Reproductive System (continued)

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Reproductive System (continued)

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University of Southern California
Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 22 Current Lecture Endocrine Events of Ovarian & Menstrual Cycles The first and third row show changes in important hormones over the course of the ovarian cycle. The average duration of the cycle is 28 days. The 2nd row shows what the follicle looks like, and the bottom is what the endometrium or innermost layer of the uterus, looks like. The 28-day cycle can be divided into two halves – a follicular phase and a luteal phase. The follicular phase is the first 14 days, dominated by the events going on in the follicle in the ovary. The luteal phase is dominated by the corpus luteum, the remnants of the follicle after ovulation has occurred. Day 1 is the onset of menstruation, which is the first appearance of blood out of the vagina. In the first few days of the cycle, levels of estrogen are falling. This estrogen has been coming from the corpus luteum, and it is degenerating so estrogen levels are falling. But then estrogen stops falling, and begins to increase because the follicle is growing (the theca cells are secreting testosterone, which the granulosa cells convert to estradiol, and that accounts for the rising levels). There is also a positive feedback cycle involved, because the estrogen itself stimulates the granulosa cells, which are estrogen dependent, greatly increasing the levels of estrogen. As the follicle grows and secretes more estrogen, estradiol rises. In particular, there are 6-10 follicles that move on to the second phase. Through mechanisms that are not understood, one follicle begins to grow more than the others. It grows faster and faster, and the others stop growing and regress. In rare cases, two of them will ovulate simultaneously, and if they both get fertilized you get fraternal twins. So the follicle increases its output of estrogen, but ovulation occurs which injures the follicle, so estradiol levels dip. But that’s just a temporary reduction because the remnant of the follicle that stays behind in the ovary will differentiate into the corpus luteum, which pumps out estrogen and progesterone in increasingly higher levels. So the estrogen levels (purple trace in the 3rd row) increase as the corpus luteum develops. Progesterone is seen to be low in the first half because it is only secreted by the corpus luteum, which develops in the second half of the cycle. So in the third week we have high levels of progesterone and moderately high levels of estradiol. But the corpus luteum has a fixed lifespan – it will last only 10 days or so, and then degenerate unless it gets continued gonadotropin support (FSH and LH). The levels of FSH and LH are pretty low, however, except for the spike in the very middle. They reach their lowest levels in the second half of the cycle, which is due to the high output of progesterone and estrogen in the second half that feeds back and inhibits the GnRH and therefore FSH and LH secretion. These ...

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