As discussed on page 1129, the ovum is fertilized in the ampullary portion of the fallopian tube several hours after ovulation (Fig. 56-2), and the conceptus remains in the fallopian tube for ~72 hours, during which time it develops to the morula stage (i.e., a mulberry-shaped solid mass of 12 or more cells), receiving nourishment from fallopian-tube secretions. During these 3 days, smooth-muscle contractions of the isthmus prevent advancement of the conceptus into the uterus while the endometrium is preparing for implantation. The mechanisms by which the ovum is later propelled through the isthmus of the fallopian tube to the uterus probably include beating of the cilia of the tubal epithelium and contraction of the fallopian tube.
FIGURE 56-2 Transport of the conceptus to the uterus.
After the morula rapidly moves through the isthmus to the uterine cavity, it floats freely in the lumen of the uterus and transforms into a blastocyst (see Fig. 56-2). A blastocyst is a ball-like structure with a fluid-filled inner cavity. Surrounding this cavity is a thin layer of trophoectoderm cells that forms the trophoblast, which develops into a variety of supporting structures, including the amnion, the yolk sac, and the fetal portion of the placenta. On one side of the cavity, attached to the trophoblast, is an inner cell mass, which develops into the embryo proper. The conceptus floats freely in the uterine cavity for ~72 hours before it attaches to the endometrium. Thus, implantation of the human blastocyst normally occurs 6 to 7 days following ovulation. Numerous maturational events occur in the conceptus as it travels to the uterus. The embryo must be prepared to draw nutrients from the endometrium on arrival in the uterine cavity, and the endometrium must be prepared to sustain the implantation of the blastocyst. Because of the specific window in time during which implantation can occur, temporal relationships between embryonic and endometrial maturation assume extreme importance.
The presence of an embryo leads to decidualization of the endometrium
During the middle to late secretory phase of the normal endometrial cycle, the endometrium becomes more vascularized and thicker, and the endometrial glands become tortuous and engorged with secretions (see pp. 1125–1126). These changes, driven by progesterone from the corpus luteum, peak at ~7 days after ovulation. Additionally, beginning 9 to 10 days after ovulation, a process known as predecidualization (see pp. 1125–1126) begins near the spiral arteries. During predecidualization, stromal cells transform into rounded decidual cells, and these cells spread across the superficial layer of the endometrium to make it more compact (zona compacta) and separating it from the deeper, more spongy layer (zona spongiosa; see Fig. 55-11). If conception fails to occur, the secretory activity of the endometrial glands decreases, followed by regression of the glands 8 to 9 days after ovulation, which is ultimately followed by menstruation.
When pregnancy occurs, the predecidual changes in the endometrium are sustained and extended, which completes the process of decidualization. The decidua is the specialized endometrium of pregnancy. Its original name was membrana decidua, a term referring to the membranes of the endometrium that are shed following pregnancy, like the leaves of a deciduous tree. Because the degree of decidualization is considerably greater in conception cycles than in nonconception cycles, it is likely that the blastocyst itself promotes decidualization. Indeed, either the presence of the embryo or a traumatic stimulus that mimics the embryo's invasion of the endometrium induces changes in the endometrium. N56-5
Onset of Decidualization
Contributed by Ervin Jones
One of the earliest signs that the blastocyst has transmitted an embryonic signal to the endometrium is a marked increase in the permeability of endometrial capillaries. One can detect this permeability increase by injecting laboratory rats intravenously with a dye, such as Evans blue, which binds to albumin. Accumulation of blue dye in the area of the blastocyst is an index of increased capillary permeability to albumin. Increased endometrial capillary permeability precedes the decidual response and may be triggered by vasoactive substances released by the blastocyst just prior to implantation. Because inhibitors of histidine decarboxylase (which converts histidine to histamine) interrupt implantation, histamine is one candidate for the vasoactive substance.
The area underneath the implanting embryo becomes the decidua basalis (Fig. 56-3). Other portions of the decidua that become prominent later in pregnancy are the decidua capsularis, which overlies the embryo, and the decidua parietalis, which covers the remainder of the uterine surface. The upper zona compacta layer and the middle zona spongiosa layer of the nonpregnant endometrium are still recognizable in the decidualized endometrium of pregnancy. The glandular epithelium within the zona spongiosa continues its secretory activity during the first trimester. Some of the glands take on a hypersecretory appearance in what has been referred to as the Arias-Stella phenomenon of early pregnancy—named after the pathologist Javier Arias-Stella. Although the decidualized endometrium is most prominent during the first trimester, prior to the establishment of the definitive placenta, elements of decidualization persist throughout gestation.
FIGURE 56-3 Three decidual zones during early embryonic development (about 13 to 14 days postfertilization). The figure shows a sagittal section through a pregnant uterus, with the anterior side to the right.
Uterine secretions nourish the preimplantation embryo, promote growth, and prepare it for implantation
Before the embryo implants in the endometrium and establishes an indirect lifeline between the mother's blood and its own, it must receive its nourishment from uterine secretions. Following conception, the endometrium is primarily controlled by progesterone, which initially comes from the corpus luteum (see p. 1124). The uterine glandular epithelium synthesizes and secretes several steroid-dependent proteins (Table 56-1) that are thought to be important for the nourishment, growth, and implantation of the embryo. The endometrium secretes cholesterol, steroids, and various nutrients, including iron and fat-soluble vitamins. It also synthesizes matrix substances, adhesion molecules, and surface receptors for matrix proteins, all of which may be important for implantation.
Endometrial Proteins, Glycoproteins, and Peptides Secreted by the Endometrial Glands during Pregnancy
• Insulin-like growth factor–binding protein 1 (IGFBP-1)
• Placental protein 14 (PP14) or glycodelin
• Pregnancy-associated endometrial α2-globulin (α2-PEG)
• Endometrial protein 15 (EP15)
• Collagen IV
• Heparin sulfate
• Acidic fibroblast growth factor
• Basic fibroblast growth factor
• Pregnancy-associated plasma protein A (PAPP-A)
• Stress response protein 27 (SRP-27)
• Cancer antigen 125 (CA-125)
• β endorphin
• Diamine oxidase
• Plasminogen activator (PA)
• Plasminogen activator inhibitor (PAI)
• Progesterone-dependent carbonic anhydrases
Pinopodes appear as small, finger-like protrusions on endometrial cells between day 19 (about the time the embryo would arrive in the uterus) and day 21 (about the time of implantation) of the menstrual cycle; they persist for only 2 to 3 days. Pinopode formation appears to be progesterone dependent and is inhibited by estrogens. Pinopodes endocytose macromolecules and uterine fluid and absorb most of the fluid in the lumen of the uterus during the early stages of embryo implantation. By removing uterine luminal fluid, the pinopodes may allow the embryo and the uterine epithelium to approximate one another more closely. Because apposition and adhesion of the embryo to the uterus are the first events of implantation, the presence and action of pinopodes may determine the extent of the implantation window.
The blastocyst secretes substances that facilitate implantation
If the blastocyst is to survive, it must avoid rejection by the maternal cellular immune system. It does so by releasing immunosuppressive agents (Table 56-2). The embryo also synthesizes and secretes macromolecules that promote implantation, the development of the placenta, and the maintenance of pregnancy.
Substances Secreted by the Blastocyst
Platelet-activating factor (PAF)
Interleukins 1α, 6, and 8
Leukemia inhibitory factor
Metalloproteases (facilitate invasion of the trophoblast into the endometrium)
Collagenases: digest collagen types I, II, III, VII, and X
Gelatinases: two forms, digest collagen type IV and gelatin
Stromelysins: digest fibronectin, laminin, and collagen types IV, V, and VII
Serine Proteases (facilitate invasion of trophoblast into the endometrium)
Other Factors or Actions
hCG: autocrine growth factor
Early pregnancy factor (EPF)
Embryo-derived histamine-releasing factor
Plasminogen activator and its inhibitors
Insulin-like growth factor 2 (IGF-2): promotes trophoblast invasiveness
Fibroblast growth factor (FGF)
Transforming growth factor-α (TGF-α)
Both short-range and long-range embryonic signals may be necessary for implantation, although the nature of some of these signals remains enigmatic. One short-range signal from the blastocyst may stimulate local changes in the endometrium at the time of its apposition to the endometrium. A long-range signal that is secreted by the early blastocyst is human chorionic gonadotropin (hCG), which is closely related to LH (see p. 1111) and sustains the corpus luteum in the face of rapidly falling levels of maternal LH.
hCG is one of the most important of the factors secreted by the trophoblast of the blastocyst, both before and after implantation. Besides rescuing the corpus luteum, hCG acts as an autocrine growth factor and promotes trophoblast growth and placental development. hCG levels are high in the area where the trophoblast faces the endometrium. hCG may have a role in the adhesion of the trophoblast to the endometrium and also has protease activity.
During implantation, the blastocyst apposes itself to the endometrium, adheres to epithelial cells, and finally invades the stroma
As noted on pages 1132–1133, the conceptus lies unattached in the uterine cavity for ~72 hours. About halfway through this period (i.e., 5 to 6 days after ovulation), the morula transforms into the blastocyst (Fig. 56-4A). Before the initiation of implantation, the zona pellucida that surrounds the blastocyst degenerates. This process, known as hatching of the embryo, occurs 6 to 7 days after ovulation. Lytic factors in the endometrial cavity appear to be essential for the dissolution of the zona pellucida. The blastocyst probably also participates in the process of zona lysis and hatching; when an unfertilized egg is placed in the uterus under the same conditions, its zona pellucida remains intact. A factor produced by the blastocyst may activate a lytic factor that is derived from a uterine precursor. Plasmin, produced from plasminogen, is a plausible candidate for this uterine factor, because plasmin exhibits a lytic effect on the zona pellucida in vitro, and inhibitors of plasmin block in vitro hatching of rat blastocysts. Implantation occurs in three stages: (1) apposition, (2) adhesion, and (3) invasion.
FIGURE 56-4 Embryo hatching, apposition, adhesion, and invasion.
The earliest contact between the blastocyst wall, the trophoectoderm, and the endometrial epithelium is a loose connection called apposition (see Fig. 56-4B). Apposition usually occurs in a crypt in the endometrium. From the standpoint of the blastocyst, it appears that apposition occurs at a site where the zona pellucida is ruptured or lysed and where it is possible for the cell membranes of the trophoblast to make direct contact with the cell membranes of the endometrium. N56-6 Although the preimplantation blastocyst is asymmetric, it seems that the entire trophoectoderm has the potential to interact with the endometrium, and the final correct orientation—with the inner cell mass pointing toward the endometrium—occurs by free rotation of the inner cell mass within the sphere of overlying trophoectoderm cells.
Mechanisms of Apposition
Contributed by Ervin Jones
A mutual reduction in the electrostatic repulsion between the blastocyst and the endometrial membranes occurs at the time of implantation. The loss of zona pellucida proteins during zona lysis and hatching, as well as changes in glycoproteins and their terminal sugars, may decrease the electrostatic repulsive forces between the conceptus and the endometrium. Inhibitors of protein glycosylation may diminish blastocyst attachment. Further evidence that changes in the zona pellucida are necessary to complete apposition is the fact that neither an unfertilized egg nor the preblastocyst-stage conceptus, each of which is still surrounded by a zona pellucida, shows evidence of adhesion. Changes in the integrity of the cytocalyx lining the surface of the uterine epithelium are also important for apposition.
The trophoblast appears to attach to the uterine epithelium via the microvilli of the trophoblast; ligand-receptor interactions are probably involved in adhesion (see Fig. 56-4C). The receptors for these ligand-receptor interactions are often members of the integrin family (see p. 17) and can be either on the blastocyst or on the endometrium. Integrins are bifunctional integral membrane proteins; on their intracellular side, they interact with the cytoskeleton, whereas on their extracellular side, they have receptors for matrix proteins such as collagen, laminin, fibronectin, and vitronectin. Therefore, ligand-receptor interactions have two possible orientations. For the first, the extracellular surface of the trophoblast has integrins for binding fibronectins, laminin, and collagen type IV. Thus, during implantation, the trophoblast binds to the laminin that is distributed around the stromal (decidual) cells of the endometrium. Fibronectin, a component of the basement membrane, probably guides the implanting embryo (see below) and is subsequently broken down by the trophoblast.
For the second orientation of matrix-integrin interactions, the extracellular surface of the glandular epithelium also expresses integrins on days 20 to 24 of the menstrual cycle, the implantation window (see p. 1126). The expression of receptors for fibronectin and vitronectin (i.e., integrins) may serve as markers of the endometrial capacity for implantation. Small peptides containing sequences that are homologous to specific sequences of fibronectin block blastocyst attachment and outgrowth on fibronectin.
In addition to the integrin-matrix interactions, another important class of ligand-receptor interactions appears to be between heparin proteoglycans or heparan sulfate proteoglycans (see p. 39), which are attached to the surface of the blastocyst, and surface receptors on the uterine epithelial cell. These endometrial proteoglycan receptors increase in number as the time of implantation approaches.
Any of the foregoing ligand-receptor interactions can lead to cytoskeletal changes. Thus, adhesion of the trophoblast via ligand-receptor interactions may dislodge the uterine epithelial cells from their basal lamina and thereby facilitate access of the trophoblast to the basal lamina for penetration.
As the blastocyst attaches to the endometrial epithelium, the trophoblastic cells rapidly proliferate, and the trophoblast differentiates into two layers: an inner cytotrophoblast and an outer syncytiotrophoblast (see Fig. 56-4D). The syncytiotrophoblast arises from the fusion of cytotrophoblast cells to form a giant multinucleated cell that constitutes the principal component of the maternal-fetal interface. During implantation, long protrusions from the syncytiotrophoblast extend among the uterine epithelial cells. The protrusions dissociate these endometrial cells by secreting tumor necrosis factor-alpha (TNF-α), which interferes with the expression of cadherins (cell-adhesion molecules; see p. 17) and β catenin (an intracellular protein that helps anchor cadherins to the cytoskeleton). N56-7 The syncytiotrophoblast protrusions then penetrate the basement membrane of the uterine epithelial cells and ultimately reach the uterine stroma.
Uterine E-Cadherin and β-Catenin at the Implantation Site
Contributed by Emile Boulpaep, and Walter Boron
The blastocyst can only implant into a receptive endometrium. Conversely, the endometrium can become receptive only when it receives the appropriate signals from the nearby blastocyst, which result in the upregulation of certain genes such as those encoding E-cadherin and β-catenin.
In the endometrial epithelial cells of the peri-implantation uterus, E-cadherin (see pp. 17 and 44–45)—a transmembrane protein that mediates cell-cell contacts—is expressed at high levels at the apical membrane at the prospective sites of implantation, and much less elsewhere. In addition, β-catenin (see pp. 44–45 and 50)—a cytoskeletal anchor protein that attaches to the cytosolic domain of the cadherin—is also expressed at the apical membrane of these same cells. These changes are part of the adhesive phenotype of the endometrium during the implantation window.
Jha RK, Titus S, Saxena D, et al. Profiling of E-cadherin, beta-catenin and Ca2+ in embryo-uterine interactions at implantation. FEBS Lett. 2006;580:5653–5660.
Mohamed OA, Jonnaert M, Labelle-Dumasi C, et al. Uterine Wnt/β-catenin signaling is required for implantation. Proc Natl Acad Sci U S A. 2005;102:8579–8584.
The trophoblast secretes several autocrine factors, which appear to stimulate invasion of the endometrial epithelium, as well as proteases (see Table 56-2). By degrading the extracellular matrix, metalloproteases and serine proteases may control both the proliferation and the invasion of the trophoblast into the endometrium.
Around the site of penetration of the syncytiotrophoblast, uterine stromal cells take on a polyhedral shape and become laden with lipids and glycogen. These are the decidual cells discussed above. The degeneration of decidual cells near the invading syncytiotrophoblast thus provides nutrients to the developing embryo. The blastocyst superficially implants in the zona compacta of the endometrium and eventually becomes completely embedded in the decidua. As the finger-like projections of the syncytiotrophoblast invade the endometrium, they reach the maternal blood supply and represent a primordial form of the chorionic villus of the mature placenta, as discussed in the next subchapter.
The blastocyst is genetically distinct from maternal tissue. Thus, implantation represents a breach of maternal immune defenses that could lead to immune attack and destruction of the embryo. The problem is confounded by the presence—within the decidual compartment—of uterus-specific large granular lymphocytes also known as decidual natural killer (NK) cells. To avoid attack, cytotrophoblasts that lie at the maternal-fetal interface produce a unique immunogenic molecule, HLA-G, on their cell surface. HLA-G may prevent immunological recognition by activated NK cells. In addition, trophoblast cells express the Fas ligand (see pp. 1241–1242), which induces apoptosis in immune cells that carry the Fas receptor. Thus, the embryo may escape attack by the maternal immune system by camouflaging itself with HLA-G and by inducing apoptosis in maternal lymphocytes that may mount an attack.
EFFECT ON EJACULATION
Upper motor neuron
Lower motor neuron