The figure above shows how (c) the ovarian cycle and (e) the uterine (menstrual) cycle are regulated by changing hormone levels in the blood, depicted in parts (a), (b), and (d). The time scale at the bottom of the figure applies to parts (b)–(e).
1 The cycle begins with the release from the hypothalamus of GnRH, which
2 stimulates the pituitary to secrete small amounts of FSH and LH.
3 The FSH (true to its name) stimulates follicle growth, aided by LH, and
4 the cells of the growing follicles start to make oestrogen.
Notice in the figure that there is a slow rise in the amount of oestrogen secreted during most of the follicular phase, the part of the ovarian cycle during which follicles are growing and oocytes maturing. (Several follicles begin to grow with each cycle, but usually only one matures; the others disintegrate.)
The low levels of oestrogen inhibit secretion of the pituitary hormones, keeping the levels of FSH and LH relatively low.
5 the secretion of oestrogen by the growing follicle begins to rise steeply. Whereas a low level of oestrogen inhibits the secretion of pituitary gonadotropins, a high concentration has the opposite effect: It stimulates the secretion of gonadotropins by acting on the hypothalamus to increase its output of GnRH.
6 You can see this response in the figure as steep increases in FSH and LH levels that occur soon after the increase in the concentration of oestrogen, indicated in the figure. The effect is greater for LH because the high concentration of oestrogen also increases the sensitivity of LH–releasing cells in the pituitary to GnRH. By now, the follicles can respond more strongly to LH because more of their cells have receptors for this hormone. The increase in LH concentration caused by increased oestrogen secretion from the growing follicle is an example of positive feedback. The LH induces final maturation of the follicle.
7 The maturing follicle develops an internal fluid–filled cavity and grows very large, forming a bulge near the surface of the ovary. The follicular phase ends, about a day after the LH surge, with ovulation: The follicle and adjacent wall of the ovary rupture, releasing the secondary oocyte.
The hormones secreted by the ovaries—oestrogen and progesterone—have a major effect on the uterus. Oestrogen secreted in increasing amounts by growing follicles signals the endometrium to thicken. In this way, the follicular phase of the ovarian cycle is coordinated with the proliferative phase of the uterine cycle. Before ovulation, the uterus is already being prepared for a possible embryo. After ovulation,
9 oestrogen and progesterone secreted by the corpus luteum stimulate continued development and maintenance of the endometrium, including enlargement of arteries and growth of endometrial glands. These glands secrete a nutrient fluid that can sustain an early embryo even before it actually implants in the uterine lining. Thus, the luteal phase of the ovarian cycle is coordinated with what is called the secretory phase of the uterine cycle.
After about 450 cycles, human females undergo menopause, the cessation of ovulation and menstruation. Menopause usually occurs between the ages of 46 and 54.
Apparently, during these years the ovaries lose their responsiveness to gonadotropins from the pituitary (FSH and LH), and menopause results from a decline in oestrogen production by the ovary. Menopause is an unusual phenomenon; in most species, females as well as males retain their reproductive capacity throughout life. Is there an evolutionary explanation for menopause? Why might natural selection have favoured females who had stopped reproducing? One intriguing hypothesis proposes that during early human evolution, undergoing menopause after having some children actually increased a woman′s fitness; losing the ability to reproduce allowed her to provide better care for her children and grandchildren, thereby increasing the survival of individuals bearing her genes.