Though related to depletion of ovarian follicles, the causes and process of menopause are poorly understood. Age-related changes in the CNS, including critical patterns of GnRH secretion, precede follicular depletion and may play an important role in menopause. Because follicles do not develop in response to LH and FSH secretion, estrogen and progesterone levels drop. Loss of the negative-feedback inhibition of estrogen on GnRH and LH/FSH results in a marked rise in serum LH and FSH. FSH levels rise more than LH levels. This could result from loss of ovarian inhibin.
Menopause typically occurs between 45 and 55 years of age. It extends over a period of several years. Initially the cycles become irregular and are periodically anovulatory. The cycles tend to shorten, primarily in the follicular phase. Eventually the woman ceases to cycle altogether. Serum estradiol levels drop to about a sixth the mean levels for younger cycling women, and progesterone levels drop to about a third those in the follicular phase of younger women. Production of these hormones does not cease entirely, but the primary source of these hormones in postmenopausal women becomes the adrenal, although interstitial cells of the ovarian stroma continue to produce some steroids. Most circulating estrogens are now produced peripherally from androgens. Because estrone is the primary estrogen produced in adipose tissue, it becomes the predominant estrogen in postmenopausal women.
Most symptoms associated with menopause result from estrogen deficiency. The vaginal epithelium atrophies and becomes dry, and bone loss is accelerated and may lead to osteoporosis. The incidence of coronary artery disease increases markedly after menopause. Hot flashes result from periodic increases in core temperature, which produces peripheral vasodilation and sweating. Hot flashes are thought to be linked to increases in LH release and are probably associated not with the pulsatile rise in LH secretion but rather with central mechanisms controlling GnRH release. Hot flashes typically subside within 1 to 5 years of the onset of menopausal symptoms.
1.
The reproductive systems are composed of gonads, an internal reproductive tract with associated glands, and external genitalia. Mammary glands are accessory reproductive glands in women.
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Gonads have two main functions: production of gametes and production of hormones. Hormones (primarily sex steroids) are absolutely necessary for normal function of the reproductive system, and their production is regulated by a hypothalamic-pituitarygonadal axis.
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Seminiferous tubules in the testis contain Sertoli cells and developing sperm cells.
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Spermatogenesis refers to the progression of sperm cells from spermatogonia through the processes of meiosis and spermiogenesis to form mature spermatozoa.
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Testosterone and pituitary FSH are required for normal sperm production. Only Sertoli cells express the androgen receptor and the FSH receptor, so these hormones regulate spermatogenesis indirectly through their actions on Sertoli cells. Sertoli cells produce the hormone inhibin, which negatively feeds back on pituitary FSH production.
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Sertoli cells have many functions, including production of androgen-binding protein (ABP) and fluid and creation of the blood-testis barrier.
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Leydig cells are stromal cells that reside outside the seminiferous tubules. They respond to LH by producing testosterone.
8.
Testosterone is an active androgen. It can be converted peripherally to DHT, which is more active in certain tissues (e.g., prostate), or to estradiol.
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