When you tutor Biology 12, you need to explain the roles of FSH and LH. Here they are in the female.
In my previous post I mentioned that in both sexes, the hypothalamus manages reproductive function by controlling the pituitary’s release of FSH and/or LH. I went on to explain the effects of FSH and LH in the male system and how negative feedback is used to regulate their release. Now, we will explore those issues in the female.
While the male reproductive system experiences little change from week to week, the female system follows a four-week cycle. Let’s imagine it begins with menstruation, when the blood lining of the uterus sloughs off because it has not received a fertilized egg. During this stage, the hypothalamus senses a lack of sex hormones in the blood. Its answer is to order the pituitary to release FSH.
FSH takes effect in the ovary, where it activates a single follicle. As the follicle starts to mature, it releases estrogen. The estrogen initiates a new blood lining in the uterus. At the same time, the estrogen is sensed by the hypothalamus, which realizes that the follicle is maturing. The hypothalamus, following a pattern of negative feedback, stops ordering the release of FSH, since it knows by the presence of estrogen that the follicle has already been stimulated to develop.
Approaching two weeks since the start of menstruation, the follicle produces a spike of estrogen. Sensing the spike, the hypothalamus sends out a new order to the pituitary – this time, for the release of LH. The LH takes effect on the follicle, causing it to burst and release the egg. This event is ovulation. The egg begins its journey down the fallopian tube.
Having released the egg, the follicle begins its new role. It now becomes the corpus luteum. Stimulated by the new flow of LH from the pituitary, the corpus luteum begins releasing progesterone. The progesterone spurs development of the blood lining of the uterus, preparing it to receive a fertilized egg if conception occurs.
Assuming conception does not occur, the hypothalamus, sensing the progesterone in the blood produced by the corpus luteum, stops ordering the release of LH by the pituitary. Losing the stimulation of the LH, the corpus luteum ceases to produce progesterone. As the progesterone in the blood wanes, the uterus releases the blood lining; menstruation begins again. In fact, it is the lack of sex hormones in the blood that results in menstruation.
Of course, if conception does occur, a different chain of events ensues. We will explore that branch of possibilities in the next post:)
Source: Mader, Sylvia S. Inquiry into Life, eleventh edition. New York: McGraw-Hill, 2006.
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