Hormonal control of reproductive cycles.docx

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  In both male and female humans, the coordinated actions of hormones from the hypothalamus, anterior pituitary, and gonads govern reproduction. The hypothalamus secretes gonadotropin-releasing hormone (GnRH), which then directs the anterior pituitary to secrete the gonadotropins: follicle stimulating hormone (FSH) and luteinizing hormone (LH). They are called gonadotropins  because they act on the male and female gonads, and they support gametogenesis by stimulating sex hormone production. The main sex hormones are steroid hormones. They consist of androgens , principally testosterone ; estrogens , principally estradiol ; and progesterone . The gonads are the major source of sex hormones. HORMONAL CONTROL OF THE MALE REPRODUCTIVE SYSTEM FSH and LH direct spermatogenesis by acting on different types of cells in the testis. FSH stimulates Sertoli cells located within the seminiferous tubules to nourish developing sperm. LH causes Leydig cells scattered in connective tissue between the tubules to produce testosterone and other androgens, which promote spermatogenesis in the tubules. Two negative-feedback mechanisms control sex hormone production in males. Testosterone regulates blood levels of GnRH, FSH, and LH through inhibitory effects on the hypothalamus and anterior pituitary. In addition, inhibin , a hormone that in males is produced by Sertoli cells, acts on the anterior pituitary gland to reduce FSH secretion. Together, these negative-feedback circuits maintain androgen levels in the normal range. Leydig cells have other roles besides producing testosterone. They in fact secrete small quantities of many other hormones and local regulators, including oxytocin, renin, angiotensin,  corticotropin-releasing factor, growth factors, and prostaglandins. These signals coordinate the activity of reproduction with growth, metabolism, homeostasis, and behavior. HORMONAL CONTROL OF FEMALE REPRODUCTIVE CYCLES  There are two closely linked reproductive cycles in human females. The uterine cycle  (or menstrual cycle) refers specifically to the changes that occur once a month in the uterus. This cyclic changes in the uterus are controlled by the ovarian cycle , cyclic events that occur in the ovaries. Thus, the female reproductive cycle is actually an integrated cycle involving two organs: the uterus and the ovaries. Hormone activity links the two cycles, synchronizing ovarian follicle growth and ovulation with the establishment of a uterine lining that can support embryonic development. The ovarian cycle begins (1) with the release of GnRH which stimulates the anterior pituitary to (2) secrete small amounts of FSH and LH. (3) These gonadotropins stimulate follicles to grow, and the (4) cells of these growing follicles start to make estradiol. During the follicular phase, there is a slow rise in estradiol concentration. The low levels of estradiol inhibit secretion of the pituitary hormones, keeping FSH and LH relatively low. (5) When estradiol secretion by the growing follicle begins to rise steeply, (6) FSH and LH levels increase markedly. This stimulates the hypothalamus to increase its output of GnRH. High estradiol concentrations also increase the GnRH sensitivity of LH-releasing cells in the pituitary gland, resulting in a further increase in LH levels. (7) The maturing follicle, containing a fluid-filled cavity, forms a bulge at the surface of the ovary. The follicular phase ends at ovulation, about a day after the LH surge. In response to both FSH and the peak in LH level, the follicle and adjacent wall of the ovary rupture, releasing the secondary oocyte. After ovulation, (8) luteinizing hormone stimulates the follicular tissue left behind in the ovary to transform into a corpus luteum. Under continued stimulation by LH, the corpus luteum secretes progesterone and estradiol, which exert negative feedback on the hypothalamus and pituitary. This feedback reduces the secretion of LH and FSH to very low levels, preventing another egg from maturing when a pregnancy may already be under way. If pregnancy does not occur, low gonadotropin levels at the end of the luteal phase cause the corpus luteum to disintegrate, triggering a sharp decline in estradiol and progesterone concentrations. The decreasing levels of ovarian steroid hormones liberate the hypothalamus and pituitary from the negative-feedback effect of these hormones. The pituitary gland can then begin to secrete enough FSH to stimulate the growth of new follicles in the ovary, initiating the next ovarian cycle. Prior to ovulation, ovarian steroid hormones stimulate the uterus to prepare for support of an embryo. Estradiol secreted in increasing amounts by growing follicles signals the endometrium to thicken. After ovulation, (9)   the estradiol and progesterone secreted by the corpus luteum stimulate maintenance and further development of the uterine lining, including enlargement of arteries and growth of endometrial glands which secrete a nutrient fluid that can sustain an early embryo. If an embryo has not implanted in the endometrium by the end of the secretory phase, the corpus luteum disintegrates. The resulting drop (10)   in ovarian hormone levels causes arteries in the  endometrium to constrict, causing the uterine lining to disintegrate. The result is menstruation — the menstrual flow phase of the uterine cycle. During this phase, a new group of ovarian follicles begin to grow. By convention, the first day of flow is designated day 1 of the new uterine (and ovarian) cycle.
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