Jul 22, 2010

The reproductive cycle of the human female

The Ovarian Cycle. 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 Figure(d) 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.

The levels of FSH and LH, however, shoot up sharply when 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 Figure (b) as steep increases in FSH and LH levels that occur soon after the increase in the concentration of oestrogen, indicated in Figure (d). 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.

8 Following ovulation, during the luteal phase of the ovarian cycle, LH stimulates the transformation of the follicular tissue left behind in the ovary to form the corpus luteum, a glandular structure (c). (LH is named for this “luteinising” function.) Under continued stimulation by LH during this phase of the ovarian cycle, the corpus luteum secretes progesterone and estrogen (see Figure(d). As the levels of progesterone and oestrogen rise, the combination of these hormones exerts negative feedback on the hypothalamus and pituitary, inhibiting the secretion of LH and FSH. Near the end of the luteal phase, the corpus luteum disintegrates, causing concentrations of estrogen and progesterone to decline sharply. The dropping levels of ovarian hormones liberate the hypothalamus and pituitary from the inhibitory effects of these hormones. The pituitary can then begin to secrete enough FSH to stimulate the growth of new follicles in the ovary, initiating the next ovarian cycle.

The Uterine (Menstrual) Cycle. 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 (see Figure (e ). 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.

10 The rapid drop in the level of ovarian hormones when the corpus luteum disintegrates causes spasms of the arteries in the uterine lining that deprive it of blood. The upper two–thirds of the endometrium disintegrates, resulting in menstruation—the menstrual flow phase of the uterine cycle—and the beginning of a new cycle. By convention, the first day of menstruation is designated day 1 of the uterine (and ovarian) cycle. Menstrual bleeding usually persists for a few days. During menstruation, a fresh batch of ovarian follicles are just beginning to grow.

Cycle after cycle, the maturation and release of egg cells from the ovary are integrated with changes in the uterus, the organ that must accommodate an embryo if the egg cell is fertilised. If an embryo has not implanted in the endometrium by the end of the secretory phase of the uterine cycle, a new menstrual flow commences, marking day 1 of the next cycle.

In addition to the roles of oestrogen in coordinating the female reproductive cycle, this hormone family is responsible for the secondary sex characteristics of the female. Oestrogen induces deposition of fat in the breasts and hips, increases water retention, affects calcium metabolism, stimulates breast development, and influences female sexual behaviour.

Menopause. 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 estrogen 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.


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