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Describe the Sequence of the Ovarian Cycle. How Do Hormones Regulate the Cycle?

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The first half of the ovarian cycle is the follicular phase shown in Figure 1. Slowly rising levels of FSH and LH cause the growth of follicles on the surface of the ovary. This process prepares the egg for ovulation. As the follicles grow, they begin releasing estrogens and a low level of progesterone. 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.

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The human male and female reproductive cycles are controlled by the interaction of hormones from the hypothalamus and anterior pituitary with hormones from reproductive tissues and organs. In both sexes, the hypothalamus monitors and causes the release of hormones from the pituitary gland. When the reproductive hormone is required, the hypothalamus sends a gonadotropin-releasing hormone (GnRH) to the anterior pituitary. This causes the release of follicle stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary into the blood. Note that the body must reach puberty in order for the adrenals to release the hormones that must be present for GnRH to be produced. Although FSH and LH are named after their functions in female reproduction, they are produced in both sexes and play important roles in controlling reproduction

Other hormones have specific functions in the male and female reproductive systems. A negative feedback system occurs in the male with rising levels of testosterone acting on the hypothalamus and anterior pituitary to inhibit the release of GnRH, FSH, and LH. The Sertoli cells produce the hormone inhibin, which is released into the blood when the sperm count is too high. This inhibits the release of GnRH and FSH, which will cause spermatogenesis to slow down. If the sperm count reaches 20 million/ml, the Sertoli cells cease the release of inhibin, and the sperm count increases. The control of reproduction in females is more complex. As with the male, the anterior pituitary hormones cause the release of the hormones FSH and LH. In addition, estrogens and progesterone are released from the developing follicles. Estrogen is the reproductive hormone in females that assists in endometrial regrowth, ovulation, and calcium absorption; it is also responsible for the secondary sexual characteristics of females. These include breast development, flaring of the hips, and a shorter period necessary for bone maturation. Progesterone assists in endometrial re-growth and inhibition of FSH and LH release. In females, FSH stimulates development of egg cells, called ova, which develop in structures called follicles. Follicle cells produce the hormone inhibin, which inhibits FSH production. LH also plays a role in the development of ova, induction of ovulation, and stimulation of estradiol and progesterone production by the ovaries. 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.

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The ovarian cycle is a series of events in the ovaries that occur during and after the maturation of the oocyte (egg or ovum). During their reproductive years, nonpregnant females usually experience a cyclical sequence of changes in their ovaries and uterus (Adashi, E.Y., 2000). Each cycle takes about one month and involves both oogenesis, the process of formation and development of oocyte, and preparation of the uterus to receive a fertilized ovum. Hormones secreted by the hypothalamus, anterior pituitary gland, and ovaries control the principal events. The uterine (menstrual) cycle is a concurrent series of changes in the endometrium of the uterus to prepare it for the arrival of a fertilized ovum that will develop in the uterus until birth

If fertilization does not occur, the lining (stratum functionalis) of the endometrium is shed during menstruation. The general term female reproductive cycle encompasses the ovarian and uterine cycles, the hormonal changes that regulate them, and also the related cyclical changes in the breasts and cervix. The postovulatory phase of the female reproductive cycle is the most constant in duration, lasting approximately from day 15 to 28 and represents the time between ovulation and the onset of the next menses. In the ovary, after ovulation, the LH stimulates the remnants of the mature follicle to develop into the corpus luteum, which secretes increasing quantities of progesterone and some estrogens. This is called the luteal phase of the ovarian cycle. Subsequent events in the ovary that ovulated an oocyte depend on whether or not the oocyte becomes fertilized. 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).

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In the long run, details of the basic physiological mechanisms controlling cyclic ovarian function in primates are known for only a few species

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.

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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.

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