Describe the sequence of events in the ovarian cycle, including the roles of FSH and LH.

The ovarian cycle has a follicular phase (days 1-14) and a luteal phase (days 14-28). In the follicular phase, FSH stimulates vesicular follicles to grow and secrete estrogens, while LH stimulates thecal cells to supply androgens that are converted to estrogens; one follicle becomes dominant. The resulting high estrogen level triggers an LH surge, which causes the dominant follicle's primary oocyte to complete meiosis I and leads to ovulation around day 14. The LH surge also converts the ruptured follicle into the corpus luteum, whose progesterone and estrogen maintain the uterine lining until it degenerates if no pregnancy occurs.

The ovarian cycle begins with the follicular phase, generally days 1-14. A rise in FSH allows several vesicular (antral) follicles to continue maturing. FSH acts mainly on granulosa cells to make them secrete estrogens, whereas LH prods thecal cells to release androgens that the granulosa cells convert into estrogens. As FSH levels drop in the middle of the follicular phase, one follicle is selected as the dominant follicle, and the other stimulated follicles undergo apoptosis. Estrogen and inhibin normally exert negative feedback, but when estrogen from the dominant follicle reaches a critical level, it briefly exerts positive feedback and triggers a midcycle LH surge. The LH surge rouses the primary oocyte of the dominant follicle to complete meiosis I, producing a secondary oocyte and first polar body. Around day 14, ovulation occurs when the follicle wall ruptures and the secondary oocyte, still surrounded by its corona radiata and arrested in metaphase II, is expelled. In the luteal phase, days 14-28, the LH surge transforms the ruptured follicle into the corpus luteum, which secretes progesterone and some estrogens, plus inhibin. Rising progesterone and estrogen levels act by negative feedback to inhibit LH and FSH release, preventing new vesicular follicles from maturing and preventing additional LH surges. If pregnancy does not occur, the corpus luteum degenerates in about 10 days, its hormonal output ends, and the resulting sharp drop in estrogen and progesterone removes the blockade on FSH and LH, allowing a new ovarian cycle to begin. If pregnancy occurs, the corpus luteum persists until the placenta takes over hormone production in about three months. If a sperm penetrates the ovulated secondary oocyte, meiosis II is completed, producing one large ovum and a tiny second polar body.