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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
  • The ovulated secondary ...

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