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C H A P T E R 3 9 Introduction to the reproductive system

point, local prostaglandins stimulate uterine contraction and the onset of labour. Once the fetus and the placenta have been expelled from the uterus, the hormone levels plummet towards the non-pregnant state. It often takes 6 to 8 weeks to reverse the effects of these hormones. This is a time of tremendous adjustment for the body as it tries to re-achieve homeostasis. Menopause The follicles contained in the ovary become depleted over time, the ovaries no longer produce oestrogen and progesterone, and menopause —the cessation of menses—occurs. The hypothalamus and pituitary produce increased levels of GnRH, FSH and LH for a while in an attempt to stimulate the ovaries to produce oestrogen and progesterone. If that does not happen, the levels of these hormones fall back within a normal range in response to their own negative feedback systems. Menopause is associated with loss of many of the effects of these two hormones on the body, including retention of calcium in the bones, lowered serum lipid levels and maintenance of secondary sex characteristics. ■■ The female ovary stores ova and produces the sex hormones oestrogen and progesterone. ■■ The hypothalamus releases GnRH at puberty to stimulate the anterior pituitary release of FSH and LH, thus stimulating the production and release of the sex hormones. Levels are controlled by a series of negative feedback systems. ■■ Female sex hormones prepare the body for pregnancy and the maintenance of the pregnancy. If pregnancy does not occur, the prepared inner lining of the uterus sloughs off as menstruation in the menstrual cycle. ■■ Menopause occurs when the supply of ova is exhausted and the woman’s body no longer produces the hormones oestrogen and progesterone. MALE REPRODUCTIVE SYSTEM The male reproductive system consists of two testes, the vas deferens, the prostate gland, the penis and the urethra. The hormone that stimulates and maintains these structures is testosterone. Structures The male reproductive system originates from the same fetal cells as in the female. The major male reproduc- tive system structure is the testes, the two endocrine glands that continually produce sperm , as well as the hormone testosterone . During fetal development, the two testes migrate down the abdomen and descend KEY POINTS

into the scrotum outside the body. There they are pro- tected from the heat of the body to prevent injury to the sperm-producing cells. The testes are made up of two distinct parts: the seminiferous tubules , which produce the sperm, and the interstitial or Leydig cells , which produce the hormone testosterone. Other components include the vas deferens, which stores produced sperm and carries sperm from the testes to be ejaculated from the body; the prostate gland, which produces enzymes to stimulate sperm maturation, as well as lubricating fluid; the penis, which includes two corpora cavernosa and a corpus spongiosum, structures that allow mas- sively increased blood flow and erection; the urethra, through which urine and the sperm and seminal fluid are delivered; and other glands and ducts that promote sperm and seminal fluid development (Figure 39.4). Hormones The primary hormone associated with the male repro- ductive system is testosterone. Testosterone is responsible for many sexual and metabolic effects in the male. Like oestrogen, testosterone enters the cell and reacts with a cytoplasmic receptor site to influence mRNA activity, resulting in the production of proteins for cell structure or function. Box 39.3 summarises the effects of testos- terone on the body. If the testes are lost before puberty occurs, there will be no development of the secondary male sex characteris- tics or the other effects seen when testosterone is released. Such a person would require testosterone replacement therapy to develop these characteristics. However, once puberty and the physical changes brought about by tes- tosterone have occurred, the androgens released by the adrenal glands are sufficient to sustain the male char- acteristics. Androgens are very similar in structure to testosterone and are able to influence cells to maintain the changes caused by testosterone. This is important information for adults undergoing testicular surgery or The activity of the male sex glands is not thought to be cyclical like that of the female. The hypothalamus in the male child is also sensitive to circulating levels of adrenal androgens and suppresses GnRH release. After the hypothalamus matures, this sensitivity is lost and the hypothalamus releases GnRH. This in turn stimu­ lates the anterior pituitary to release FSH and LH, or what is sometimes called interstitial cell–stimulating hormone (ICSH) in males. FSH directly stimulates the seminiferous tubules to produce sperm, a process called spermatogenesis. FSH also stimulates the Sertoli cells in the seminiferous tubules to produce oestrogens, which provide negative feedback to the pituitary and hypothal- amus to cause a decrease in the release of GnRH, FSH and LH. chemical castration. Control mechanisms