Describe the Sequence of the Ovarian Cycle. How Do Hormones Regulate the Cycle?
Progesterone maintains the endometrium to help ensure pregnancy. The trip through the fallopian tube takes about seven days. At this stage of development, called the morula, there are 30-60 cells. If pregnancy implantation does not occur, the lining is sloughed off. After about five days, estrogen levels rise and the menstrual cycle enters the proliferative phase. The endometrium begins to regrow, replacing the blood vessels and glands that deteriorated during the end of the last cycle.
Estradiol and progesterone are steroid hormones that prepare the body for pregnancy. Estradiol produces secondary sex characteristics in females, while both estradiol and progesterone regulate the menstrual cycle.
If the oocyte is not fertilized, the corpus luteum has a lifespan of only two weeks, after which it degenerates into a corpus albicans. As the levels of progesterone, estrogens, and inhibin decrease during this phase, GnRH, FSH, and LH release increases because of the lack of feedback suppression by the ovarian hormones. Then follicular growth resumes and a new ovarian cycle begins (Grudzinskas, J.G., 1995).
Concentrating on information derived from studies in women and in rhesus and marmoset monkeys, this paper examines some of the hormonal mechanisms underlying the primate ovarian cycle with particular reference to the factors controlling preovulatory follicular development during the follicular phase.
Adashi, E.Y. Ovulation: Evolving Scientific and Clinical Concepts. New York: Springer Verlag, 2000.
Grudzinskas, J.G., and J. Yovich, eds. Gametes: The Oocyte. Cambridge: Cambridge University Press, 1995.
Heffner, Linda G. Human Reproduction at a Glance. Oxford: Blackwell Science, 2001.