Regulation of Testicular Temperature
Thermal Regulation of the Testes
Successful spermatogenesis is achieved at temperatures a few degrees below core body temperature. The extra-abdominal location of the testes is critical to spermatogenesis. The testes may be several Celsius degrees cooler than core body temperature.
In a number of mammalian species, including humans, the testes are extra-abdominal. In other species, the testes are intra-abdominal, located partially or entirely in the inguinal canal; in some species, the testes actually migrate from the scrotum to the inguinal canal between breeding and non-breeding season. In humans, failure of the testes to descend into the scrotum results in a condition referred to as cryptorchidism. In persons with bilateral cryptorchidism, both testes are retained within the abdomen; in persons with unilateral cryptorchidism, a single testis remains in the abdomen while the second descends to the scrotum. Males with bilateral cryptorchidism are unable to produce sperm, but my well produce androgens at normal levels, and will generally have a normal masculinizing puberty.
The gubernaculum is a ligament connecting the epididymis and the base of the scrotum. Under the influence of androgens, and perhaps gonadotropins (LH), the gubernaculum thickens and shortens to draw the testis through the inguinal canal and into the scrotum in the male fetus near term. It may be possible to induce testicular descent with androgens or hCG. hCG may act directly on the gubernaculum as well as indirectly by stimulating androgen production by the male fetus. Remember, hCG is gonadotropic and binds and activates the same receptor as LH (ie. the so called LH/hCG receptor).
Musculature of the Testis & Scrotum:
The testes are contained within a pouch of skin, known as the scrotum, outside of the abdominal cavity. Within the walls of the scrotum is a smooth muscle called the tunica dartos. Within the spermatic cord is a skeletal muscle called the cremaster muscle. While the cremaster muscle is composed of skeletal muscle fibers, its control, like that of the tunica dartos, is involuntary. These muscles work to contract the scrotum and to draw the testes towards the abdominal cavity when the testes are cold. They relax when the testes are too warm.
The Pampiniform Plexus:
The testicular artery entering the scrotum from the abdomen and the testicular vein entering the abdomen from the scrotum lie in a tortuous plexus, formed by the testicular veins above the testes. Blood entering the scrotum from the abdomen is warmer than blood leaving the testes. Countercurrent heat exchange, from blood in the testicular artery to blood in the plexus formed by the testicular veins, decreases the temperature of blood entering the testes. By the same token, blood leaving the testes is warmed contact of the testicular veins with the testicular artery along the plexus.
The skin of the scrotum tends to be rich with large apocrine sweat glands.David Currie.