Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
270 Comparative Anatomy and Histology 60 50 20 40 Progesterone 10 Estradiol 30 5 20 Diestrus Proestrus Estrus Metestrus Diestrus FIGURE 17 Mouse estrus cycle. Morphologic changes in the vaginal mucosa associated with hormonal changes during the mouse estrus cycle. Endometrial morphology also varies with cycle, although not as dramatically as in the human (see Figures 13 and 14). Source: © Elsevier, Inc., www.netterimages.com. clitoral glands are located anterolaterally in the subcutaneous tissue, each with a single duct that opens into the lateral walls of the clitoral fossa. The 6- to 7.5-cm human vagina is also muscular and extends from the cervix to the vulva. However, there are a series of grossly apparent ridges produced by folding of the wall of the outer third of the vagina termed rugae. These rugae allow significant extension and stretching during sexual intercourse and parturition. The vaginal introitus opens into the vestibule of the vulva (Figure 2). The vulva is composed of the mons pubis anteriorly, which separates into two folds of hairbearing skin termed the labia majora. Medial to the labia majora are the labia minora, two folds of tissue covered by squamous mucosa that form the anterolateral borders to the vestibule of the vulva. The vestibule contains the clitoris, the urethral opening, and the vaginal introitus. The clitoris is located in the anterior portion of the vestibule, where the labia minora meet. The urethral opening is immediately posterior to the clitoris and anterior to the vaginal introitus. Posterior to the vagina is the perineum, the area between the vagina and the anus. Microscopic Anatomy The vaginal vault in the mouse is lined by a keratinized stratified squamous epithelium that undergoes measurable changes during the mouse estrus cycle (Figures 17 and 18). The mucosa is folded with no glands. The lamina propria is composed of connective tissue with circular and longitudinal muscularis layers, whereas the outermost layer is formed by adventitia that is continuous with the rectal and urethral adventitia (Figure 19). The human vagina is lined by a nonkeratinized squamous epithelium and does not undergo the significant morphologic changes seen in the mouse during the menstrual cycle (Figure 20). Small changes in the amount of glycogen accumulation and nuclear maturation do occur but are minor compared with the changes seen in the mouse. The outer aspects of the labia majora are lined by hear-bearing skin, whereas the inner portions of the labia majora, the labia minor, and the vestibule are lined by nonkeratinized squamous mucosa. The clitoris of the mouse is covered by skin and hair, whereas that of the human is covered C h a p t e r 1 7 Female Reproductive System FIGURE 18 Mouse vaginal epithelial changes during the estrus cycle. Vaginal mucosa morphology, particularly the numbers of layers and differentiation, changes during the estrus cycle. Histologically, four stages of the estrus cycle are easily determined: proestrus, estrus, metestrus, and diestrus. (A) During proestrus, the mucosa is 10–13 cells thick and the outer layers stain lightly with eosin, whereas the granulosa layer shows increasing cornification. Mitoses are frequent, but few leukocytes are present. (B) In estrus, the mucosa is approximately 12 cells thick. The superficial nucleated layer is lost, and the cornified layer is superficial. Mitoses are decreasing, and leukocytes are absent. (C) In metestrus, the cornified layer is delaminated, and leukocytes begin to appear under the epithelium. (D) During diestrus, the mucosa is 4–7 cells thick . Surface epithelial cells are mucified, and mucus, leukocytes, and desquamated cells are present in the lumen. by squamous mucosa. In mice, paired clitoral glands are sebaceous glands that empty into the clitoral fossa via excretory ducts lined by stratified squamous epithelium (Figure 21). Also opening into the clitoral fossa is the urethra, which is lined 271 l Need-to-know Mouse vaginal mucosa changes morphology at each phase of the estrus cycle, whereas human vaginal mucosa is relatively stable in morphologic appearance. n by transitional epithelium. In humans, there are periurethral (Skene’s) glands and major (Bartholin’s) and minor vestibular glands, which open into the vestibule. Skene’s glands are lined with pseudostratified, mucus-secreting columnar 272 Comparative Anatomy and Histology CF CF VM B LP E M U E A FIGURE 19 Mouse vaginal mucosa. The mouse vaginal mucosa (VM) is composed of stratified squamous epithelium and is folded into longitudinal elevations with no glands. The morphology of the vaginal epithelium changes during the different stages of the estrus cycle. The lamina propria (LP) is fibrous, and the muscularis (M) layer is thin admixed with significant fibrous connective tissue. Adventitia (A) makes up the outermost layer. (A) F PC CC CC S VM FIGURE 20 Human vaginal mucosa. The human vaginal mucosa (VM) is composed of stratified squamous epithelium with no glands. Unlike that of the mouse, the human vagina is not keratinized, and the morphology of the human vaginal mucosa does not change significantly during the menstrual cycle. (B) FIGURE 21 The mouse and human clitoris. (A) The mouse clitoris is a ventrally extending elevation located on the anterior wall of the vaginal opening. The tip of the clitoris is covered by vaginal epithelium, and the anterior lateral surfaces are covered by haired skin. The urethra (U) opens near the tip of the clitoris. Erectile tissue (E) similar to penile erectile tissue surrounds the urethra near the tip of the clitoris. A small bone (B) homologous with the os penis is surrounded by connective tissue on the anterior face. The paired clitoral gland ducts (asterisks)open laterally at the clitoral fossa (CF). (B) Micrograph of human clitoris. Note the two corpora cavernosa (CC), arranged side-by-side and engorged with blood, the incomplete central septum (S), and the fibrocollagenous sheath (F), outside which are prominent nerve endings (mainly Pacinian touch corpuscles (PC)). Source: Reprinted from Human Histology, 3e, Stevens, A., Lowe, J.S., 2004, with permission from Mosby, © Elsevier. www.netterimages.com C h a p t e r 1 7 Female Reproductive System 273 cells that empty via bilateral periurethral ducts that are lined by transitional-type epithelium. Bartholin’s glands are lined by mucus-secreting columnar cells and empty via bilateral ducts adjacent to the vaginal introitus that are also lined by transitional epithelium. U (A) The urethra is lined by transitional epithelium that merges with the nonkeratinized squamous epithelium at the urethral orifice. Erectile tissue surrounds the urethra near the tip of the clitoris in mice; similar erectile tissue surrounds the clitoris in humans. The clitoral erectile tissue is composed of a fine vascular network similar to that present in the male penis (Figures 21 and 22). Mice have a small bone in the clitoral connective tissue, homologous to the os penis; humans do not have such a structure (Figure 22). The lamina propria of the clitoris and urethra consists of connective tissue that blends into the adjacent connective tissue and dermis. Placenta The placenta is the most morphologically diverse organ across mammalian species (Figures 23–31). Theories to explain this diversification over evolutionary history include maternal–fetal genetic conflict over nutrient allocation. U (B) FIGURE 22 Mouse erectile tissue. Clitoral erectile tissue (asterisks), similar to penile erectile tissue, surrounds the urethra (U) near the tip of the clitoris. (A) A small bone (arrow) homologous with the os penis is surrounded by connective tissue on the anterior face. (B) Higher magnification view of vascular erectile tissue. l Need-to-know The mouse clitoris is covered by haired skin on the anterior and lateral surfaces. n The mouse clitoris has a small bone homologous to the os penis. n Gross Anatomy In mice, the placenta is discoid and approximately 2 0.6 cm. The uteroplacental circulation is similar to that of humans, although the maternal– fetal interdigitation is labyrinthine rather than villous (Figure 23 and Table 4). In humans, the placenta is a disc approximately 20 cm in diameter that has a fetal surface and a maternal surface that is attached to the uterus (Figure 24). Maternal blood enters the intervillous space through the spiral arteries, which have been modified by invasive fetal trophoblast cells and transformed into low-resistance vessels. Maternal blood exits the intervillous space through uterine veins. Fetal blood enters the placenta through the umbilical artery, which branches over the fetal 274 Comparative Anatomy and Histology E DC E M YS L CP T M DB E FIGURE 23 Subgross of mouse gravid uterus. The embryo (E) is contained within the yolk sac (YS). The placental layers are partially visible at this low magnification and include the chorionic plate (CP), trophoblast (T) that includes spongiotrophoblasts and giant cells, labyrinth (L), decidua basalis (DB), and capsularis (DC). Myometrium (M) and portions of additional embryos (E) are indicated. C h a p t e r 1 7 Female Reproductive System 275 YS YS CP L S G MV MV D l Need-to-know (A) Mice and humans share hemochorial placentation. n L S G (B) FIGURE 24 Mouse placenta. (A) Humans and mice share hemochorial placentation. In mice, this organ is fully functional at embryonic day 12.5, which is equivalent to month 5.6 in humans. In hemochorial placentas, maternal blood comes in direct contact with fetal membranes through a labyrinth layer (L). Embryonic-derived layers include the labyrinth, the spongiotrophoblasts (S), and the giant cell trophoblasts (G). The decidual (D) layer is completely derived from maternal tissue. (B) Embryonal components of the placenta, the labyrinth layer (L), the spongiotrophoblast layer (S), and the giant cell trophoblast layer (G). CP, allantochorionic plate; MV, maternal vessels; YS, yolk sac. 276 Comparative Anatomy and Histology surface of the placenta, and further branches enter the chorionic villi containing capillary trees, with the blood returning in a venous path to the umbilical vein. The yolk sac is an extraembryonic membrane that surrounds the developing embryo. The yolk sac is derived from embryonic endoderm and mesoderm, and it provides the primitive circulatory system and erythropoiesis prior to placenta development. In the mouse, the yolk sac plays a minor role in embryonic support following placental development (Figure 25). In humans, the chorionic vessels are entirely derived from the allantois, and the yolk sac does not contribute to maternal–fetal exchange and is present only as a small remnant at delivery. It is lined by a single endodermal layer of tall columnar cells on a thin basal lamina separating endodermal epithelium from the mesoderm that gives rise to the blood islands. Blood islands lined by elongated endothelial-like cells contain embryonic nucleated erythrocytes (Figure 25). The yolk sac maintains communication with the embryo via intralabyrinth sinuses and the margins of the chorioallantoic plate. In both mice and humans, the chorioallantoic plate is formed by the fusion of the allantois and the chorionic plate (Figures 23 and 26). This fusion initiates the formation of the placenta. In mice, the chorioallantoic plate is composed of umbilical vessels and loosely arranged spindle cells originating from the extraembryonal ectoderm (Figure 23). Both humans and mice have hemochorial placentas, where the fetal trophoblast is directly exposed to maternal blood. The hemochorial placenta is fully developed in the mouse by day 12 of the 18.5- to 21-day gestation period. In humans, the hemochorial placenta is fully developed by week 5 of the 37- to 42-week gestation period (Table 4). Microscopic Anatomy In the mouse, layers of placenta include the chorioallantoic plate, labyrinth layer, Reichert’s membrane, giant cell trophoblasts, spongiotrophoblasts, and the decidua (Figure 23). The embryonal components include Reichert’s membrane, spongiotrophoblasts, giant cell trophoblasts, the labyrinth layer, and chorioallantoic plate. Layers of the human placenta include the amnion, chorionic plate, intervillous space, basal plate, and deciduas (Figure 24). The decidua is the only completely maternal component in both species. In mice, the decidua is divided into the basalis and the capsularis. The heavily vascularized basalis is located on the mesometrial side, where it interacts with the spongiotrophoblasts, whereas capsularis on the antimesometrial aspect is thinner and less well vascularized. Likewise, in humans, the decidua at the implantation site, which later underlies the placenta, is known as the decidua basalis. The human decidua is further divided into the decidua capsularis, which covers the surface of the gestational sac, and the decidua parietalis, which comprises the decidua on the opposite uterine wall. C h a p t e r 1 7 Female Reproductive System 277 Placenta I — Form and Structure Cotyledons Connective tissue septa Full-term placenta Maternal aspect Fetal aspect A C F B D E G B A H FIGURE 25 Human placenta showing the maternal and fetal surfaces as well as the insertion of the umbilical cord. Fetal villi project into the lacuna system, which is filled with maternal blood. Gas exchange occurs across the syncytiotrophoblast and the endothelial lining of the fetal vessel within the villous. (Bottom left) Section through deep portion of placenta, early gestation. A, villus; B, trophoblast; C, intervillous space; D, anchoring villus; E, villus invading blood vessel; F, fibrinoid degeneration; G, decidua basalis; H, gland. (Bottom right) Appearance of placental villi at term. A, syncytial cell mass becoming trophoblastic embolus; B, fetal blood vessel endothelium against a thinned syncytiotrophoblast, where they share a basal lamina. The cytotrophoblast has disappeared. Source: © Elsevier, Inc., www.netterimages.com. In mice, Reichert’s membrane (Figure 25), a thick acellular basement membrane that divides the trophoblast giant cells and the yolk sac, is unique to the rodent placenta. The mouse labyrinth layer is a highly vascularized structure composed of labyrinth trophoblasts; embryonal endothelium, which forms blood vessels; and embryonal erythrocytes (Figure 27). Maternal blood cells fill spaces lined by labyrinth trophoblasts. It is not uncommon to see mineralization in the labyrinth layer as a normal finding (Figure 28). The spongiotrophoblast layer is composed of large cells with abundant, often vacuolated, eosinophilic cytoplasm (Figure 29). This layer forms just 278 Comparative Anatomy and Histology BI BI BI l Need-to-know (A) Beyond embryonic day 12.5, when the placenta is fully developed, the yolk sac plays a minor role in embryonic support. n YS Reichert’s membrane is unique to rodents. n T (B) FIGURE 26 Mouse placenta. (A) Yolk sac from day 16.5 of gestation. The parietal and visceral layers of the yolk sac are lined by tall columnar cells. A thin basal lamina separates the epithelium from the mesoderm and blood islands (BI), which are lined by endothelial-like cells and contain immature and mature erythrocytes. (B) Reichert’s membrane is unique to the rodent placenta. Reichert’s membrane (asterisk) forms an acellular basement membrane that separates the trophoblasts (T) and the yolk sac (YS). below the labyrinth and is traversed by maternal vessels. Spongiotrophoblasts may phagocytize maternal erythrocytes or have pink hyaline cytoplasmic globules (Figure 29). The globules may dramatically increase with trophoblast degeneration. Between the spongiotrophoblasts and the maternal decidua is the giant cell trophoblast layer. These cells have abundant eosinophilic cytoplasm and large nuclei. Giant cell trophoblasts are frequently greater than 100 μm in diameter, and individual nuclei can be greater than 50 μm in diameter (Figure 30). The decidua makes up the maternal portion of the placenta (Figure 30). Cells are haphazardly arranged and typically have diffuse cytoplasmic vacuolation and irregularly shaped nuclei. Large maternal vessels develop to support the pregnancy. At Reichert’s membrane, giant cell trophoblasts extend into the C h a p t e r 1 7 Female Reproductive System AE AM FV CM IVS FIGURE 27 Human placenta. The human chorionic plate. The plate is lined by amniotic epithelium (AE), amniotic mesoderm (AM), and chorionic mesoderm (CM). FV, fetal vessel; IVS, intervillous space. 279 280 Comparative Anatomy and Histology SK V M IVS M V V V SK (A) FIGURE 28 Mouse and human placenta. (A) The labyrinth layer is highly vascularized with embryonic and maternal blood. The embryonic blood channels (V) are lined by embryonic endothelial cells and contain immature erythrocytes. Labyrinth trophoblasts line vascular channels filled by maternal blood (M). Mineral (asterisk) is not uncommon in normal animals. (B) The human intervillous space (IVS) is perfused by maternal blood. Highly vascularized fetal villi are lined by villous trophoblast (arrow). SK, syncytial knot; V, fetal vessel. Source: Courtesy of T. B. Treuting. decidua to form a sinusoidal network, the vitelline meshwork. This meshwork is filled with maternal blood. In humans, the chorion laeve or fetal membranes are formed at the edge of the placental disc by fusion of the basal plate and chorionic plate by obliteration of the intervillous space (Figure 26). The fetal surface is lined by amniotic epithelium composed of a single layer of cuboidal to columnar cells, an unkeratinized stratified squamous epithelium over the umbilical cord. (B) l Need-to-know Mineralization is commonly observed in the labyrinth and trophoblast layers in normal pregnancies. n Connective tissue subjacent to the amnion contains fetal vessels and residual trophoblast. Subjacent to the chorion laeve is the maternally derived decidua capsularis, which is derived from decidualized maternal endometrium. There are two major populations of human trophoblast. The first population is the villous trophoblast that lines the chorionic villi, the site of maternal–fetal exchange (Figures 24, 27, and 28), and is composed of two layers. The outmost layer, the syncytiotrophoblast, is