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SUMMARY OF KEY POINTS
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The Female Reproductive System SUMMARY OF KEY POINTS Ovaries, Follicles, and Oocytes
The female gonads, the paired ovaries, each have an outer cortex containing many hundreds of ovarian follicles and an inner medulla of dense connective tissue and large blood vessels.
The ovary’s cortex is covered by a cuboidal mesothelium, the surface epithelium (or germinal epithelium) that overlies a layer of connective tissue, the tunica albuginea.
Before puberty, all follicles are primordial follicles, formed in the developing fetal gonad, with each having one primary oocyte arrested in meiotic prophase I and a surrounding layer of squamous follicular epithelial cells.
After puberty, some primordial follicles develop each month as growing primary follicles, with an enlarging primary oocyte surrounded by larger epithelial cells now called granulosa cells.
During follicular growth, the granulosa cells, surrounded by a basement membrane, become stratified and actively engage in fluid secretion and steroid hormone metabolism.
Between the oocyte and the granulosa cells, a thin layer forms called the zona pellucida, which contains glycoproteins (ZO proteins) to which the sperm surface must bind to reach the oocyte at fertilization.
Antral or vesicular follicles are larger and have developed fluid-filled spaces among their granulosa cells, but the growing oocyte is still in prophase I.
While the primary follicle grows, mesenchymal cells immediately around it form the highly vascular layer, the theca interna, and a more fibrous theca externa, with smooth muscle cells.
Endocrine cells of the theca interna secrete both progesterone and estrogen precursors, which are converted by granulosa cells into estrogen.
Antral follicles continue developing as mature, graafian follicles, which have a large antrum filled with fluid, with the large primary oocyte enclosed by granulosa cells of the cumulus oophorus.
Each month only one graafian follicle becomes a dominant follicle and undergoes ovulation; most other developing follicles arrest and degenerate with apoptosis in a process called atresia.
Ovulation and the Corpus Luteum Ovulation involves movement of a very large, dominant graafian follicle to the ovary surface to form a bulge, completion of meiosis I, and release of a polar body from the oocyte.
Rupture of the follicle and ovarian coverings releases the secondary oocyte, arrested now in metaphase II, and a layer of attached granulosa cells that make up the corona radiata.
Cells of the granulosa and thecal layers left in the ovary after ovulation are reorganized under the influence of LH to form the endocrine gland called the corpus luteum.
The cells of the corpus luteum are granulosa lutein cells, producing estrogen and comprising 80% of the gland, and theca lutein cells producing progesterone.
LH levels drop about 2 weeks after ovulation, causing the corpus luteum to lose activity, degenerate, and be removed by macrophages, leaving a temporary collagen-filled region called a corpus albicans.
Uterine Tubes or Oviducts