Although the brain is not usually considered to be a component of the reproductive system, part of the brain is, in fact, essential to the function of the reproductive organs because of the hormones produced there. This part of the brain is the hypothalamus, a relatively small area that acts without conscious control. The hypothalamus is located in the lower middle of the brain; it contains centers that control eating, drinking, body temperature, and other essential functions.
Hypothalamic control over reproduction in the male is primarily by way of the hormone called gonadotropin-releasing hormone (GnRH). GnRH is released from the hypothalamus to enter blood vessels that carry it to the pituitary gland, a small gland suspended just below the hypothalamus. When GnRH arrives at the pituitary, it stimulates the pituitary to produce and release two more hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH and LH in the male are identical to hormones of the same names in the female. The names of these hormones describe their functions in the female. Like other hor mones, FSH and LH are released into the blood and circulate throughout the body. FSH and LH are called gonadotropin hormones: gonadotropin means "gonad stimulating." These are the hormones that stimulate the gonads (testes in the male, ovaries in the female) to produce sperm or eggs and to secrete gonadal hormones. In the male, the gonadal hormones are primarily testosterone and related hormones. There is a chain of hormonal commands, with GnRH from the hypothalamus at the top of the chain. GnRH stimulates the pituitary to secrete FSH and LH, which in turn stimulate the testes to produce sperm and testosterone.
In addition to the chain leading from the brain to the pituitary to the testes, information is sent back to the brain from the testes, a checks-and-balances system using principles of negative feedback to ensure that the hormones are produced in the appropriate quantities. If, for example, the hormone system gets slightly out of balance, leading to too much testosterone being produced, this excess of testosterone will be sensed by the hypothalamus. It will cause a temporary shutdown of GnRH production, leading to the system's correcting itself, because then a little less testosterone will be produced. If testosterone levels fall too low, the opposite will happen: GnRH, and then FSH and LH, and then finally testosterone, will all increase, again resulting in a correction of the original aberration. The hormonal system is a delicately balanced network that ensures the proper functioning of the testes.
The testes are the sites of sperm production. Within the testes are hundreds of tiny tubes, the seminiferous tubules, that are responsible for sperm production. The sperm develop gradually from round cells called spermatogonia, which are located in the walls of the seminiferous tubules. As a sperm matures, it develops a long, whiplike tail attached to an oval head. The head of the sperm contains chromosomes, the genetic information of the male that will be passed on to his offspring. The sperm of some mammals can be dis tinguished under the microscope by characteristic differences in their appearance.
Between the seminiferous tubules are clusters of hormone-producing cells, the interstitial or Leydig cells. The Leydig cells produce testosterone and related hormones. Testosterone is essential for proper sperm development. In addition, testosterone is responsible for the development of male body features, including, in most species, a large muscle mass, and for the growth of the reproductive organs during puberty. In some animals, testosterone is also linked to aggressive and reproductive behaviors.
The testes of most mammals are located in the scrotum, a pouch of skin and muscle that is suspended outside the abdomen. In some animals, the testes may be withdrawn into the abdomen when the animal is startled or when it is not in the breeding condition.
The function of the scrotum is to maintain the temperature of the testes at a few degrees lower than average body temperature. The capability to maintain this temperature of the scrotum is rooted in the fact that the muscles within the scrotum are responsive to temperature. Under warm conditions, the scrotum relaxes, allowing the testes to move away from the body and lose heat. In cool temperatures, the opposite occurs: The scrotum wrinkles, pulling the testes closer to the body and allowing them to stay warmer. The reduced temperature maintained by the scrotum is mandatory for the production of normal fertile sperm. Fever or other situations that raise the temperature of the scrotum can interfere with sperm production, even resulting in temporary infertility. In a few large mammals (such as elephants, whales, and dolphins), the testes are not located within a scrotum, but instead occupy a position in the abdomen. It is not known why these species apparently do not require a temperature lower than that of the body for sperm production.
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