The Ovaries: Anatomy, Oogenesis and Hormonal Function

The ovaries are paired gonads with two linked functions. They house the oocyte supply and coordinate reproductive physiology through steroid and peptide hormones. Oogenesis is not a monthly process of creating new oocytes. The available pool is established before birth and progressively depleted through atresia, the natural degeneration of follicles. The fetal ovary contains approximately 6–7 million oocytes at 20 weeks of gestation. About 1–2 million remain at birth, 300,000–500,000 remain at puberty, and only a few hundred may persist by menopause. Ovulation accounts for only a small portion of this loss. This finite reserve explains why increasing age is associated with declining fertility, but ovarian reserve and fertility are not identical. Reserve refers primarily to the remaining follicle pool; it does not guarantee that an oocyte will ovulate, fertilize, implant, or produce a chromosomally normal embryo. The endocrine function is equally important. Follicles and the corpus luteum produce estradiol, progesterone, inhibins, and smaller amounts of androgens. These hormones act on the endometrium, cervix, breasts, hypothalamus, pituitary, and myometrium. Their changing concentrations create the menstrual cycle, support early pregnancy, and...