Development and differentiation of the external genitalia (also from the sexually indifferent stage) occur at the same time and result from the action of dihydrotestosterone (DHT), a product of the conversion of testosterone by 5reductase. Without DHT, regardless of the genetic or gonadal sex, the external genitalia will develop along the female template. The appearance of testosterone, MIF, and DHT in the developing male embryo determines its hormonal sex. (Folder 22.1)
At approximately the 26th week of gestation, the testes descend from the abdomen into the scrotum. This migration of testes is caused by differential growth of the abdominal cavity combined with the action of testosterone that causes shortening of the gubernaculum, the testosterone-sensitive ligament connecting the inferior pole of each testis with the developing scrotum. The testes descend into the scrotum by passing through the inguinal canal, the narrow passage  between the abdominal cavity and the scrotum. During  descent, the testes carry their blood vessels, lymphatic vessels, and nerves as well as their principal ex-current duct system, the ductus deferens with them. Descent of the testis is sometimes obstructed, resulting in cryptorchidism, or undescended testes. This condition is common (30%) in premature newborns and about 1% of fullterm newborns. Cryptorchidism can lead to irreversible histologic changes in the testis and increases the risk of testicular cancer. Therefore, an undescended testis requires surgical correction. Orchiopexy (placement in the scrotal sac) should be performed, preferably before histologic changes become irreversible at approximately 2 years of age.
Spermatogenesis requires that the testes be maintained below normal body temperature.
As the testes descend from the abdominal cavity into the scrotum, they carry with them blood vessels, lymphatic vessels, autonomic nerves, and an extension of the abdominal peritoneum called the tunica vaginalis, which covers their anterolateral surface. Within the scrotum the temperature of the testes is 2C to 3C below body temperature. This lower temperature is essential for spermatogenesis, but is not required for hormone production (steroidogenesis), which can occur at normal body temperature. If the testes are maintained at higher temperatures (e.g., because of fever) or if they fail to descend into the scrotum, sperm are not produced.
Each testis receives blood through a testicular artery, a direct branch of the abdominal aorta. It is highly convoluted near the testis, where it is surrounded by the pampiniform venous plexus, which carries blood from the testis to the abdominal veins. This arrangement allows heat exchange between the blood vessels and helps maintain the testes at a lower temperature. The cooler venous blood returning from the testis cools the arterial blood before it enters the testis through a countercurrent heat exchange mechanism. In addition, the cremaster muscle, whose fibers originate from the internal abdominal oblique muscle of the anterior abdominal wall, responds to changes in ambient temperature. Its contraction moves the testes closer to the abdominal wall, and its relaxation lowers the testes within the scrotum. Cold temperatures also cause contraction of a thin sheet of smooth muscle (dartos muscle) in the superficial fascia of the  scrotum. Contraction of dartos
FIGURE 22.3 • Schematic diagram of male sex development and hormonal infuence on developing reproductive organs.
This diagram illustrates three levels on which the sex of the developing embryo is determined. The genetic sex is determined at the time of fertilization; gonadal sex is determined by activation of the SRY gene located on the short arm of chromosome Y; and hormonal sex is determined by a hormone secreted by the developing gonad. The diagram shows the influence of Mlerianinhibiting factor (MIF), testosterone, and dihydrotestosterone (DHT) on the developing structures.
 FOLDER 22.1 Functional Considerations: Hormonal Regulation of Spermatogenesis