrupted by pregnancy or terminated by menopause. All the cyclic physiological changes prepare the female reproductive tract for sperm and ovum transport, fertilization, implantation, and pregnancy.

Female reproductive organs include the ovaries and accessory sex organs

The ovaries lie on the sides of the pelvic cavity (Fig. 55-1A). A layer of mesothelial cells covers the surface of the ovary. The ovary itself consists of an inner medulla and an outer zone, or cortex, that surrounds the medulla except at the hilar area. The cortex of the ovary in a mature woman contains developing follicles and corpora lutea in various stages of development (Fig. 55-1B). These elements are interspersed throughout the stroma, which includes connective tissue, interstitial cells, and blood vessels. The medulla comprises large blood vessels and other stromal elements.

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Figure 55-1 The anatomy of the female internal genitalia and accessory sex organs.

The female accessory sex organs include the fallopian tubes, the uterus, the vagina, and the external genitalia. The fallopian tube provides a pathway for the transport of ova from the ovary to the uterus. The distal end of the fallopian tube expands as the infundibulum, which ends in multiple fimbriae. The infundibulum is lined with epithelial cells that have cilia that beat toward the uterus. The activity of these cilia and the contractions of the wall of the fallopian tube, particularly around the time of ovulation, facilitate transport of the ovum.

The uterus is a complex, pear-shaped, muscular organ that is suspended by a series of supporting ligaments. It is composed of a fundus, a corpus, and a narrow caudal portion called the cervix. The external surface of the uterus is covered by serosa, whereas the interior, or endometrium, of the uterus consists of complex glandular tissue and stroma. The uterus is continuous with the vagina through the cervical canal. The cervix is composed of dense fibrous connective tissue and muscle cells. The cervical glands lining the cervical canal produce a sugar-rich secretion, the viscosity of which is conditioned by estrogen and progesterone.

The human vagina is ~10 cm in length and is a single, expandable tube. The vagina is lined by stratified epithelium and is surrounded by a thin muscular layer. During development, the lower end of the vagina is covered by the membranous hymen, which is partially perforated during fetal life. In some instances, the hymen remains continuous. The external genitalia include the clitoris, the labia majora, and the labia minora, as well as the accessory secretory glands (including the glands of Bartholin), which open into the vestibule. The clitoris is an erectile organ, which is homologous to the penis (see Chapter 53) and mirrors the cavernous ends of the glans penis.

PUBERTY

Puberty marks the transition to cyclic, adult reproductive function

Puberty is the transition from a noncyclic, relatively quiescent reproductive endocrine system to a state of cyclic reproductive function that allows procreation. Puberty is the transition between the juvenile state and adulthood during which time secondary sexual characteristics appear, the adolescent growth spurt occurs, and the ability to procreate is achieved. Table 55-1 summarizes the stages of puberty in the female. Puberty in girls involves the beginning of menstrual cycles (menarche), breast development (thelarche), and an increase in adrenal androgen secretion (adrenarche).

Table 55-1 Stages in Female Puberty

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The precise cause of the onset of puberty is not completely understood, although multiple intrinsic and extrinsic factors play a role. Genetic factors are major determinants of pubertal onset. Other factors, such as nutrition, geographic location, and exposure to light, also play a role. Over the last century, the age of girls at menarche in the United States and Europe has gradually decreased. Although the reason that menarche now occurs at a younger age remains incompletely understood, it is probably because of improved nutritional status. However, better nutritional status alone cannot completely explain the decreased age of pubertal onset. Distance from the equator and lower altitudes are associated with early onset of puberty. A loose correlation is also seen between the onset of menarche in the mother and the onset of menarche in the daughter. The onset of puberty is also related to body composition and to fat deposition. Severe obesity and heavy exercise delay puberty.

Gonadotropin levels are low during childhood

As shown in Figure 55-2A, a surge in the levels of the pituitary gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), occurs during intra-uterine life. A second peak takes place in the immediate postnatal period. However, gonadotropin levels tend to decrease at ~4 months of age; thereafter, they decline further and remain low until just before puberty. Gonadotropin levels are lowest between 6 and 8 years of age. Although the reason for low gonadotropin secretion by the pituitary in childhood remains unknown, it was once thought to result from feedback inhibition by high levels of gonadal steroids. However, an experiment of nature has revealed that such is not the case. Indeed, girls with gonadal dysgenesis, like physiologically normal girls, have low levels of LH and FSH, even though their ovaries produce low levels of steroids. Thus, it is likely that the low levels of gonadotropins in the prepubertal period do not reflect high levels of steroids, but rather a high sensitivity to feedback inhibition of the hypothalamic-pituitary system by these steroids. I discuss this feedback mechanism in the next section.

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Figure 55-2 Gonadotropin function during life. A, The levels of both LH and FSH peak during fetal life and again during early infancy, before falling to low levels throughout the rest of childhood. At the onset of puberty, LH and FSH levels slowly rise and then begin to oscillate at regular monthly intervals. At menopause, gonadotropin levels rise to very high levels. The four insets show daily changes in gonadotropin levels. B, This is a highly schematic plot of how estrogen levels negatively feed back on gonadotropin secretion by the gonadotroph cells of the anterior pituitary. In childhood, even very low estrogen levels are sufficient to suppress gonadotropin output fully. In adolescence, higher levels of estrogens are required. In the adult woman, estrogens must be at very high levels to suppress gonadotropin release. C, This is a plot—versus age—of the midpoints of curves such as those in B.

During puberty, gonadotropin-releasing hormone secretion becomes pulsatile, and the sensitivity of the gonadotrophs to feedback inhibition by estrogens decreases

As shown by the insets to Figure 55-2A, one of the earliest events of puberty is the onset of pulsatile gonadotropin secretion from the pituitary during rapid eye movement (REM) sleep; this pulsatile gonadotropin secretion reflects the pulsatile release of gonadotropin-releasing hormone (GnRH) from the hypothalamus. The development of secondary sexual characteristics follows the onset of sleep-associated pulsatility. With maturation, these pulses occur throughout the day. It is not understood why pulsatile behavior should occur initially only during REM sleep. The precipitating event that is responsible for initiating pulsatile GnRH release is also unknown, although it may reflect the maturation of hypothalamic neurons. Once a pulsatile pattern of gonadotropin secretion is established, it continues throughout reproductive life into menopause.

The increased pulsatility of GnRH release eventually leads to a marked increase in plasma LH levels—the LH surge that marks the initiation of the first menstrual cycle. During early pubescence, the LH surges do not occur in a regular pattern, so menstrual cycles are generally irregular. As the reproductive system matures, the LH surges gradually come at regular intervals, and cyclic reproductive function becomes firmly established.

The appearance of GnRH pulsatility early in puberty is associated with decreased sensitivity of the hypothalamic-pituitary system to circulating sex steroids. In young girls, even low levels of sex steroids are sufficient to feed back on the hypothalamic-pituitary system and to block the release of gonadotropins (Fig. 55-2B). As a girl goes through puberty, the levels of steroids required to block gonadotropin release progressively become higher and higher. At about the same time, the levels of sex steroids also rise. Eventually, a situation is reached in which the monthly oscillations in sex steroid levels produce the full range of feedback inhibition of gonadotropin release. Thus, during maturation, the sensitivity of the hypothalamic-pituitary system to inhibition by sex steroids falls to reach the low level that is characteristic of the adult (Fig. 55-2C). As discussed later, in addition to the negative feedback of sex steroids on gonadotropin release, positive feedback also occurs near the midpoint of the menstrual cycle.

During puberty, basal levels of LH and FSH increase (Fig. 55-2A). Concentrations of androgens and estrogens also increase many-fold as a result of gonadal stimulation by FSH and LH. The LH surge that occurs at midcycle is thus superimposed on an already high basal level of circulating LH.

HYPOTHALAMIC-PITUITARY-GONADAL AXIS AND CONTROL OF THE FEMALE MENSTRUAL RHYTHM

The menstrual cycle includes both the ovarian and endometrial cycles

The menstrual cycle actually involves cyclic changes in two organs: the ovary and the uterus (Fig. 55-3). The ovarian cycle includes the follicular phase and the luteal phase, separated by ovulation. The endometrial cycle includes the menstrual, the proliferative, and the secretory phases.

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Figure 55-3 The ovarian and endometrial cycles. The menstrual cycle comprises parallel ovarian and endometrial cycles. The follicular phase of the ovarian cycle and the menses start on day 0. In this idealized example, ovulation occurs on day 14, and the entire cycle lasts 28 days.

Although menstrual cycles are generally regular during the reproductive years, the length of the menstrual cycle may be highly variable because of disturbances in neuroendocrine function. The mean menstrual cycle is 28 days long, but considerable variation occurs during both the early reproductive years and the premenopausal period. Irregular menses during adolescence and the premenopausal period occur primarily because of the increased frequency of anovulatory cycles.

The first phase of the ovarian cycle is the follicular phase—during which FSH stimulates a follicle to complete its development (i.e., folliculogenesis). The follicular phase begins with the initiation of menstruation and averages ~14 days in length. The duration of the follicular phase is the most variable of the cycle. During folliculogenesis, the granulosa cells of the follicles increase production of the estrogen estradiol, which stimulates the endometrium to undergo rapid and continuous growth and maturation. This period is the proliferative phase of the endometrial cycle. A rapid rise in ovarian estradiol secretion eventually triggers a surge in LH, which causes ovulation.

After releasing its ovum, the follicle transforms into a corpus luteum, which is why the second half of the ovarian cycle is called the luteal phase. The luteal cells produce progesterone and estrogen, which stimulate further endometrial growth and development. This period is the secretory phase of the endometrial cycle. For unknown reasons, the corpus luteum rapidly diminishes its production of estrogens and progestins, thereby resulting in a catastrophic degeneration of the endometrium that leads to menstrual bleeding. This period is the menstrual phase of the endometrial cycle.

The hypothalamic-pituitary-ovarian axis drives the menstrual cycle

Neurons in the hypothalamus synthesize, store, and release GnRH. Long portal vessels carry the GnRH to the anterior pituitary, where the hormone binds to receptors on the surface of gonadotrophs. The results are the synthesis and release of both FSH and LH from the gonadotrophs.

These trophic hormones, LH and FSH, stimulate the ovary to synthesize and secrete the sex steroids estrogens and progestins. The ovaries also produce peptides called inhibins and activins. Together, these ovarian steroids and peptides exert both negative and positive feedback on both the hypothalamus and the anterior pituitary. This complex interaction is unique among the endocrine systems of the body inasmuch as it generates a monthly pattern of hormone fluctuations. Because the cyclic secretion of estrogens and progestins primarily controls endometrial maturation, menstruation reflects these cyclic changes in hormone secretion.

Neurons in the hypothalamus release GnRH in a pulsatile fashion

At the rostral end of the hypothalamic-pituitary-ovarian axis (Fig. 55-4), neurons in the arcuate nucleus and the preoptic area of the hypothalamus synthesize GnRH. They transport GnRH to their nerve terminals for storage and subsequent release. As discussed later, each of the aforementioned two groups of neurons is responsible for a very different kind of rhythm of GnRH secretion. Axons of the GnRH neurons project directly to the median eminence, the extreme basal portion of the hypothalamus, and terminate near portal vessels. These vessels carry GnRH to the gonadotrophs in the anterior pituitary.

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Figure 55-4 Hypothalamic-pituitary-ovarian axis. Small-bodied neurons in the arcuate nucleus and the preoptic area of the hypothalamus secrete GnRH, a decapeptide that reaches the gonadotrophs in the anterior pituitary through the long portal veins. GnRH binds to a G-protein-coupled receptor on the gonadotroph membrane, triggering the IP3/DAG pathway, raising [Ca2+]i and phosphorylation. Stimulation causes the gonadotrophs to synthesize and release two gonadotropins—FSH and LH—that are stored in secretory granules. Both FSH and LH are glycoprotein heterodimers comprising common α subunits and unique β subunits. The LH binds to receptors on theca cells, thus stimulating Gαs, which, in turn, activates adenylyl cyclase. The resultant rise in [cAMP]i stimulates protein kinase A (PKA), which increases the transcription of several proteins involved in the biosynthesis of progestins and androgens. The androgens enter granulosa cells, which convert the androgens to estrogens. The dashed arrow indicates that the granulosa cells also have LH receptors. FSH binds to receptors on the basolateral membrane of granulosa cells, also activating PKA, thereby stimulating gene transcription and synthesis of the relevant enzymes (e.g., aromatase), activins, and inhibins. Negative feedback on the hypothalamic-pituitary-ovarian axis occurs by several routes. The activins and inhibins act only on the anterior pituitary. The estrogens and progestins act on both the anterior pituitary and on the hypothalamic neurons, by exerting both positive and negative feedback controls. CNS, central nervous system.

The gene encoding GnRH is located on chromosome 9 (Fig. 55-5). The mature mRNA for GnRH encodes a preprohormone composed of 92 amino acids. After removing the 23–amino acid signal sequence (residues −23 to −1), the neuron produces a prohormone (residues 1 to 69). Cleavage of this prohormone yields the decapeptide GnRH (residues 1 to 10), a 56–amino acid peptide (residues 14 to 69) referred to as GnRH-associated peptide (GAP), and three amino acids that link the two. The neuron transports both GnRH and GAP down the axon for secretion into the portal circulation. The importance of GAP is unknown, but it may inhibit prolactin secretion.

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Figure 55-5 Map of the gonadotropin-releasing-hormone gene. The mature mRNA encodes a preprohormone with 92 amino acids. Removal of the 23–amino acid signal sequence yields the 69–amino acid prohormone. Cleavage of this prohormone yields GnRH.

GnRH is present in the hypothalamus at 14 to 16 weeks’ gestation, and its target, the gonadotropin-containing cells (gonadotrophs), are present in the anterior pituitary gland as early as 10 weeks’ gestation. The hypothalamic-pituitary system is functionally competent by ~23 weeks’ gestation, at which time fetal tissues release GnRH.

The GnRH neurons do not release GnRH continuously, but rather in rhythmic pulses (Fig. 55-6). GnRH is released in bursts into the portal vessels about once per hour, thereby intermittently stimulating the gonadotrophs in the anterior pituitary. Because the half-life of GnRH in blood is only 2 to 4 minutes, these hourly bursts of GnRH cause clearly discernible oscillations in portal plasma GnRH levels that result in hourly surges in release of the gonadotropins LH and FSH. Early in the follicular phase of the cycle, when the gonadotrophs are not very GnRH sensitive, each burst of GnRH elicits only a small rise in LH (Fig. 55-6A). Later in the follicular phase, when the gonadotrophs in the anterior pituitary become much more sensitive to the GnRH in the portal blood, each burst of GnRH triggers a much larger release of LH (Fig. 55-6B).

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Figure 55-6 Pulsatile release of GnRH and pulsatile secretion of LH. (Data from Wang CF, Lasley BL, Lein A, Yen SS: J Clin Endocrinol Metab 1976; 42:718-728.)

Although the mechanisms controlling the hourly pulses of GnRH remain unclear, the pulse generator for GnRH is thought to be located in the arcuate nucleus of the medial basal hypothalamus, where one group of GnRH neurons resides. In rodents, bursts of nerve impulses from neurons in these nuclei correspond in time with the pulsatile release of GnRH from the hypothalamus and with the episodic release of LH from the anterior pituitary. These data suggest that a built-in system within the hypothalamus controls the pulsatile discharge of GnRH from nerve terminals. The pulse-generating mechanism is key to control of cyclic reproductive function and to regulation of the menstrual cycle. The frequency of GnRH release, and thus LH release, determines the specific response of the gonad. Pulses spaced 60 to 90 minutes apart upregulate the gonadotrophs’ GnRH receptors and thus stimulate the release of gonadotropins. However, continuous administration of GnRH (or an analogue) causes downregulation of the gonadotrophs’ GnRH receptors and thus suppresses gonadotropin release and gonadal function (see the box titled Therapeutic Uses of GnRH). (See Note: Frequency versus Amplitude of Hypothalamic Releasing Hormones)

In addition to the hourly rhythm of GnRH secretion, orchestrated by the arcuate nucleus, a monthly rhythm of GnRH secretion also occurs—in rhesus monkeys. A massive increase in GnRH secretion at midcycle is, in part, responsible for the LH surge, which, in turn, leads to ovulation. Which neurons produce the massive surge in GnRH that leads to the LH surge? These are not the GnRH neurons in the arcuate nucleus but, rather, those in the preoptic area. The preoptic GnRH neurons have inhibitory γ-aminobutyric acid (GABA) receptors, whereas the arcuate GnRH neurons have inhibitory opioid receptors. Later in this chapter, I discuss how these two sets of GnRH neurons may underlie the negative and positive feedback produced by estrogens.

GnRH stimulates gonadotrophs in the anterior pituitary to secrete FSH and LH, which stimulate ovarian cells to secrete estrogens and progestins

GnRH enters the anterior pituitary through the portal system and binds to GnRH receptors on the surface of the gonadotroph, thus initiating a series of cellular events that result in the synthesis and secretion of gonadotropins (Fig. 55-7). GnRH binds to a G protein–linked receptor coupled to Gαq. The result is activation of phospholipase C (PLC), which, in turn, hydrolyzes phosphatidylinositol 4,5-biphosphonate (PIP2) to inositol 1, 4, 5-triphosphate (IP3), and diacylglycerol (DAG) (see Chapter 3). Both IP3 and DAG are second messengers. Release of Ca2+ from the endoplasmic reticulum by IP3 causes an increase in [Ca2+]i. This Ca2+ induces the Ca2+ channels at the cell membrane to open and allows an influx of extracellular Ca2+ that sustains the elevated [Ca2+]i. The rise in [Ca2+]i triggers exocytosis and gonadotropin release.

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Figure 55-7 Gonadotropin secretion. PKC, protein kinase C.

Therapeutic Uses of GnRH

Continuous administration of GnRH leads to downregulation (suppression) of gonadotropin secretion, whereas pulsatile release of GnRH causes upregulation (stimulation) of FSH and LH secretion. Clinical problems requiring upregulation of gonadotropin secretion, which leads to stimulation of the gonads, are therefore best treated by a pulsatile mode of GnRH administration. In contrast, when the patient requires gonadal inhibition, a continuous mode of administration is necessary.

An example of a disease requiring pulsatile GnRH administration is Kallmann syndrome. Disordered migration of GnRH cells during embryologic development causes Kallmann syndrome, which in adults results in hypogonadotropic hypogonadism and anosmia (loss of sense of smell). Normally, primordial GnRH cells originate in the nasal placode during embryologic development. These primitive cells then migrate through the forebrain to the diencephalon, where they become specific neuronal groups within the medial basal hypothalamus and preoptic area. In certain individuals, both male and female, proper migration of GnRH cells fails to occur. The cause of Kallmann syndrome was confirmed in humans when researchers studied a fetus at 19 weeks’ gestation that had complete deletion of the X-linked Kallmann locus. The GnRH cells were found along their known migration route, but not in the brain. Girls and women with Kallmann syndrome generally have amenorrhea (no menstrual cycles). However, the pituitary and gonads of these individuals can function properly when appropriately stimulated. Thus, women treated with exogenous gonadotropins or GnRH analogues—pulsatile administration with a programmed infusion pump—can have normal folliculogenesis, ovulation, and pregnancy.

An example of a disease requiring continuous GnRH administration to downregulate gonadal function is endometriosis. Endometriosis is a common condition caused by the aberrant presence of endometrial tissue outside the uterine cavity. This tissue responds to estrogens during the menstrual cycle and is a source of pain and other problems, including infertility. In patients with endometriosis, continuous administration of GnRH analogue inhibits replenishment of the receptor for GnRH in the gonadotrophs in the anterior pituitary. As a result, insufficient numbers of GnRH receptors are available for optimum GnRH action, thereby diminishing gonadotropin secretion and producing relative hypoestrogenism. Because estrogen stimulates the endometrium, continuous administration of GnRH or GnRH analogues causes involution and diminution of endometriotic tissue.

Leiomyomas (smooth muscle tumors) of the uterus (also called a uterine fibroid) are also estrogen dependent. When estrogen levels are decreased, the proliferation of these lesions is decreased. Therefore, leiomyomas of the uterus can also be effectively treated by continuousadministration of GnRH analogues.

In addition to the IP3 pathway, GnRH also acts through the DAG pathway. The DAG formed by PLC stimulates protein kinase C, which indirectly leads to increases in gene transcription. The net effect is an increase in synthesis of the gonadotropins FSH and LH. In addition, GnRH increases mRNA levels for certain immediate early response genes (e.g., c-Fos, c-Jun, and JunB).

The GnRH receptor is internalized and partially degraded in the lysosomes. However, a portion of the GnRH receptor is shuttled back to the cell surface. Return of the GnRH receptor to the cell membrane is referred to as receptor replenishment and is related to the upregulation of receptor activity discussed earlier. The mechanism through which GnRH receptor replenishment occurs remains unclear.

FSH and LH are in the same family as thyroid-stimulating hormone (TSH; see Chapter 49) and human chorionic gonadotropin (hCG; see Chapter 56). All four are glycoprotein hormones with α and β chains. The α chains of all four of these hormones are identical; in humans, they have 92 amino acids and a molecular weight of ~20 kDa. The β chains of FSH and LH are unique and confer the specificity of the hormones. The rhythm of GnRH secretion influences the relative rates of expression for genes encoding the synthesis of the α, βFSH, and βLH subunits of FSH and LH. GnRH pulsatility also determines the dimerization of the α and βFSH subunits, or α and βLH, as well as their glycosylation.

Differential secretion of FSH and LH is also affected by several other hormonal mediators, including ovarian steroids, inhibins, and activins. I discuss the role of these agents in the section on feedback control of the hypothalamic-pituitary-ovarian axis. Thus, depending on the specific hormonal milieu produced by different physiological circumstances, the gonadotroph produces and secretes the α and β subunits of FSH and LH at different rates. The secretion of LH and FSH is further modulated by neuropeptides, amino acids such as aspartate, neuropeptide Y, corticotropin-releasing hormone (CRH), and endogenous opioids.

Before ovulation, the LH and FSH secreted by the gonadotrophs act on cells of the developing follicle. The theca cells of the follicle have LH receptors, whereas the granulosa cells have both LH and FSH receptors. Both LH and FSH are required for estrogen production because neither the theca cell nor the granulosa cell can carry out all the required steps. After ovulation, LH acts on the cells of the corpus luteum; recall that after ovulation, the cells of the follicle give rise to the corpus luteum.

LH and FSH bind to specific receptors on the surface of their target cells. Both the LH and the FSH receptors are coupled through Gαs to adenylyl cyclase (see Chapter 3), which catalyzes the conversion of ATP to cAMP. cAMP stimulates protein kinase A, which not only stimulates the enzymes involved in steroid biosynthesis but also induces the synthesis of certain proteins and increases cell division. Among the proteins whose synthesis is promoted by gonadotropins is the low-density lipoprotein (LDL) receptor required for cholesterol uptake and the aromatase required for estrogen synthesis.

Ovaries also produce peptide hormones: inhibins, which inhibit FSH secretion, and activins, which activate it

The inhibins and the activins are peptides that modulate FSH secretion by the gonadotrophs. The transforming growth factor β (TGF-β) supergene family is a group of molecules that are structurally related and include TGF-β, antimüllerian hormone (AMH; see Chapter 53), the activins, the inhibins, and other glycoproteins. These growth factors modulate growth and differentiation during development. The inhibins and activins are dimers constructed from a related set of building blocks: a glycosylated 20-kDa α subunit and two nonglycosylated 12-kDa β subunits, one called βA and the other called βB (Fig. 55-8). The inhibins are always composed of one α subunit and either a βA or a βB subunit; the α and β subunits are linked by disulfide bridges. The α-βA dimer is called inhibin A, whereas the α-βB dimer is called inhibin B. The activins, however, are composed of two β-type subunits. Thus, three kinds of activins are recognized: βAA, βBB, and the heterodimer βAB.

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Figure 55-8 The inhibins and activins. The inhibins and activins are peptide hormones that are made up of a common set of building blocks. For both the inhibins and the activins, disulfide bonds link the two subunits.

The inhibins are produced by the granulosa cells of the follicle, as well as other tissues, including the pituitary, the brain, the adrenal gland, the kidney, the bone marrow, the corpus luteum, and the placenta. FSH specifically stimulates the granulosa cells to produce inhibins. Also involved in the regulation of inhibin production are certain other factors, including hormones and growth-stimulating factors. Estradiol may stimulate inhibin production through an intraovarian mechanism. Just before ovulation, after the granulosa cells acquire LH receptors, LH also stimulates the production of inhibin by granulosa cells. The biological action of the inhibins is primarily confined to the reproductive system. As discussed later, the inhibins inhibit FSH production by gonadotrophs. The activins are produced in the same tissues as the inhibins, but they stimulate—rather than inhibit—FSH release from pituitary cells.

Both the ovarian steroids (estrogens and progestins) and peptides (inhibins and activins) feed back on the hypothalamic-pituitary axis

As summarized in Figure 55-4, the ovarian steroids—the estrogens and progestins—exert both negative and positive feedback on the hypothalamic-pituitary axis. Whether the feedback is negative or positive depends on both the concentration of the gonadal steroids and the duration of the exposure to these steroids (i.e., the time in the menstrual cycle). In addition, the ovarian peptides—the inhibins and activins—also feed back on the anterior pituitary.

Negative Feedback by Ovarian Steroids Throughout most of the menstrual cycle, the estrogens and progestins that are produced by the ovary feed back negatively on both the hypothalamus and the gonadotrophs of the anterior pituitary. The net effect is to reduce the release of both LH and FSH. The estrogens exert negative feedback at both low and high concentrations, whereas the progestins are effective only at high concentrations.

Although estrogens inhibit the GnRH neurons in the arcuate nucleus and preoptic area of the hypothalamus, this inhibition is not direct. Rather, the estrogens stimulate interneurons that inhibit the GnRH neurons. In the arcuate nucleus, these inhibitory neurons exert their inhibition through opiates. However, in the preoptic area, the inhibitory neurons exert their inhibitory effect through GABA, a classic inhibitory neurotransmitter (see Chapter 13).

Positive Feedback by Ovarian Steroids Although ovarian steroids feed back negatively on the hypothalamic-pituitary axis during most of the menstrual cycle, they have the opposite effect at the end of the follicular phase. Levels of estrogen, mainly estradiol, rise gradually during the first half of the follicular phase of the ovarian cycle and then steeply during the second half (Fig. 55-9). After the estradiol levels reach a certain threshold for a minimum of 2 days—and perhaps because of the accelerated rate of estradiol secretion—the hypothalamic-pituitary axis reverses its sensitivity to estrogens; that is, estrogens now feed back positively on the axis. One manifestation of this positive feedback is that estrogens now increase the sensitivity of the gonadotrophs in the anterior pituitary gland to GnRH. As discussed in the next section, this switch to positive feedback promotes the LH surge. Indeed, pituitary cells that are cultured in the absence of estrogen have suboptimal responses to GnRH. Once high levels of estrogens have properly conditioned the gonadotrophs, rising levels of progesterone during the late follicular phase also produce a positive feedback response and thus facilitate the LH surge.

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Figure 55-9 Hormonal changes during the menstrual cycle. The menstrual cycle is a cycle of the hypothalamic-pituitary-ovarian axis, as well as a cycle of the targets of the ovarian hormones: the endometrium of the uterus. Therefore, the menstrual cycle includes both an ovarian cycle—which includes the follicular phase, ovulation, and the luteal phase—and an endometrial cycle—which includes the menstrual, the proliferative, and the secretory phases.

Negative Feedback by the Inhibins The inhibins inhibit FSH secretion by the gonadotrophs of the anterior pituitary (hence the name inhibin) in a classic negative feedback arrangement. The initial action of inhibin appears to be beyond the Ca2+-mobilization step in FSH secretion. In cultured pituitary cells, even very small amounts of inhibin markedly reduce mRNA levels for both the αLH/FSH and the βFSHsubunits. As a result, inhibins suppress FSH secretion. In contrast, inhibins have no effect on the mRNA levels of βLH. In addition to their actions on the anterior pituitary, the inhibins also have the intraovarian effect of decreasing androgen production, which can have secondary effects on intrafollicular estrogen production.

Positive Feedback by the Activins Activins promote marked increases in βFSH mRNA and FSH release, with no change in βLH formation. The stimulatory effect of activins on FSH release is independent of GnRH action. Like the inhibins, the activins also have the intraovarian action of stimulating the synthesis of estrogens. Thus, by their actions on both the gonadotrophs and the ovaries, the activins and inhibins regulate the activity of the follicular cells during the menstrual cycle.

Modulation of gonadotropin secretion by positive and negative ovarian feedback produces the normal menstrual rhythm

We already saw in Figure 55-6 that the pulsatile release of GnRH from the hypothalamus, generally occurring every 60 to 90 minutes, triggers a corresponding pulsatile release of LH and FSH from the gonadotrophs of the anterior pituitary. Because the gonadotropins elicit the release of ovarian steroids, and these steroids modulate the hypothalamic-pituitary axis, the interaction between the ovarian steroids and gonadotropin release is an example of feedback. This feedback is especially interesting because it is bidirectional in that it elicits negative feedback throughout most of the menstrual cycle but positivefeedback immediately before ovulation.

Figure 55-9 illustrates the cyclic hormonal changes during the menstrual cycle. The time-averaged records of LH and FSH levels mask their hour-by-hour pulsatility. The follicular phase is characterized by a relatively high frequency of GnRH—and thus LH—pulses. Early in the follicular phase, when levels of estradiol are low but rising, the frequency of LH pulses remains unchanged, but their amplitude gradually increases with time. We see this increase in amplitude in Figure 55-6, in which the early and late follicular phases are compared. Later in the follicular phase of the menstrual cycle, the higher estrogen levels cause both the frequency and the amplitude of the LH pulses to increase gradually. During this time of high estradiol levels, the ovarian steroids are beginning to feed back positively on the hypothalamic-pituitary axis. Late in the follicular phase, the net effect of this increased frequency and amplitude of LH and FSH pulses is an increase in their time-averaged circulating levels (Fig. 55-9).

The LH surge is an abrupt and dramatic rise in the LH level that occurs around the 13th to 14th day of the follicular phase in the average woman. The LH surge peaks ~12 hours after its initiation and lasts for ~48 hours. The peak concentration of LH during the surge is ~3-fold greater than the concentration before the surge (Fig. 55-9). The LH surge is superimposed on the smaller FSH surge. Positive feedback of estrogens, progestins, and activins on the hypothalamic-pituitary axis is involved in the induction of this LH surge. The primary trigger of the gonadotropin surge is a rise in estradiol to very high threshold levels just before the LH surge. The rise in estrogen levels has two effects. First, the accelerated rate of increase in estradiol levels in the preovulatory phase sensitizes the gonadotrophs in the anterior pituitary to GnRH pulses (Fig. 55-6). Second, the increasing estrogen levels also modulate hypothalamic neuronal activity and induce a GnRH surge, presumably through GnRH neurons in the preoptic area of the hypothalamus. Thus, the powerful positive feedback action of estradiol induces the midcycle surge of LH and, to a lesser extent, FSH. Gradually rising levels of the activins—secreted by granulosa cells—also act in a positive feedback manner to contribute to the FSH surge. In addition, gradually increasing levels of LH trigger the preovulatory follicle to increase its secretion of progesterone. These increasing—but still “low”—levels of progesterone also have a positive feedback effect on the hypothalamic-pituitary axis that is synergistic with the positive feedback effect of the estrogens. Thus, although progesterone is not the primary trigger for the LH surge, it augments the effects of estradiol.

The gonadotropin surge causes ovulation and luteinization. The ovarian follicle ruptures, probably because of weakening of the follicular wall, and expels the oocyte and with it the surrounding cumulus and corona cells. This process is known as ovulation, and it is discussed in more detail in Chapter 56. As discussed later, a physiological change—luteinization—in the granulosa cells of the follicle causes these cells to secrete progesterone rather than estradiol. The granulosa and theca cells undergo structural changes that transform them into luteal cells, a process known as luteinization. The pulsatile rhythm of GnRH release and gonadotropin secretion is maintained throughout the gonadotropin surge.

As the luteal phase of the menstrual cycle begins, circulating levels of LH and FSH rapidly decrease (Fig. 55-9). This fall-off in gonadotropin levels reflects negative feedback by three ovarian hormones—estradiol, progesterone, and inhibin. Moreover, as gonadotropin levels fall, so do the levels of ovarian steroids. Thus, immediately after ovulation we see more or less concurrent decreases in the levels of both gonadotropins and ovarian hormones.

Later, during the luteal phase, the luteal cells of the corpus luteum gradually increase their synthesis of estradiol, progesterone, and inhibin (Fig. 55-9). The rise in concentration of these hormones causes—in typical negative feedback fashion—the continued decrease of gonadotropin levels midway through the luteal phase. One of the mechanisms of this negative feedback is the effect of progesterone on the hypothalamic-pituitary axis. Recall that at the peak of the LH surge, both the frequency and the amplitude of LH pulses are high. Progesterone levels rise, and high levels stimulate inhibitory opioidergic interneurons in the hypothalamus, thus inhibiting the GnRH neurons. This inhibition decreases the frequency of LH pulses, although the amplitude remains rather high.

By ~48 hours before onset of the menses, the pulsatile rhythm of LH secretion has decreased to one pulse every 3 to 4 hours. As a result, circulating levels of LH slowly fall during the luteal phase. During the late luteal phase, the gradual demise of the corpus luteum leads to decreases in the levels of progesterone, estradiol, and inhibin (Fig. 55-9). After the onset of menstruation, the hypothalamic-pituitary axis returns to a follicular-phase pattern of LH secretion (i.e., a gradual increase in the frequency of GnRH pulses).

OVARIAN STEROIDS

Starting from cholesterol, the ovary synthesizes estradiol, the major estrogen, and progesterone, the major progestin

Estrogens in female humans are derived from the ovary and the adrenal gland and from peripheral conversion in adipose tissue. In a nonpregnant woman, estradiol, the primary circulating estrogen, is secreted principally by the ovary. The precursor for the biosynthesis of the ovarian steroids, as it is for all other steroid hormones produced elsewhere in the body, is cholesterol. Cholesterol is a 27-carbon sterol that is both ingested in the diet and synthesized in the liver from acetate (see Chapter 46). Ovarian cells can synthesize their own cholesterol de novo. Alternatively, cholesterol can enter cells in the form of LDL cholesterol and can bind to LDL receptors. (See Note: Cytochrome P-450 Enzymes)

As shown in Figure 55-10, a cytochrome P-450 enzyme (see Table 50-2) known as the side-chain–cleavage enzyme (or 20, 22-desmolase) catalyzes the conversion of cholesterol to pregnenolone. This reaction is the rate-limiting step in estrogen production. Ovarian cells then convert pregnenolone to progestins and estrogens. The initial steps of estrogen biosynthesis from pregnenolone follow the same steps as synthesis of the two so-called adrenal androgens dehydroepiandrosterone (DHEA) and androstenedione, both of which have 19 carbon atoms. These steps are discussed in connection with both substances (see Figs. 50-2 and 54-5). The Leydig cells in the testis can use either of two pathways to convert these weak androgens to testosterone. Cells in the ovaries are different because, as shown in Figure 55-10, they have an aromatase that can convert androstenedione to estrone and testosterone to estradiol. This aromatization also results in loss of the 19-methyl group (thus, the estrogens have only 18 carbons), as well as conversion of the ketone at position 3 to a hydroxyl in the A ring of the androgen precursor. Once formed, estronecan be converted into the more powerful estrogen estradiol, and vice versa, by 17β-hydroxysteroid dehydrogenase (17β-HSD). Finally, the liver can convert both estradiol and estrone into the weak estrogen estriol.

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Figure 55-10 Biosynthesis of the ovarian steroids. This scheme summarizes the synthesis of the progestins and estrogens from cholesterol. The individual enzymes are shown in the horizontal and vertical boxes; these enzymes are located in either the smooth endoplasmic reticulum (SER) or the mitochondria. The side-chain–cleavage enzyme that produces pregnenolone is also known as 20, 22-desmolase. The chemical groups modified by each enzyme are highlighted in the reaction product. The ovary differs from the testis in having aromatase, which converts androgens to estrogens. Certain of these pathways are shared in the biosynthesis of the glucocorticoids and mineralocorticoids (see Fig. 50-2) and estrogens (see Fig. 54-5).

The two major progestinsprogesterone and 17α-hydroxyprogesterone, are formed even earlier in the biosynthetic pathway than the adrenal androgens. Functionally, progesterone is the more important progestin, and it has higher circulating levels.

Estrogen biosynthesis requires two ovarian cells and two gonadotropins, whereas progestin synthesis requires only a single cell

In the follicular phase of the menstrual cycle, the follicle synthesizes estrogens, whereas in the luteal phase, the corpus luteum does the synthesis. A unique aspect of estradiol synthesis is that it requires the contribution of two distinct cell types: the theca and granulosa cells within the follicle and the theca-lutein and granulosa-lutein cells within the corpus luteum (Fig. 55-11). I discuss these cells—as well as development of the follicle and corpus luteum—in the next major section.

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Figure 55-11 Two-cell, two-gonadotropin model. During the follicular phase, the major product of the follicle is estradiol, whereas during the luteal phase, the major products of the corpus luteum are the progestins, although estradiol synthesis is still substantial. In the follicular phase, LH primes the theca cell to convert cholesterol to androstenedione. Because the theca cell lacks aromatase, it cannot generate estradiol from this androstenedione. Instead, the androstenedione diffuses to the granulosa cell, whose aromatase activity has been stimulated by FSH. The aromatase converts the androstenedione to estradiol. In the luteal phase, the vascularization of the corpus luteum makes low LDL available to the granulosa-lutein cells. Thus, both the theca-lutein and the granulosa-lutein cells can produce progesterone, the major product of the corpus luteum. For production of 17α-hydroxyprogesterone (17α-OH progesterone), some of the progesterone diffuses into the theca-lutein cell, which has the 17α-hydroxylase activity needed for converting the progesterone to 17α-hydroxyprogesterone. The theca-lutein cell can also generate the androstenedione, which diffuses into the granulosa-lutein cell for estradiol synthesis. AC, adenylyl cyclase.

The superficial theca cells and theca-lutein cells can take up cholesterol and produce the adrenal androgens, but they do not have the aromatase necessary for estrogen production. However, the deeper granulosa cells and granulosalutein cells have the aromatase, but they lack the 17α-hydroxylase and 17, 20-desmolase (which are the same protein) necessary for making the adrenal androgens. Another difference between the two cell types is that—in the follicle—the superficial theca cell is near blood vessels and is hence a source of LDL cholesterol. The granulosa cell, conversely, is far from blood vessels and instead is surrounded by LDL-poor follicular fluid. Thus, in the follicular stage, the granulosa cells obtain most of their cholesterol by de novo synthesis. However, after formation of the corpus luteum, the accompanying vascularization makes it possible for the granulosa-lutein cell to take up LDL cholesterol from the blood and to thus synthesize large amounts of progesterone. A final difference between the two cell types is that theca cells have LH receptors, and granulosa cells have both LH and FSH receptors.

Because of their unique physiological properties, neither the theca/theca-lutein cells nor the granulosa/granulosalutein cells can make estrogens by themselves. According to the two-celltwo-gonadotropin hypothesis, estrogen synthesis occurs in the following steps:

Step 1. LH stimulates the theca cell, through the adenylyl cyclase pathway, to increase its synthesis of LDL receptors and the side-chain–cleavage enzyme.

Step 2. Thus stimulated, the theca cell increases its synthesis of androstenedione.

Step 3. The androstenedione synthesized in the theca cells freely diffuses to the granulosa cells.

Step 4. FSH, also acting through the adenylyl cyclase pathway, stimulates the granulosa cell to produce aromatase.

Step 5. The aromatase converts androstenedione to estrone (Fig. 55-10). 17β-HSD then converts the estrone to estradiol. Alternatively, 17β-HSD can first convert the same androstenedione to testosterone, and then the aromatase can convert this product to estradiol. By these pathways, theca-derived androgens are converted to estrogens in the granulosa cell.

Step 6. The estradiol diffuses into the blood vessels.

At low concentrations, the weak androgens produced by the theca cells are substrates for estrogen synthesis by the granulosa cells, in addition to enhancing the aromatase activity of granulosa cells. However, at high concentrations, conversion of androgens to estrogens is diminished. Instead, the weak androgens are preferentially converted by 5α-reductase (see Fig. 54-5) to more potent androgens, such as dihydrotestosterone, a substance that cannot be converted to estrogen. Furthermore, these 5α-reduced androgens inhibit aromatase activity. Thus, the net effect of a high-androgen environment in the follicle is to decrease estrogen production. These androgens also inhibit LH receptor formation on follicular cells.

In the luteal phase of the cycle, luteinization of the follicle substantially changes the biochemistry of the theca and granulosa cells. As part of the formation of the corpus luteum, blood vessels invade deep toward the granulosa-lutein cells. Recall that in the follicle, the granulosa cells had been surrounded by follicular fluid, which is poor in LDL cholesterol. The increased vascularity facilitates the delivery of LDL cholesterol to the granulosa-lutein cells. In addition, LH stimulates the granulosa-lutein cell to take up and process cholesterol—as it does in theca cells. The net effect is the increased progesterone biosynthesis that is characteristic of the midluteal phase. Indeed, the major products of the corpus luteum are progesterone and 17α-hydroxyprogesterone, although the corpus luteum also produces estradiol. As indicated in Figure 55-11, the granulosa-lutein cells cannot make either 17α-hydroxyprogesterone or estradiol directly because these cells lack the protein that has dual activity for 17α-hydroxylase and 17, 20-desmolase (Fig. 55-10). Thus, 17α-hydroxyprogesterone synthesis necessitates that progesterone first moves to the theca-lutein cell (Fig. 55-11), which can convert progesterone to 17α-hydroxyprogesterone, as well as androstenedione. Furthermore, estradiol synthesis necessitates that androstenedione from the theca-lutein cell moves to the granulosa-lutein cell for aromatization and formation of estradiol.

The Birth Control Pill

Hormonal contraception is the most commonly used method of contraception in the United States; ~30% of sexually active women take the oral contraceptive pill (OCP). Numerous combination (i.e., estrogen and progestin) oral contraceptives and progestin-only pills are available. The estrogens and progestins used in OCPs have varying potencies. In the United States, two estrogen compounds are approved for oral contraceptive use: ethinyl estradiol and mestranol. The progestins used in OCPs are modified steroids in which the methyl at position 19 (Fig. 55-10) is removed; these progestins include norethindrone, norgestrel, norethynodrel, norethindrone acetate, and ethynodiol diacetate. A new generation of progestins—including gestodene and norgestimate—have reduced androgenic effects.

The woman takes the OCP daily for 21 days out of the 28-day cycle; she takes no pill, a placebo, or an iron pill during days 22 to 28. No medication is usually given during this fourth week, to allow withdrawal bleeding to occur. Three regimens of contraceptive steroid administration are used:

1. Monophasic or fixed-combination OCPs. The pills taken for the first 21 days of the cycle are identical.

2. Multiphasic or varying-dose OCPs. The pills contain two or three different amounts of the same estrogen and progestin, the dosages of which vary at specific intervals during the 21-day medication period. Multiphasic OCPs generally maintain a low dose of estrogen throughout the cycle, combined with varying amounts of progestin. The rationale for this type of formulation is that the woman takes a lower total dose of steroid but is not at increased risk of breakthrough endometrial bleeding.

3. Progestin-only OCPs (“minipill”). The woman takes these estrogen-free OCPs daily for 3 weeks of a 4-week cycle. This regimen may be associated with irregular, low-grade, breakthrough endometrial bleeding. The progestin-only OCP is a good option for nursing mothers, as well as women for whom estrogens are contraindicated (e.g., those with thromboembolic disease, cerebral vascular incidents, and hypertension).

Biological Action of Oral Contraceptives

The contraceptive effectiveness of OCPs accrues from several actions. Like natural ovarian steroids, contraceptive steroids feed back both directly at the level of the hypothalamus (decreasing secretion of GnRH) and at the level of the gonadotrophs in the anterior pituitary (Fig. 55-4). The net effect is suppressed secretion of the gonadotropins, FSH and LH. The low FSH levels are insufficient to stimulate normal folliculogenesis; the low LH levels obviate the LH surge and therefore inhibit ovulation. However, in the commonly used doses, contraceptive steroids do not completely abolish either gonadotropin secretion or ovarian function.

The progestin effect of the OCP causes the cervical mucus to thicken and become viscid and scant. These actions inhibit sperm penetration into the uterus. The progestins also impair the motility of the uterus and oviducts and therefore decrease transport of both ova and sperm to the normal site of fertilization in the distal fallopian tube (see Chapter 56). Progestins also produce changes in the endometrium that are not conducive for implantation of the embryo. These changes include decreased glandular production of glycogen and thus diminished energy for the blastocyst to survive in the uterus.

Progestin-only OCPs do not effectively inhibit ovulation, as do the combination pills. However, they do produce the other actions: mucus thickening, reduced motility, and impaired implantation. Because they are inconsistent inhibitors of ovulation, the progestin-only OCPs have a substantially higher failure rate than does the combined type of OCP.

Side effects of the compounds in OCPs are those associated with estrogens and progestins and include nausea, edema, headaches, and weight gain. Side effects of progestins include depression, mastodynia, acne, and hirsutism. Many of the side effects associated with the progestin component of the pill are the result of the androgenic actions of the progestins used, particularly the acne and hirsutism. The potential benefits of the newer progestins include decreased androgenic effects, such as increased sex hormone–binding globulin, improved glucose tolerance (see Chapter 51), and increased high-density lipoprotein and decreased LDL cholesterol (see Chapter 46). The clinical impact of these changes remains to be determined. Table 55-2 lists the major benefits and risks of OCPs.

Table 55-2 Benefits and Risks of Oral Contraceptives

Oral Contraceptives Decrease the Risk of

Ovarian cancer

Endometrial cancer

Ovarian retention cysts

Ectopic pregnancy

Pelvic inflammatory disease

Benign breast disease

Oral Contraceptives Increase the Risk of

Benign liver tumors

Cholelithiasis (gallstones)

Hypertension

Heart attack

Stroke

Deep vein thrombosis

Pulmonary embolus

The principal functions of estrogens are stimulation of cellular proliferation and growth of sex organs and other tissues related to reproduction

Most estrogens in blood plasma are bound to carrier proteins, as are testosterone and other steroid hormones. In the case of estradiol, 60% is bound to albumin and 38% to sex hormone–binding globulin (SHBG)—also known as testosterone-binding globulin (TeBG; see Chapter 54). The latter name is doubly a misnomer; not only does TeBG bind estradiol, but also TeBG levels are twice as high in women as they are in men. At least one reason for the higher levels in women is that estrogens (including birth control pills) stimulate the synthesis of SHBG. Only 2% of total plasma estradiol circulates as the free hormone. Because of their lipid solubility, estrogens readily cross cell membranes. Although it was once believed that estrogens bound to cytoplasmic receptors, more recently it became clear that the receptor for estradiol resides in the cell nucleus (see Chapter 3). The estrogen receptor (ER) functions as a homodimer (see Table 4-2). The estrogen–estrogen receptor complex interacts with steroid response elements on chromatin and rapidly induces the transcription of specific genes to produce mRNA. The RNA enters the cytoplasm and increases protein synthesis, which modulates numerous cellular functions. Over the next several hours, DNA synthesis increases, and the mitogenic action of estrogens becomes apparent. Estrogens almost exclusively affect particular target sex organs that have the estrogen receptor. These organs include the uterus and the breasts.

The progestins, particularly progesterone, stimulate glandular secretion in reproductive tissue and promote the maturation of certain estrogen-stimulated tissue. One of the most prominent actions of progesterone, which binds to the dimeric progesterone receptor (PR; see Table 4-2), is the induction of secretory changes in the endometrium. The endometrium must be conditioned by estrogen for progesterone to act effectively. During the latter half of the menstrual cycle, progesterone induces final maturation of the uterine endometrium for reception and implantation of the fertilized ovum.

THE OVARIAN CYCLE: FOLLICULOGENESIS, OVULATION, AND FORMATION OF THE CORPUS LUTEUM

Follicles mature in stages from primordial to graafian (or preovulatory) follicles

Oocyte maturation—the production of a haploid female gamete capable of fertilization by a sperm—begins in the fetal ovary. The primordial germ cells migrate from the hind gut to the gonadal ridge. These primordial germ cells develop into oogonia, or immature germ cells, which, in turn, proliferate in the fetal ovary by mitotic division (see Chapter 53). By 6 to 7 weeks of intrauterine life, ~10,000 oogonia are present. This figure is the result of migration and rapid mitotic division; up until this time, no atresia occurs. By ~8 weeks’ gestation, ~600,000 oogonia are present, and they may enter prophase of the first meiosis and become primary oocytes. From this point onward, the number of germ cells is determined by three ongoing processes: mitosis, meiosis, and atresia. By middle fetal life, all the mitotic divisions of the female germ cells have been completed, and the number of germ cells peaks at 6 to 7 million around 20 weeks’ gestation. At this point, oogonia enter their first meiotic division. During prophase of the first meiosis, when the primary oocytes have a duplicated set of 23 chromosomes (4N DNA)—22 duplicated pairs of autosomal chromosomes and 1 pair of duplicated X chromosomes)—crossing over occurs (see Fig. 53-1C). Meiosis arrests in prophase I. This prolonged state of meiotic arrest is known as the dictyotene stage. During the remainder of fetal life and childhood, the number of primary oocytes gradually declines to ~2 to 2.5 million at birth and to ~400,000 just before puberty. As we shall see, they remain primary oocytes—arrested in prophase I of meiosis—until just before ovulation, many years later, when meiosis is completed and the first polar body is extruded. Oocyte maturation is complete when the resulting haploid oocyte is capable of fertilization by a sperm.

The few primary oocytes that survive are those that are surrounded by flat, spindle-shaped follicular or pregranulosa cells (Fig. 55-12). This oocyte-pregranulosa cell complex is enclosed by a basement lamina. At this stage of development, the primary oocyte with its surrounding single layer of pregranulosa cells is called a primordial follicle. Primordial follicles are 30 to 60 μm in diameter. The first primordial follicle usually appears ~6 weeks into intrauterine life, and the generation of primordial follicles is complete by ~6 months after birth.

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Figure 55-12 The maturation of the ovarian follicle.

The ovarian follicle is the primary functional unit of the ovary. Throughout reproductive life, some 90% to 95% of all follicles are the primordial (i.e., “nongrowing”) follicles discussed earlier. The growing follicles that are recruited from this pool of primordial follicles undergo a striking series of changes in size, morphology, and physiology. This follicular development, as well as the subsequent ovulation, is central to control of the menstrual cycle.

The first step in follicular growth is that a primordial follicle becomes a primary follicle. The primary follicle (Fig. 55-12) forms as the spindle cells of the primordial follicle become cuboidal cells. In addition, the oocyte enlarges. Thus, the primary follicle contains a larger primary oocyte that is surrounded by a single layer of cuboidal granulosa cells.

The secondary follicle (Fig. 55-12) contains a primary oocyte surrounded by several layers of cuboidal granulosa cells. The granulosa cells of a primary follicle proliferate and give rise to several layers of cells. In addition, stromal cells differentiate, surround the follicle, and become the theca cells. These theca cells are on the outside of the follicle’s basement membrane. The oocyte increases in size to a diameter of ~120 μm. As the developing follicle increases in size, the number of granulosa cells increases to ~600, and the theca cells show increasing differentiation. The progression to secondary follicles also entails the formation of capillaries and an increase in the vascular supply to developing follicular units.

As the increasingly abundant granulosa cells secrete fluid into the center of the follicle, they create a fluid-filled space called the antrum. At this stage, the follicle is now a tertiary follicle (Fig. 55-12), the first of the two antral stages. In contrast to this tertiary follicle, the primordial, primary, and secondary follicles are solid masses of cells that lack an antrum; they are therefore referred to as preantral follicles. In tertiary follicles, gap junctions are located among both theca cells and granulosa cells. In addition, tight junctions and desmosomes exist between adjacent cells. Gap junctions may also exist between the oocyte and the granulosa cells closest to the oocyte and may function as thoroughfares to transport nutrients and information from the granulosa cells to the oocyte and vice versa. The granulosa cells closest to the oocyte also secrete the mucopolysaccharides that form the zona pellucida immediately surrounding the oocyte.

As the antrum enlarges, it nearly encircles the oocyte, except for a small mound or cumulus that attaches the oocyte to the rest of the follicle, whose diameter increases to 20 to 33 mm. This preovulatory or graafian follicle (Fig. 55-12) is the second of the two antral stages.

The granulosa cells of the tertiary and graafian follicles are of three types: (1) mural granulosa cells, which are the farthest from the center of the follicle, are the most metabolically active, and contain large quantities of LH receptors and enzymes that are necessary for the synthesis of steroids; (2) cumulus granulosa cells are shed with the oocyte at the time of ovulation; and (3) antral granulosa cells, which face the antrum, are left behind within the follicle to become the large luteal cells of the corpus luteum. The capacity of the three types of granulosa cells to generate steroids differs. Cumulus cells contain neither the side-chain–cleavage enzyme (P-450scc) nor aromatase (P-450arom) and therefore cannot generate estrogens. Moreover, cumulus cells respond less to LH and have a low overall LH receptor content. The exact role of the cumulus layer has not been definitively established, although investigators have postulated that the cumulus layer may function as a feeder layer and may provide stem cells that differentiate into other follicular cell types.

Both FSH and LH stimulate follicular growth

Even in fetal life and childhood, some primordial follicles can develop all the way to the antral stage. However, these follicles all undergo atresia (death of the ovum, followed by collapse of the follicle and scarring) at some stage in their development. At the time of puberty, the increase in levels of gonadotropins and ovarian steroids produces a marked increase in the rate of follicular development. During the luteal phase of each cycle, a cohort of primordial follicles is recruited for further development into graafian follicles, a process that occurs during the follicular phase of the next cycle. Thus, primordial follicles may remain in a nongrowing state for 50 years before they develop into primary follicles. All along the course of this development, follicular units undergo atresia until only one dominant graafian follicle remains at the time of ovulation. Some controversy exists about the length of this developmental process. Some investigators believe that the entire developmental process takes three to four monthly cycles. However, the predominant view is that a cohort of primary follicles is recruited during the end of one cycle, and one of these follicles develops into the dominant graafian follicle.

Primary Follicles Appropriate structural and functional development of the follicle necessitates that the follicle is exposed to the appropriate sequence of three hormones: FSH, estradiol, and LH. FSH secretion occurs during early fetal life, and FSH can be detected as early as 5 months into gestation (Fig. 55-2). Three pieces of evidence suggest that gonadotropins and estrogens are essential for early follicular growth, that is, for the progression from primordialto primary follicles. First, hypophysectomy (i.e., removal of the pituitary) causes depletion of primordial follicular units in primates. Thus, with the absence of gonadotropins—as well as other pituitary hormones—and a reduction in estrogens, no follicular development takes place in females. Second, patients with resistant ovary syndrome have a normal complement of primordial follicles, but the follicles do not develop beyond the primordial stage. Although these patients have high circulating levels of FSH, they have no receptors for FSH and LH in their ovaries. Thus, FSH or LH must be able to act on the ovary for follicular development to occur. Third, in individuals with 17α-hydroxylase deficiency—and thus low estrogen levels—follicular development does not progress beyond the primordial stage. Circulating levels of LH in these individuals are normal or high. These three lines of evidence suggest that removal of the gonadotropins, the gonadotropin receptors, or the estrogens halts early follicular growth.

Secondary Follicles As primary follicles form secondary follicles, the theca cells proliferate and acquire LH receptors, as well as the ability to synthesize steroids. Moreover, the granulosa cells acquire receptors for FSH, androgens, and estrogens. When the granulosa cells acquire FSH receptors, the follicular unit becomes a functional steroid-producing entity.

Tertiary Follicles Formation of the antrum, which leads to the development of a tertiary follicle, also requires gonadotropins. FSH, acting in concert with estrogens, causes the proliferation of granulosa cells after development of the antrum, and as a result, the total number of receptors for FSH is increased. FSH, along with estradiol, also induces the proliferation of LH receptors in granulosa cells.

Graafian Follicles When human granulosa cells from graafian follicles are studied in vitro, the mitotic and steroid-synthesizing characteristics of these cells reflect the hormonal condition of the follicle from which they came. Both FSH and estradiol are needed for mitosis to occur in granulosa cells. FSH, LH, and estradiol are necessary for maximum progesterone production by granulosa cells. Premature exposure of developing follicles to LH inhibits mitosis, as well as steroidogenesis. Therefore, the follicle must be exposed to the appropriate sequence of hormones (e.g., FSH, followed by estradiol and then LH) for appropriate maturational and functional development.

Each month, a cohort of follicles is recruited, one of which achieves dominance

The consensus is that the monthly cycle of folliculogenesis actually begins from the primary follicle stage 2 to 3 days before onset of the menses of the previous cycle. At this time, FSH levels begin to increase (Fig. 55-9) because of decreasing inhibin concentrations, thus inducing folliculogenesis, which is completed in the next cycle.

Although we do not understand why some primordial follicles—and not others—join a developing cohort of follicles, FSH is thought to be at least partly responsible for continued development of a cohort of follicles each cycle. The number of follicles in a cohort depends on the residual pool of remaining follicles in the ovary. As the cycle continues, only some of the cohort of follicles continue to develop in response to gonadotropin secretion. The other members of the cohort of follicles undergo atresia.

The one follicle destined to ovulate is recruited during the early days of the current menstrual cycle and eventually achieves dominance. Although the mechanism of selection of the dominant follicle is not completely understood, it is thought to be caused by estrogen-induced events within the follicles. As estrogen levels rise during the follicular phase of the cycle, the pituitary gradually lowers its secretion of FSH (Fig. 55-9). Rising inhibin levels also feed back on the anterior pituitary to decrease FSH secretion. Peak inhibin levels correlate with the number of follicles present and rise in parallel with circulating estradiol levels.

Decreased levels of FSH cause a decline in FSH-dependent aromatase activity in granulosa cells. As a result, estrogen production decreases in the less mature follicles. Conversely, estrogen increases the effectiveness of FSH in the more mature follicles by increasing the number of FSH receptors. Although the dominant follicle continues to be dependent on FSH, it has more FSH receptors, a greater rate of granulosa cell proliferation, more FSH-dependent aromatase activity, and more estrogen production than the less dominant follicles. Because the less dominant follicles have less aromatase activity, the androstenedione in the theca cells cannot be converted as readily to estrogens in the granulosa cell. Instead, the androstenedione either builds up or is converted to other androgens. The less dominant follicles consequently undergo atresia under the influence of androgens in their local environment. In contrast, the production of estrogens and inhibins allows the dominant follicle to become prominent and to gain an even greater edge over its competitors. The vascular supply to the thecal layer of the dominant follicle increases rapidly, so that during the late follicular phase, the vasculature of the dominant follicle is several-fold greater than that of other follicles in the cohort. Increased vascularity may allow greater FSH delivery to the dominant follicle and may thus help to maintain dominance of the follicle selected for ovulation.

Estradiol secretion by the dominant follicle triggers the LH surge, which, in turn, signals ovulation

Ovulation occurs at the midpoint of every normal menstrual cycle and is triggered by the LH surge, which, in turn, is stimulated by rapidly rising levels of estradiol. Estradiol secretion by the dominant follicle increases rapidly near the end of the late follicular phase (Fig. 55-9). This dramatic rise in circulating estradiol exerts positive feedback on the anterior pituitary and sensitizes it to GnRH. The net effect of a rising estradiol level is induction of the LH surge. The LH surge is generally initiated 24 to 36 hours after peak estradiol secretion is achieved, and ovulation usually occurs ~36 hours after onset of the LH surge and ~12 hours after its peak. Thus, it appears that the developing follicle, through its increased estradiol secretion, signals the hypothalamic-pituitary system that follicular maturation is complete and that the hypothalamic-pituitary axis can now release a bolus of gonadotropin to induce ovulation. The LH surge appears to terminate in part as a result of rising levels of progesterone, through negative feedback, and in part as a result of loss of the positive feedback that is derived from estradiol. Depletion of gonadotropin stores in the anterior pituitary gland may also contribute to termination of the LH surge.

At the time of the LH surge, the primary oocyte (4N DNA), which had been arrested in the prophase of its first meiotic division since fetal life (see Chapter 53), now resumes meiosis and completes its first meiotic division several hours before ovulation. The result of this first meiotic division is a small first polar body, which degenerates or divides to form nonfunctional cells, and a much larger secondary oocyte. Both the first polar body and the secondary oocyte, like secondary spermatocytes (see Chapter 54), have a haploid number of duplicated chromosomes (2N DNA): 22 duplicated somatic chromosomes and 1 duplicated X chromosome. This secondary oocyte begins its second meiotic division, but it becomes arrested in metaphase until the time of fertilization (see Chapter 56). The secondary oocyte is surrounded by the zona pellucida and one or more layers of follicular cells, the corona radiata. Before ovulation, the cumulus oophorus expands under the influence of LH, and eventually a complex consisting of the cumulus, the oocyte, and its surrounding cells breaks free with its “stalk” and floats inside the antrum, surrounded by follicular fluid. Breaking away of the oocyte-cumulus complex is probably facilitated by increased hyaluronidase synthesis that is stimulated by FSH.

Release of the oocyte from the follicle—ovulation—follows thinning and weakening of the follicular wall, probably under the influence of both LH and progesterone. Both LH and progesterone enhance the activity of proteolytic enzymes (e.g., collagenase) within the follicle and thus lead to the digestion of connective tissue in the follicular wall. Prostaglandins, particularly those in the E and F series, may also contribute to ovulation, perhaps by triggering the release of lysosomal enzymes that digest the follicular wall. Ultimately, a stigma—or spot—forms on the surface of the dominant follicle. As this stigma balloons out and forms a vesicle, it ruptures, and the oocyte is expelled. Ovulation is apparently facilitated by increased intrafollicular pressure and contraction of smooth muscle in the theca as a result of prostaglandin stimulation.

The expelled oocyte, with its investment of follicular cells, is picked up by the fimbriae of the fallopian tube (Fig. 55-1) as they move over the surface of the ovary. The oocyte is then transported through the infundibulum into the ampulla by means of ciliary movement of the tubal epithelium, as well as by muscular contractions of the tube. Fertilization, if it occurs, takes place in the ampullary portion of the fallopian tube. The resulting zygote subsequently resides there for ~72 hours, followed by rapid transport through the isthmus to the uterine cavity, where it floats free for an additional 2 to 3 days before attaching to the endometrium.

After ovulation, the theca and granulosa cells of the follicle differentiate into the theca-lutein and granulosa-lutein cells of the highly vascularized corpus luteum

After expulsion of the oocyte, the granulosa and theca cells are thrown up into folds that occupy the follicular cavity and form the corpus luteum, a temporary endocrine organ whose major product is progesterone. The corpus luteum is highly vascularized, and surrounding blood vessels penetrate the theca and granulosa layers. Blood accumulates in the resealed antral cavity of the corpus luteum soon after ovulation. Formation of the corpus luteum occurs as a result of transformation of the granulosa and theca cells under the influence of LH (Fig. 55-12). The theca cells at the periphery of the follicle differentiate into stroma and give rise to thecalutein cells—also known as small luteal cells. The granulosa cells, in contrast, enlarge and give rise to granulosa-lutein cells—also known as large luteal cells. Therefore, the mature corpus luteum is composed of two cell types: theca-lutein and granulosa-lutein cells.

During the luteal phase of the menstrual cycle, estrogens and progestins inhibit folliculogenesis. Luteal function begins to decrease ~11 days after ovulation. The mechanisms responsible for luteal regression—or luteolysis—remain open to speculation. One school of thought postulates that withdrawal of trophic support results in demise of the corpus luteum, whereas the second school maintains that local factors induce luteal regression. For example, prostaglandin F2α inhibits luteal function and terminates the life of the corpus luteum.

Growth and involution of the corpus luteum produce the rise and fall in progestins and estrogens during the luteal phase of the menstrual cycle

Although the corpus luteum produces both estrogen and progesterone, the luteal phase is primarily dominated by progesterone secretion. Estrogen production by the corpus luteum is largely a function of the theca-lutein or the small cells, which also produce androgens. Progestin production in the corpus luteum is primarily a function of the granulosa-lutein or large cells (Fig. 55-11), which also produce estrogens.

As shown earlier in Figure 55-9progesterone production rises before follicular rupture. After ovulation, progesterone levels rise sharply and peak in ~7 days. Progesterone acts locally to inhibit follicular growth during the luteal phase. In addition, progesterone may act centrally by inhibiting gonadotropin secretion. Progestins are also antiestrogens. As a result, progestins acting locally may downregulate ERs and may reduce the effectiveness of estradiol. Therefore, increasing progesterone production may have adverse effects on folliculogenesis.

Estradiol levels also rise during the luteal phase (Fig. 55-9) and reflect production by the corpus luteum. The estradiol produced during the luteal phase is necessary for the occurrence of progesterone-induced changes in the endometrium.

Unless rescued by hCG (see Chapter 56)—produced by the syncytial trophoblasts of the blastocyst—luteal production of progesterone ceases toward the end of the menstrual cycle. hCG produced by the developing conceptus maintains steroidogenic function of the corpus luteum until approximately the ninth week of gestation, at which time placental function is well established. If not rescued by pregnancy, the hormone-producing cells of the corpus luteum degenerate and leave behind a fibrotic corpus albicans.

THE ENDOMETRIAL CYCLE

In the human female fetus, the uterine mucosa is capable of responding to steroid hormones by 20 weeks’ gestation. Indeed, some of the uterine glands begin secreting material by the 22nd week of gestation. Endometrial development in utero apparently occurs in response to estrogens derived from the maternal placenta. At ~32 weeks’ gestation, glycogen deposition and stromal edema are present in the endometrium. As estrogenic stimulation is withdrawn after delivery, the endometrium regresses, and at ~4 weeks after birth, the glands are atrophic and lack vascularization. The endometrium remains in this state until puberty.

The ovarian hormones drive the morphological and functional changes of the endometrium during the monthly cycle

The ovarian steroids—estrogens and progestins—control the cyclic monthly growth and breakdown of the endometrium. The three major phases in the endometrial cycle are the menstrual, proliferative, and secretory phases.

The Menstrual Phase If the oocyte was not fertilized and pregnancy did not occur in the previous cycle, a sudden diminution in estrogen and progesterone secretion will signal the demise of the corpus luteum. As hormonal support of the endometrium is withdrawn, the vascular and glandular integrity of the endometrium degenerates, the tissue breaks down, and menstrual bleeding ensues; this moment is defined as day 1 of the menstrual cycle (Fig. 55-13). After menstruation, all that remains on the inner surface of most of the uterus is a thin layer of nonepithelial stromal cells and some remnant glands. However, epithelial cells remain in the lower uterine segments, as well as regions close to the fallopian tubes.

image

Figure 55-13 The endometrial cycle. The ovarian cycle includes the follicular phase—in which the follicle develops—and the luteal phase—in which the remaining follicular cells develop into the corpus luteum. The endometrial cycle has three parts: the menstrual, the proliferative, and the secretory phases.

The Proliferative Phase After menstruation, the endometrium is restored by about the fifth day of the cycle (Fig. 55-13) as a result of proliferation of the basal stromal cells on the denuded surface of the uterus (the zona basalis), as well as the proliferation of epithelial cells from other parts of the uterus. The stroma gives rise to the connective tissue components of the endometrium. Increased mitotic activity of the stromal and glandular epithelium continues throughout the follicular phase of the cycle and beyond, until ~3 days after ovulation. Cellular hyperplasia and increased extracellular matrix result in thickening of the endometrium during the late proliferative phase. The thickness of the endometrium increases from ~0.5 to as much as 5 mm during the proliferative phase.

Proliferation and differentiation of the endometrium are stimulated by estrogen that is secreted by the developing follicles. Levels of estrogen rise early in the follicular phase and peak just before ovulation (Fig. 55-9). ER levels in the endometrium also increase during the follicular phase of the menstrual cycle. Levels of endometrial ER are highest during the proliferative phase and decline after ovulation in response to changing levels of progesterone. As already discussed, estradiol binds to a nuclear receptor on the endometrial cell, and the activated receptor interacts with the hormone response elements of specific genes and modulates their transcription rates.

Estrogen is believed to act on the endometrium in part through its effect on the expression of proto-oncogenes (see Chapter 3) that are involved in the expression of certain genes. Part of the effect of estrogens is to induce the synthesis of growth factors such as the insulin-like growth factors (IGFs, also called somatomedins; see Chapter 48), TGFs, and epidermal growth factor (EGF). These autocrine and paracrine mediators are necessary for maturation and growth of the endometrium. Estrogen causes the stromal components of the endometrium to become highly developed. Estrogen also induces the synthesis of progestin receptors in endometrial tissue. Levels of progestin receptors peak at ovulation, when estrogen levels are highest, to prepare the cells for the high progestin levels of the luteal phase of the cycle.

Progesterone, in contrast, opposes the action of estrogen on the epithelial cells of the endometrium and functions as an antiestrogen. Although progesterone inhibits epithelial cell proliferation, it promotes proliferation of the endometrial stroma. Progesterone exerts its primary antiestrogen effects by stimulating 17β-HSD and sulfotransferase, enzymes that convert estradiol to weaker compounds. Thus, 17β-HSD may convert the estradiol captured from the extracellular environment to estrone (Fig. 55-10), and sulfotransferase may conjugate estrogens to sulfate and produce derivatives such as estradiol 3-sulfate. Estrone is far less active than estradiol, and sulfated estrogens are by themselves biologically inactive.

The Secretory Phase During the early luteal phase of the ovarian cycle, progesterone further stimulates the 17β-HSD and sulfation reactions. These antiestrogenic effects halt the proliferative phase of the endometrial cycle. Progesterone also stimulates the glandular components of the endometrium and thus induces secretory changes in the endometrium. The epithelial cells exhibit a marked increase in secretory activity, as indicated by increased amounts of endoplasmic reticulum and mitochondria. These increases in synthetic activity occur in anticipation of the arrival and implantation of the blastocyst. The early secretory phase of the menstrual cycle (Fig. 55-13) is characterized by the development of a network of interdigitating tubes within the nucleolus of the endometrial epithelial cells. These tubes are referred to as the nucleolar channel system, and progesterone apparently stimulates their development. The cytoplasm invaginates near the nucleolar channel system, although it remains unclear whether actual connections exist between the channel system and the cytoplasm. The nucleolar channel system may provide a route of transport for mRNA to the cytoplasm.

During the middle to late secretory phase, evidence of increased secretory capacity of the endometrial glands becomes more apparent. Vascularization of the endometrium increases, the glycogen content increases, and the thickness of the endometrium increases to 5 to 6 mm. The endometrial glands become engorged with secretions. They are no longer straight; instead, they become tortuous and achieve maximal secretory activity at approximately day 20 or 21 of the menstrual cycle.

The changes in the endometrium are not limited to the glands; they also occur in the stromal cells between the glands. Beginning 9 to 10 days after ovulation, stromal cells that surround the spiral arteries of the uterus enlarge and develop eosinophilic cytoplasm, with a prominent Golgi complex and endoplasmic reticulum. This process is referred to as predecidualization. The rounded decidual cells differentiate from spindle-shaped fibroblast-like stromal cells under the influence of progesterone. As the stromal cells differentiate into decidual cells, their biochemical activity changes, and they form secretory products typical of decidual cells. Laminin, fibronectin, heparin sulfate, and type IV collagen surround matrices of decidualizing cells. Multiple foci of these decidual cells spread throughout the upper layer of the endometrium and form a dense layer called the zona compacta (Fig. 55-13). This spreading is so extensive that the glandular structures of the zona compacta become inconspicuous. Inflammatory cells accumulate around glands and blood vessels. Edema of the midzone of the endometrium distinguishes the compact area from the underlying zona spongiosa, where the endometrial glands become more prominent.

Together, the superficial zona compacta and the midlevel zona spongiosa make up the so-called functional layer of the endometrium. This functional layer is the region that proliferates early in the monthly endometrial cycle, that later interacts with the embryo during pregnancy, that is shed after pregnancy, and that is also shed each month during menstruation. The deepest layer of the endometrium—the zona basalis—is the layer left behind after parturition or menstruation. The cells of the zona basalis give rise to the proliferation at the beginning of the next endometrial cycle.

During the late luteal phase of the menstrual cycle, just before the next menstruation, levels of both estrogens and progestins diminish, and these decreased ovarian steroid levels lead to eventual demise of the upper two thirds of the endometrium. During this period, the spiral arteries rhythmically go into spasm and then relax. This period of the cycle is sometimes referred to as the ischemic phase. As cells begin to die, hydrolases are released from lysosomes and cause further breakdown of the endometrium. Prostaglandin production increases as a result of the action of phospholipases liberated from lysosomes. Necrosis of vascular cells leads to microhemorrhage. The average loss of blood, tissues, and serous fluid amounts to ~30 mL. Menstrual blood does not clot because of the presence of fibrolysins released from necrotic endometrial tissue.

The effective implantation window is 3 to 4 days

From studies of embryo transfer to recipient mothers in oocyte donation programs (see the box in Chapter 56 on in vitro fertilization), when both the age of the donated embryo and the time of the endometrial cycle of the recipient are known, the period of endometrial receptivity for implantation of the embryo is estimated to extend from as early as day 16 to as late as day 19 of the menstrual cycle. Of course, because implantation must normally follow the ovulation that occurs on day 14 and because fertilization normally occurs within 1 day of ovulation, the effective window is less than 4 days, from day 16 to day 19. In contrast, when embryos are transferred on cycle days 20 through 24, no pregnancies are achieved. (See Note: Implantation Window)

Although the mechanisms underlying endometrial receptivity remain unclear, several changes in the endometrium are believed to be associated with increased receptivity of the endometrium for the embryo. The formation of microvilli and pinopods (i.e., protrusions of endometrial cells near gland openings) during the midluteal phase and the secretion of extracellular matrix composed of such materials as glycoproteins, laminin, and fibronectin may provide a surface that facilitates attachment of the embryo (see Chapter 56).

THE FEMALE SEX ACT

Female sexual desire—libido—is a complex phenomenon that consists of physical and psychological effects, all modulated by circulating sex steroids. Libido varies during the ovarian cycle, and the frequency of female sexual activity increases around the time of ovulation. There may also be an increase in the rate of initiation of sexual activity by women around the time of ovulation. These changes may, in part, reflect the increased secretion of androgenic steroids that occurs just before and during ovulation secondary to the LH surge.

The female sex response, which is elicited by physical, psychic, and hormonal stimuli, occurs in four distinct phases

Although sexual function has a strong physiological basis, it is not possible to separate sexual response from the other emotional and contributing factors involved in sexual relationships. Masters and Johnson published, in their now classic work Human Sexual Response, a discussion of data obtained on the sexual cycles of 700 subjects. Our current understanding of the female sexual response is based on their findings. Masters and Johnson described four stages of the sex response in women: the excitement or seduction phase, plateau, orgasm, and resolution. Following is a brief description of each phase.

Excitement The excitement or arousal phase of the female sex response may be initiated by a multitude of internal or external stimuli, including psychological factors, such as thoughts and emotions, and physical factors, such as sight and tactile stimuli. Table 55-3 summarizes the responses of the excitement phase, many of which reflect activity of the parasympathetic division of the autonomic nervous system (ANS). Sexual intensity rises in crescendo fashion.

Table 55-3 Female Sexual Response Cycle

Excitement

Warmth and erotic feelings

Increased sexual tension

Deep breathing

Increased heart rate

Increased blood pressure

Generalized vasocongestion

Skin flush

Breast engorgement

Nipple erection (myotonic effect)

Engorgement of labia and clitoris

Vaginal “sweating” (transudative lubrication)

Secretions from Bartholin glands

Uterine tenting into pelvis

Plateau

Marked vasocongestion

“Sex flush” (maculopapular rash on breasts, chest, and epigastrium)

Nipple erection

Engorgement of the labia

Engorgement of lower third of the vagina, with narrowing of diameter

Dilation of upper two thirds of vagina

Clitoral swelling and erection

Vaginal “sweating”

Uterine tenting

Orgasm

Release of tension

Generalized, rhythmic myotonic contractions

Contractions of perivaginal muscles and anal sphincter

Uterine contractions

Resolution

Return to pre-excitement state

Personal satisfaction and well-being

New excitement cycles may be initiated

Plateau The plateau stage is the culmination of the excitement phase as it reaches its peak. It is associated with a marked degree of vasocongestion throughout the body.

Orgasm During orgasm, the sexual tension that has built up in the entire body is released. The climax, or orgasm, is intense and includes a myotonic response throughout the body. Muscle contractions start 2 to 4 seconds after the woman begins to experience orgasm, and they repeat at 0.8-second intervals. The actual number of contractions, as well as their intensity, varies from woman to woman. Some women observed to have orgasmic contractions are not aware that they are having an orgasm. Masters and Johnson suggested that prolonged stimulation during the excitement phase leads to more pronounced orgasmic activity. Whereas the excitement phase is under the influence of the parasympathetic division of the ANS, as is the erection phase in men, orgasm seems to be related to the sympathetic division, as is the emission phase in men (see Chapter 54).

Resolution The last phase of the female sex response is a return of the woman’s physiological state to the pre-excitement level. During the resolution phase, the woman generally experiences a feeling of personal satisfaction, well-being, and relaxation of sexual desire. A new sexual excitement cycle may be initiated at any time after orgasm without the refractory phase that occurs in men.

Both the sympathetic and the parasympathetic divisions control the female sex response

Much of the response in the excitement phase results from stimulation of the parasympathetic fibers of the ANS. In some cases, anticholinergic drugs may interfere with a full response in this stage. Dilatation of blood vessels in the erectile tissues causes engorgement with blood and erection of the clitoris, as well as distention of the peri-introital tissues and subsequent narrowing of the lower third of the vagina. Parasympathetic fibers emanating from the sacral plexus (see Chapter 14) innervate these erectile tissues, just as in men (see Chapter 54). In addition, the parasympathetic system innervates Bartholin’s glands, which empty into the introitus, as well as the vaginal glands. Adequate lubrication is necessary to minimize the friction of intercourse and thus maximize the stimulation to achieve orgasm.

Both physical stimulation and psychic stimuli are important for female orgasm. Psychic stimuli are coordinated through the cerebrum, which causes the generalized tension throughout the body, as discussed earlier, and also modulates the autonomic response. The female orgasm is also coordinated through a spinal cord reflex that results in rhythmic contractions of the perineal muscles. The afferent pathways for this spinal cord reflex follow the pudendal nerves, which emanate through sacral segments 2 to 4 and are the primary innervation to the perineum and the female external genitalia. This spinal cord reflex is similar to that observed in men.

The female sex response facilitates sperm transport through the female reproductive tract

The spinal reflexes previously discussed may also increase uterine and cervical activity and may thus promote transport of gametes. The cervix dilates during orgasm, thereby facilitating sperm transport into the upper part of the reproductive tract. The release of oxytocin at the time of orgasm stimulates uterine contractility, which also facilitates gamete transport. Although 150 to 600 million sperm cells (see Chapter 54) are normally deposited in the vagina during sexual intercourse, ~100,000 reach the cavity of the uterus, and only 50 to 100 viable sperm reach the distal fallopian tube where fertilization occurs. Aside from the one or more sperm that will fertilize the ovum (or ova), most sperm degenerate, to be disposed of by the female genital tract. Sperm transport is accomplished by swimming movements of the sperm tail through the mucus of the cervical canal. The sperm reach the ampulla of the fallopian tubes within 5 minutes of ejaculation. Clearly, this rapid rate of transport could not be achieved by the swimming activity of the sperm alone. Therefore, uterine or tubal activity must serve a major role in sperm transport.

MENOPAUSE

Menopause, or the climacteric, signals the termination of reproductive function in women. Cyclic reproductive function ceases, menstruation comes to an end, and childbearing is generally no longer possible. Also occurring are significant physiological changes (Table 55-4) that have a major impact on health.

Table 55-4 The Menopausal Syndrome and Physical Changes in Menopause

Menopausal Syndrome

Physical Changes in Menopause

Vasomotor instability

Atrophy of the vaginal epithelium

Hot flashes

Changes in vaginal pH

Night sweats

Decrease in vaginal secretions

Mood changes

Decrease in circulation to vagina and uterus

Short-term memory loss

Pelvic relaxation

Sleep disturbances

Loss of vaginal tone

Headaches

 

Loss of libido

Cardiovascular disease

 

Osteoporosis

 

Alzheimer disease

Only a few functioning follicles remain in the ovaries of a menopausal woman

Progressive loss of ovarian follicular units occurs throughout life. Approximately 6 to 7 million germ cells are present in the two ovaries of the developing female fetus at 20 weeks’ gestation. At birth, only ~1 to 2 million follicular units remain in the ovaries. At puberty ~400,000 remain, a finding again reflecting the continued process of atresia during the prepubertal years. Puberty generally occurs in American girls at ~12.5 years of age. The average age of menopause in American women is 51.5 years. Thus, it is estimated that more than 400 oocytes are ovulated during the reproductive life of a woman. At menopause and during the ensuing 5 years, the ovary contains only an occasional secondary follicle and a few primary follicles in a prominent stroma. The massive loss of oocytes over the reproductive life of a woman—from 400,000 at puberty to virtually none at menopause—is the result of the rapid, continuous process of atresia during reproductive life. During each cycle, a large cohort (~10 to 30) of follicles is recruited, but only one follicle reaches dominance and ovulates. The others become atretic. However, even if we multiply the number of follicles in a cohort by the total number of menstrual cycles, we cannot account for all 400,000 of the prepubertal units. Thus, most of the primordial and primary ovarian follicles are lost as a result of atresia during the reproductive life of the individual.

During menopause, levels of the ovarian steroids fall, whereas gonadotropin levels rise

The loss of functional ovarian follicles is primarily responsible for menopause in primates. Even before the onset of menopause, significant hormonal changes occur very early during reproductive life. Because of a gradual decline in the number of follicles, the decreased ovarian production of estrogen reduces the negative feedback to the anterior pituitary and leads to increased levels of FSH. Increased levels of FSH are seen as early as 35 years of age, even though cyclic reproductive function continues. When compared with younger women, older—but premenopausal—women have diminished estradiol production and decreased luteal function during natural cycles. Diminished inhibin production by the aging ovary may also contribute to the sharp rise in FSH levels that occurs in the perimenopausal period of life.

Hormone Replacement Therapy During Menopause

Although the mean age at menopause is ~51.5 years, changes in hormone secretion patterns are seen much earlier. Increases in levels of FSH occur as early as 35 years of age. The mechanisms responsible for this change remain to be elucidated. However, it is clear that ovarian function begins to diminish far in advance of a woman’s last menstrual period. The increase in gonadotropin secretion is probably a result of decreased folliculogenesis leading to decreased secretion of sex steroids and inhibin and thus lowered negative feedback on the gonadotrophs during the perimenopausal period.

The characteristic changes associated with menopause are primarily the result of low circulating estrogen levels. Estrogen is a very important regulatory hormone in girls and women. In addition to the role of estrogen in reproductive processes, this hormone has profound effects on several other physiological systems (Table 55-4).

Hormone replacement therapy is indicated during menopause to alleviate the menopausal syndrome and to prevent or diminish the physical changes that occur as a result of estrogen deficiency. Menopausal hormone replacement therapy consists of estrogen and progestin administration. The reason for administering progestins is that the endometrium is at significant risk of neoplasia from the unopposed actions of estrogens. Thus, progestins are not generally administered to women who have had hysterectomies. Estrogen replacement is very effective against the menopausal syndrome, as well as against osteoporosis and cardiovascular disease. However, because of side effects (e.g., menstruation) and concern about endometrial and breast cancer, compliance with hormone replacement therapy is often compromised.

The selective ER modulators (SERMS) comprise a group of structurally dissimilar compounds that interact with ERs. However, these agents act as either estrogen agonists or estrogen antagonists, depending on the target tissue and hormonal status of the individual. The exact mechanisms through which SERMS elicit their effects in specific tissues remain unclear and constitute an area of active research. The estrogen antagonist effects of SERMS may be mediated by classic competition for the ER. SERMS such as tamoxifen and raloxifene have beneficial effects, similar to those of estrogens, on bone and the cardiovascular system, whereas they antagonize estrogen in reproductive tissue. Clearly, the ideal SERM would have all the beneficial effects of estrogen without the negative and potentially dangerous side effects. For example, the perfect SERM would alleviate the menopausal syndrome, protect against cardiovascular and Alzheimer disease, and act as estrogen agonists in certain reproductive tissues and as antagonists in others.

During menopause, estradiol levels are generally less than 30 pg/mL, and progesterone levels are often less than 1 ng/mL of plasma. Both these values are somewhat less than the lowest levels seen during the menstrual cycle of a younger woman (Fig. 55-9). Ovarian production of androstenedione is minimal during menopause, although androstenedione production by the adrenal cortex remains normal.

Because the output of estrogens, progestins, and inhibins from the ovaries falls to very low levels during menopause, negative feedback on the hypothalamic-pituitary-ovarian axis (Fig. 55-4) becomes minimal. As a result, levels of FSH and LH may be higher than those seen during the midcycle surge in premenopausal women—the futile attempt of the axis to stimulate follicular development and production of the female sex steroids. During menopause, the anterior pituitary still secretes FSH and LH in pulses, presumably after cyclic release of GnRH from the hypothalamus (Fig. 55-2A). Although gonadotropins cannot generally stimulate the postmenopausal ovary, it appears that the gonadotrophs in the anterior pituitary can respond to exogenous GnRH.

REFERENCES

Books and Reviews

Adashi EY, Rock JA, Rosenwaks Z (eds): Reproductive Endocrinology, Surgery and Technology. Philadelphia: Lippincott-Raven, 1996.

de Kretser DM, Robertson DM, Risbridger GP: Recent advances in the physiology of human inhibin. J Endocrinol Invest 1990; 13:611-624.

Dufau ML: The luteinizing hormone receptor. Annu Rev Physiol 1998; 60:461-496.

Marshall WA, Tanner JM: Puberty. In Falkner F, Tanner JM (eds): Human Growth, vol 2, New York: Plenum; 1986: 171-209.

Masters WH, Johnson VE: Human Sexual Response. Boston: Little, Brown, 1966.

Wilson JD, Foster DW, Kronenberg HM, Larsen PR (eds): Williams Textbook of Endocrinology, 9th ed. Philadelphia: WB Saunders, 1998.

Woodruff TK, Mather JP: Inhibin, activin and the female reproductive axis. Annu Rev Physiol . 1995; 57:214-244.

Journal Articles

Erickson GF, Wang C, Hsueh AJW: FSH induction of functional LH receptors in granulosa cells cultured in a chemically defined medium. Nature 1979; 279:336-338.

Fiddes JC, Goodman HM: The gene encoding the common alpha subunit of the four human glycoprotein hormones. J Mol Appl Genet 1981; 1:3-18.

Ryan KJ: Granulosa-thecal cell interaction in ovarian steroidogenesis. J Steroid Biochem 1979; 11:799-800.

Schwanzel-Fukuda M, Pfaff DW: Origin of luteinizing hormone-releasing hormone neurons. Nature 1989; 338:161-164.

Veldhuis JD, Dufau ML: Estradiol modulates the pulsatile secretion of bioactive luteinizing hormone in vivo. J Clin Invest 1987; 80:631-638.

Wang CF, Lasley BL, Lein A, Yen SS: The functional changes of the pituitary gonadotrophs during the menstrual cycle. J Clin Endocrinol Metab 1976; 42:718-728.