We will combine our discussion of the ovarian and menstrual cycles to show how hormones coordinate the activities of the ovaries and the uterus. The brain controls reproduction, acting via hormones produced by the pituitary gland. While a woman is menstruating, her hypothalamus begins secreting a releasing hormone, which stimulates the anterior pituitary to secrete FSH and LH into the blood. FSH (follicle-stimulating hormone) stimulates the growth of an ovarian follicle in the ovary. The follicle consists of a developing egg cell surrounded by cells that nourish and protect it. The developing follicle begins secreting estrogen, a female sex hormone. Estrogen shapes development of the female reproductive system and female secondary sex characteristics such as broad hips and development of the breasts. Its immediate role is to stimulate regrowth of the lining of the uterus, in preparation for possible pregnancy. Low levels of estrogen also exert negative feedback on the hypothalamus, keeping blood levels of FSH and LH relatively low. Basically what is going on here is that the brain triggers development of an egg, and the follicle containing the egg signals the uterus to prepare to support the development of an embryo. Negative feedback assures that only one follicle develops at a time. As the follicle grows, it secretes more and more estrogen. This stimulates further development and thickening of the uterine lining. Estrogen level reaches a peak just before the mid-point of the cycle. Now response of the hypothalamus and pituitary to estrogen reverses. The higher level of estrogen actually stimulates the hypothalamus to signal the pituitary to secrete a burst of FSH and LH. It is the burst of LH that triggers ovulation on about day 14 of the cycle. The ovum develops, the follicle ruptures, and the nearly mature ovum is released from the follicle and swept into the oviduct. LH (or luteinizing hormone) also causes the ruptured follicle to develop into a glandular structure called the corpus luteum. After ovulation, the corpus luteum continues to secrete estrogen and increasing amounts of a second female hormone called progesterone. Both hormones contribute to further thickening of the uterine lining, in preparation for possible pregnancy. Estrogen and progesterone also exert negative feedback on the hypothalamus and pituitary, suppressing FSH and LH secretion. This keeps additional follicles from developing after ovulation. We can summarize ovulation and the following events like this: When the follicle is nearly mature, it signals the hypothalamus to trigger ovulation. The ruptured follicle, now called the corpus luteum, prevents additional follicles from developing, and signals the uterus to prepare for implantation and development of an embryo. What happens next depends on whether the ovum is fertilized or not. If the egg is not fertilized, the corpus luteum has a short life span. Suppression of LH after ovulation causes the corpus luteum to degenerate. As the corpus luteum's output of estrogen and progesterone drops, two changes occur. The lining of the uterus begins to slough off, the beginning of the menstrual period and start of the next cycle. The drop in estrogen and progesterone also reduces feedback inhibition of the hypothalamus and pituitary. This triggers secretion of FSH and LH, which stimulates the development of a new follicle. So, if fertilization doesn't occur, the corpus luteum degenerates. Without its hormones, the lining of the uterus breaks down and a new follicle starts to grow. If the egg is fertilized, it starts to develop and implants in the wall of the uterus. The embryo secretes a hormone called human chorionic gonadotropin, HCG for short. HCG functions like LH to signal the corpus luteum to continue its secretion of estrogen and progesterone. (Later the placenta takes over this function.) Progesterone and estrogen maintain the lining of the uterus and suppress the development of any more follicles. In effect, the embryo signals the corpus luteum to stick around, maintaining the uterus for embryonic development and stopping the monthly cycle. Human reproduction is complicated! It depends on precise coordination of the ovaries and uterus, in response to hormonal signals orchestrated by the brain.