Reproductive tracts of selected species are shown in Figures 4-1, 4-2 (human), 4-3 (cow), 4-4 (pig), 4-5 (rat), and 4-6 (marsupial); while all females have the same complement of parts, architectural designs are somewhat varied. In vertebrates below the level of placental mammals there is a common portal (cloaca) shared by the digestive, urinary, and reproductive systems.
Ovary. Ovaries contain three fundamental morphological components - follicular, luteal, and interstitial (stromal). Follicles and CL are mainly dispersed within the interstitium of the ovarian cortex. Connective tissue of the cortical region, particularly the (outermost) tunica albuginea, is more densely oriented than within the medulla. Blood vessels and nerves traverse through the supportive ligament of the ovary, the mesovarium, and enter/ exit the ovary through the hilus (Figure 4-7).
Follicles house ova and are the site of production of steroid hormones and inhibin. Follicles are classified in ascending progression of maturity as primordial, primary, secondary, tertiary, or preovulatory (Graafian) (Table 4-1; Figures 4-8, 4-9, 4-10, and 4-11). Follicles do not form without an ovum (eg., when immature ova are destroyed in the fetal ovary by X-irradiation).
The wall of the preovulatory follicle is composed of three distinctive layers - theca externa, theca interna, and membrana granulosa. The theca externa is the outer layer of the mature follicle; it contains smooth muscle-like cells interdispersed within a dense connective tissue matrix. Blood vessels and nerves that serve the follicle terminate within the theca interna. The theca interna also contains polygonal shaped steroidogenic cells. A basement membrane separates the thecal layers from the granulosa (Figures 4-12 and 4-13). Granulosa cells play an important role in follicular endocrinology and interact with the ovum.
The ovum of developed follicles is situated within a fluid-filled cavity (antrum) on a pedicle of specialized granulosa cells, the cumulus oophorus (Figure 4-14). In some species the ovum is projected into the antrum by numerous cumulus extensions (Figure 4-15). The plasma (vitelline) membrane of the ovum is immediately encircled by a sulfated glycoprotein coat called the zona pellucida (Figure 4-16). Granulosa cells that contact the zona pellucida, the corona radiata, have projections extending to the surface of the egg. Junctions between granulosa cells shield the ovum from immunological recognition (blood-follicle barrier). Most ovarian follicles contain a single egg - although there have been reports of polyovular follicles, especially among juvenile animals.
Follicular atresia (degeneration) can occur at any step during the developmental life of a follicle (ie., primordial up to the preovulatory stage). The ovum of atretic follicles begins to shrink and may divide. Dissociative granulosa cells have a low mitotic index and a preponderance of pyknotic (condensed) nuclei (Figure 4-17). The theca of atretic follicles undergoes hypertrophy (increased size) and sometimes luteinizes (CL atretica). Scavenger cells are often observed within atretic follicles.
The follicle destined to ovulate protrudes from the surface of the ovary and ruptures along its apical aspect. Just before the time of ovulation the ovum, normally connected at the basal wall in apposition to the follicular apex, becomes detached from the underlying granulosa layer. Follicular fluid provides a suspending medium for the ovum during its exit from the follicle (Figure 4-18).
Ovulation in most mammals takes place at random locations over the surface of either ovary. The site of ovulation in the mare and nine-banded armadillo is restricted to a depression in the ovary called the fossa (Figure 4-19). In some species ovulations occur more frequently from one ovary than the other (eg., pig and camel = left; cattle = right). The left ovary of cetaceans produces the majority of ovulations early in their reproductive life; it is then superseded by the right ovary. Only the left ovary is active in the platypus (akin to birds).
Most unusual is the mature follicle of the hedgehog, which is without an antrum altogether. Spermatozoa actually invade the follicle and fertilize the ovum in situ. The pronucleate egg then moves toward the ovarian surface and is shed (Figure 4-20).
The ovary is covered by a layer of epithelial cells continuous with the mesovarium (Figure 4-7). During the mechanism of ovulation most surface cells in apposition to the follicular apex degenerate and are sloughed from the ovary (this phenomenon is reminiscent of morphological alterations associated with programmed cell death). The majority of ovarian cancers are thought to arise from a (persistent) surface epithelial cell that is traumatized (into a malignant phenotype) at ovulation and propagated during the clonal postovulatory "wound-repair" process. Common epithelial ovarian neoplasia ranks among the most lethal of all cancers; it usually goes undiagnosed until intraperitoneal carcinomatosis is far advanced.
Ovaries of some animals are surrounded by a thin membranous sac, the bursa, and are isolated from the peritoneal cavity (Figure 4-21). The bursa is continuous with the orifice of the oviduct. The bursa is not continuous in other mammals (eg., farm animals and human beings); ovaries of these species can be exposed within the abdomen, and thus ectopic pregnancy is possible.
The ovulated follicle is transformed into a new steroidogenic (luteal) organ. The developing luteal structure is called the corpus hemorrhagicum ("bloody body") (Figure 4-22). At follicular rupture the antrum becomes filled with some blood. The blood clot within the antrum is soon resorbed and replaced by luteinized granulosa cells (Figure 4-23). Corpora lutea (Figures 4-24 and 4-25) of some species contain high concentrations of carotene and have a characteristic yellowish color. The regressed CL, a connective tissue scar, is called the corpus albicans ("white body") (Figures 4-26 and 4-27). Remnants of corpora albicantia can persist within the ovaries for considerable periods.
In many species the interstitium is functional only from a supportive standpoint. In some animals the ovarian stroma is highly differentiated (Figure 4-28) and capable of steroidogenesis.
Tubular organs. Tubular organs of the female system include the oviducts (Fallopian tubes), uterus (womb), cervix (birth canal), and vagina (copulatory organ). Fertilization occurs within the oviduct. The embryo develops into a fetus within the uterus. The cervix intercedes between the vagina and uterus; it becomes sealed during pregnancy and thus protects the unborn from insult by external contaminants. Distinguishing anatomical features of the reproductive tracts among various female mammals are contrasted in Table 4-2 and Figure 4-29. Also refer back to Figures 4-2 through 4-6.
The oviduct can be segmented into three regions - infundibulum ("funnel"), ampulla, and isthmus (Figure 4-30). At ovulation the oocyte is captured by the ovarian bursa or is retrieved by infundibular finger-like projections (fimbriae) and passed into the oviduct. Epithelium of the oviduct is ciliated (Figure 4-31) for moving the oocyte toward the site of fertilization within the ampulla (Figure 4-32). The isthmus is the point at which the oviduct joins the uterus. Each oviduct is cut and tied during a tubal ligation (Figure 4-33).
Like other tubular organs, the wall of the uterus is composed of a mucosa (endometrium), muscularis (myometrium) (Figure 4-34), and serosal epithelial covering (perimetrium) continuous with the peritoneum. The uterus has a tremendous capacity to expand during pregnancy and revert to a nongravid state (involute) following parturition. Subdivisions of the broad ligament suspend the oviducts (mesosalpinx) and uterus (mesometrium) within the abdominal cavity.
The cervix is a sphincter that forms the neck of the uterus; it is thick-walled and has a narrow, tortuous lumen (Figure 4-35). The cervix projects partially into the vagina. The cul-de-sac surrounding the cervical os (mouth) is called the fornix (Figure 4-1). The epithelial lining of the cervix contains numerous invaginations, or crypts (Figure 4-36).
The cervix is the primary site of precancerous growth (dysplasia) and malignancy within the female reproductive tract. Cervical diseases are a problem mainly in younger women (age 30 through 50), particularly those who engage in frequent intercourse with multiple partners. Cervical cancer is diagnosed by Pap smear (Figure 4-37). It is critical to detect cervical cancer before it can spread.
The uterine body is also susceptible to carcinoma and a variety of benign growths. Endometriosis is a common noncancerous condition in women of reproductive age; endometrial cells are displaced during menstruation to locations other than the uterine mucosa, usually the upper urogenital tract and other nearby pelvic organs. Symptoms of endometriosis include dysmenorrhea (painful menstruation), abdominal bleeding (often leading to adhesions), and infertility. Definitive diagnosis of endometriosis is by laparoscopic and(or) microscopic verification of ectopic endometrium. Menstrual bleeding and degree of endometrial desquamation can be minimized with steroidal contraceptives and related drugs (eg., danazol). Surgical removal of ectopic tissue is indicated in advanced cases of endometriosis, but care must be taken not to aggravate the ailment further. Fibroids are benign tumors of the myometrium (leiomyomata) of prevalence in premenopausal women.
Most mammals have a single vagina. Some noneutherians have a septate vagina (to conform, the penis of males is bifurcated). The medial vagina (Figure 4-6) of marsupials remains closed, except at parturition. The vagina is lined by stratified squamous epithelium (Figure 4-38); it lacks true glands. Vaginal lubrication is provided mainly by cervical mucus and epithelial and plasma transudates (some mucus is secreted from vestibular glands located around the vaginal opening). The hymen is a thin membranous fold that partially occludes the external opening of the vagina in virgin females; it is usually torn before sexual maturity.