Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger,
Peer Review Status: Externally Peer Reviewed
The female reproductive system includes the ovaries, the uterine tubes, uterus, vagina, and external genitalia. The ovaries perform both an exocrine function by producing ova and an endocrine function by producing estrogen and progesterone.
The ovaries are ovoid structures, approximately 3 cm in length, lying on each side of the uterus within the pelvis. The ovary consists of a cortical zone composed of a specialized stroma, which contains follicles with ova. In the mature functional ovary, many follicles are quiescent, whereas others exhibit a wide range of histomorphology, depending upon their stage of maturation or regression. The medulla consists primarily of connective tissue and an extremely rich vascular supply.
Immature ova or oocytes are spherical cells, about 30 µm in diameter; when fully mature, they have increased in size to about 120 µm and are designated ova. The nucleus of an oocyte is large and vesicular and contains a prominent nucleolus. The cytoplasm is rich in nutritive material, the yolk.
The events of meiosis and the maturation stages of the female gamete parallel those events occurring in spermatogenesis discussed in Section 14, entitled Male Reproductive System. During human fetal development, the primordial germ cells migrate to and are incorporated within the developing ovary and are termed oogonia. The oogonia multiply by mitosis, but early in fetal life, they enter meiosis. However, the meiotic events are arrested by a mechanism not understood in prophase (diplotene stage) of the first meiotic division. These cells, about 40 µm in diameter and termed primary oocytes, are enclosed within a single layer of squamous cells, forming a primordial follicle. The primordial follicles in each human fetal ovary number more than 200,000 and decline in number until very few or none remain at about the 50th year.
The transition from an inactive primordial follicle to a growing and maturing primary follicle involves changes in the oocyte, the follicular cells, and the adjacent connective tissue. As the oocyte enlarges, the single layer of follicular cells increases in size through mitotic division and gives rise to cells (granulosa cells) that eventually form a stratified epithelium termed the granulosa. A distinctive feature of the multilaminar follicle is the elaboration of a highly refractile zona pellucida interposed between the oocyte and granulosa cells; the zona is secreted by both the egg and surrounding follicular cells. Concomitant with the development of the granulosa cells, a sheath of stromal cells (theca folliculi) develops around the follicle and subsequently forms two layers. The inner layer exhibits a well- developed capillary plexus and secretory cells and is termed the theca interna. The cells of the theca interna are believed to secrete androstenedione, which is subsequently converted to estradiol by the granulosa cells. Secondary follicles can be identified when they are about 0.2 mm in diameter and are recognized by the presence of irregular spaces among the granulosa cells filled with a clear liquid (liquor folliculi), which increases with continued growth of the follicle. Eventually, the oocyte comes to be eccentrically placed within the follicle upon a pedestal of follicular cells, the cumulus oophorus. The oocyte is intimately surrounded by a crown of follicular cells, the corona radiata. The cumulus projects into a single large fluid-filled space, the antrum, formed from the coalescence of the smaller spaces noted previously.
Even after the primary oocyte has reached full size, the follicle may continue to enlarge until it reaches approximately 10 mm in diameter. Follicles that have matured to maximal size, exhibit a large antrum, and extend through the entire thickness of the cortex are termed graafian follicles*. just prior to ovulation, a bulge on the surface of the ovary (the stigma) marks the site where ovulation will occur. The growth of a primordial follicle to full maturity takes about 10 to 14 days. The thecae folliculi, particularly the theca interna, reach their highest development in relation to the mature follicle.
At mid-menstrual cycle (approximately day 14), the surge of pituitary luteinizing hormone (LH) induces ovulation. At this time, the primary oocyte's first meiotic division occurs, resulting in the formation of the first polar body and the secondary oocyte. In the human female, the secondary oocyte completes its second meiotic division at the time of fertilization, and the male and female haploid genomes fuse in the formation of the zygote.
Following ovulation and discharge of the liquor folliculi and the oocyte within its cumulus mass, the walls of the follicle collapse and the granulosa cell lining becomes folded. Rupture of blood vessels in the theca interna is associated with bleeding into the partially collapsed follicle, and a clot is formed. The cells of the granulosa layer and the theca interna undergo transformation and are renamed granulosa lutein and theca lutein cells, respectively. These changes in the follicle following ovulation result in a new but transitory organ, the corpus luteum (yellow body, for its appearance in fresh specimens). The corpus luteum secretes the hormone progesterone. If the ovulated oocyte fails to be fertilized, the corpus luteum remains functional for only about 14 days and then regresses and is reduced eventually to a scar within the ovary termed the corpus albicans (white body). In the event of fertilization, the corpus luteurn enlarges and persists as a functional endocrine gland throughout most of the pregnancy but begins to involute after the sixth month. Its ultimate fate after the termination of pregnancy is to become a large corpus albicans.
Most follicles never develop into mature follicles, since that number is limited to about 400 (or 1 of every 1000 follicles) during the reproductive span of the human female. The process by which follicles degenerate and disappear is little understood and is termed follicular atresia. This process can begin at any stage of follicular development. The smallest follicles leave no trace of their dissolution, but the larger follicles may leave a remnant of the zona pellucida as a persistent marker within the ovary. In larger secondary follicles, the earliest signs of atresia include the loosening and shedding of the granulosa cells, the invasion of the granulosa layers by vascular tissue and wandering cells, and the collapse or partial collapse of the follicle.
At the time of ovulation, the oocyte is shed upon the surface of the ovary, from which it must be transported to the interior of the ovarian (fallopian) tube. The oviduct possesses a highly specialized, flared terminal portion, the infundibulum, which bears long, frond-like extensions of the mucosa (termed fimbriae), which sweep over the surface of the ovary; the ovulated oocyte within its cumulus mass is transported by means of ciliary action along the surface of the fimbriae toward the ostium, or opening of the oviduct. The ostium leads to the second portion of the oviduct, the ampulla, which is the duct's dilated mid-portion where fertilization usually occurs. A constricted isthmic portion joins the ampulla to the uterine wall; the length of the oviduct that passes through the wall of the uterus is termed the intramural portion of the organ. The epithelium lining the oviduct is principally simple columnar; many of the lining cells are ciliated. Transport of the cumulus and oocyte within the oviduct is facilitated through vigorous peristaltic action of the oviduct; the muscularis is composed of two layers of smooth muscle, which become progressively more well developed as the uterine tube approaches the uterus.
The uterus lies within the pelvis in relation to the bladder anteriorly and the rectum posteriorly and is a hollow pear-shaped organ that opens into the vagina. The uterus is composed of a mucosa, given the special name of endometrium; a muscularis termed the myometrium; and a serosa, or perimetrium. The uterine mucosa undergoes cyclic changes, which are synchronized with ovarian secretory activity. The surface epithelium is simple columnar with patches of ciliated columnar cells. Uterine glands, lined with a similar columnar epithelium, open to the surface and secrete mucus. The endometrial stroma has a framework consisting of reticular fibers and stromal cells. Lymphocytes and granular leucocytes are also found in the stroma. The endometrium is composed of two parts, the superficial functionalis, which undergoes changes during the menstrual cycle and is shed during menstruation, and the basalis, which does not undergo cyclic changes and remains intact during menstruation.
The myometrium is a thick coat containing smooth muscle and abundant connective tissues. The smooth muscle of the uterus, in response to female sex hormones, undergoes cyclic variation in length and diameter and in functional activity. Three layers of smooth muscle are recognized: an inner longitudinal layer, a middle circular and oblique layer, and an outer longitudinal layer.
The following cyclic changes occur in the uterine endometrium during an idealized 28-day menstrual cycle: (1) The proliferative or estrogenic phase extends from about day 4 to day 14 of the cycle. This period involves re-epithelialization of the denuded endometrial surface and growth in thickness of the endometrium and glands. The glands are initially straight but begin to coil toward the end of this phase. Estrogen is the dominant hormonal influence during this phase. (2) The secretory, progestational, or luteal phase constitutes days 15 to 28 of the cycle. During this period, the uterine glands become highly coiled and , irregularly sacculated in the middle of the endometrium. The glandular epithelium secretes a mucoid fluid rich in glycogen. The endometrium becomes edematous and may reach a thickness of 5 mm.
Progesterone is the dominant hormonal influence during this phase. On day 27 or 28, the uterus enters the ischemic phase, during which the arterial supply constricts intermittently. At this point, glandular secretion is interrupted. (3) The menstrual phase involves the extravasation of blood and the detachment of patches of hemorrhagic endometrium until the entire functionalis is sloughed. The basal layer remains intact during this phase and is the source of the regenerating functional layer during the ensuing proliferative phase. The menstrual phase lasts from days 1 to 4 or 5.
The outlet of the uterus is the vagina, a fibromuscular sheath lined with thick stratified squamous epithelium. The underlying lamina propria is also thick and contains numerous lymphocytes and other wandering cells that invade the epithelium. The muscularis is irregularly arranged in two layers: an inner circular or spiral layer and an outer longitudinal layer. The vagina does not possess a muscularis mucosae or glands in the lamina propria. The adventitia is a dense collagenous tissue that merges with the adventitia of the bladder and rectum and is highly vascular.
In pregnancy, an important but temporary organ, the placenta, is formed in the uterus. The placenta is derived from both maternal (uterine) and fetal components. The maternal component is the endometrium; the fetal contribution consists of the chorionic plate and its branching villi, which are of two types: (1) the anchoring villi, which extend from the chorionic plate to the decidua basalis; and (2) the free villi, formed by branches from the anchoring villa. A villus has a fibromuscular core with fetal blood vessels and is covered by an epithelium termed the trophoblast. The trophoblast is arranged in two layers: (1) an outer layer without cell boundaries, the syncytiotrophoblast; and (2) an inner cuboidal cell layer, the cytotrophoblast. In the later stages of pregnancy, the cellular layer disappears. The source of human chorionic gonadotropin (HCG) and other placental hormones is the syncytiotrophoblast.
The fetal and maternal blood do not mix within the placenta. In the human, the fetal blood is contained entirely within small vessels and capillaries, whereas the maternal blood within the placenta leaves arterial vessels and gains free access to the space surrounding the fetal villi. Thus, what is termed the maternal blood lake is formed. It is across this interface of trophoblastic epithelium and maternal blood that oxygen, carbon dioxide, nutrients, and waste products are exchanged. Maternal blood is drained by veins from the intervillous spaces of the placenta.
*de Graaf, 1641 -1673, was a Dutch physician and anatomist.
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