(2) Follicle growth. Growth of the primary follicle is initiated by the
folliclestimulating hormone (FSH). The FSH causes a proliferation of the granulosa
cells and the production of the fluid filling the antrum. The luteinizing hormone (LH)
causes a further production of fluid that continues until the follicle bursts. The ovum is
then expelled and the remainder of the follicle undergoes a transformation into a mass
of yellow cells known as the corpus luteum.
(3) Release of FSH. The release of FSH by the adenohypohysis, in addition
to causing the growth of the follicle, also causes the follicles to secrete one of the two
female hormones--estrogen. Estrogen is the principal female hormone. Estrogen is a
composite of several hormones called estradiol, estriol, and estrone. These three
substances have slightly different molecular structures, but they produce the same
activity in the body. Estrogens are responsible for the secondary sexual characteristics
of the female. Estrogens also cause the lining of the uterus, the endometrium, to
increase in thickness by about threefold. The corpus luteum, under the stimulation of
the luteotropic hormone secreted by the pituitary gland, begins to secrete large amounts
of estrogen and progesterone. Unless fertilization of the ova occurs, the corpus luteum
persists for about two weeks, after which time it begins to degenerate. Progesterone is
the other female hormone. Its principal effect is on the endometrium. Progesterone
causes the endometrium to secrete a nutrient fluid to nourish the ovum under its
implantation, to deposit fats and glycogen in the endometrium, and to increase the
blood supply to the endometrium. Progesterone also prepares the breasts for the
secretion of milk and inhibits contractions of the uterus, since contractions might expel
the ovum. Thus, if fertilized, the ovum would be able to stay in the uterus.
6-13. THE FEMALE'S MENSTRUAL CYCLE
The rhythmical cycle of events in the female's reproductive system is known as
the menstrual cycle. The menstrual cycle depends on the interplay of the hypophyseal
gonadotropins and the estrogens. At the beginning of the cycle, estrogen levels are
low. Because estrogens act to inhibit the pituitary's production of the follicle-stimulating
hormone (FSH), the FSH level is allowed to increase. The increase in the FSH acts on
the ovaries to stimulate the production of estrogens. The level of estrogens as
produced by the follicles then increases, causing a drop in the FSH level. At midcycle,
the luteinizing hormone (LH) is secreted by the pituitary gland. The luteinizing hormone
stimulates ovulation, followed by the conversion of the follicle to a corpus luteum and
the secretion of estrogen and progesterone by the corpus luteum. The high levels of
progesterone cause a decrease in secretion of the luteinizing hormone. If the egg is not
fertilized by a sperm cell, the corpus luteum degenerates, causing a drop in levels of
both estrogen and progesterone that completes the cycle. This drop in estrogen and
progesterone levels causes the endometrium to degenerate and slough off and also
causes small hemorrhages in the uterus. This is the cause of the periodic menstrual
flow in women.
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