Module 3S2 Chemistry of Reproduction

2.3.3 S2 page 3

Going Beyond

There are as many differences in menstrual cycles as there are females in the world. Take some time now to do some research on common menstrual disorders such as: menorrhagia, hypomenorrhea, polymenorrhea and oligomenorrhea.

See if you can find what the difference is, specifically related to hormone control, between an estrus, or “heat period”, and a menstrual cycle.

Lesson Summary

Sexual maturation in females occurs with the onset of puberty. Puberty begins when GnRH is released from the hypothalamus and acts on the anterior pituitary to produce the reproductive pituitary hormones of FSH and LH. FSH and LH, then activate the female gonads, the ovaries, to produce the ovarian hormones, progesterone and estrogen, throughout the course of the ovarian and uterine events of the menstrual cycle.

The menstrual cycle is said to begin on the first day of bleeding and is usually a 28 day cycle. Both the ovarian and uterine cycles begin on this day. The ovarian cycle, which takes place in the ovaries, is divided into the follicular stage and the luteal stage. During the follicular stage, levels of FSH are high while stimulating normally one follicle to mature. The maturing follicle releases the ovarian hormones estrogen and progesterone. Estrogen then negatively feeds back to the pituitary to cause a reduction in the release of FSH.  Estrogen also causes the hypothalamus to secret GnRH, which then leads to an increase in the pituitary hormone of LH. LH activates ovulation of the primary oocyte from the follicle.

The end of ovulation is considered the end of the follicular stage and the start of the luteal stage.  With the release of the egg, the follicle develops into the corpus luteum under the influence of LH.  Both ovarian hormones are secreted from the corpus luteum and their increased levels feedback negatively to the anterior pituitary to inhibit production of FSH and LH. As the corpus luteum disintegrates, the ovarian hormone levels decrease in the blood. The low blood levels of progesterone and estrogen are detected by the anterior pituitary and cause an increase in FSH and LH; the cycle starts again. 

On the first day of menstrual flow, the corpus luteum degeneration causes estrogen and progesterone to be at low levels in the blood. Estrogen blood levels begin to rise as a new follicle begins to mature, releasing the ovarian hormone. When estrogen levels are high, it causes the endometrium thicken. A more rapid thickening of the endometrium lining occurs once the corpus luteum begins to produce progesterone, after ovulation has occurred. If fertilization takes place, the zygote travels through the oviduct and implants in the freshly, blood-engorged endometrial lining of the uterus. If fertilization does not occur, then there is a disintegration of the endometrium lining, and it is expelled from the body during menstruation.

Without this intricate dance of pituitary and ovarian hormones, the menstrual cycle would not take place, and there would be no possibility of fertilization.