The Testis Is the Site of Sperm Formation

During embryonic stages of development, the testes lie attached to the posterior abdominal wall. As the embryo elongates, the testes move to the inguinal ring. Between the seventh month of pregnancy and birth, the testes descend through the inguinal canal into the scrotum. The location of the testes in the scrotum is important for sperm production, which is optimal at 2 to 3°C lower than core body temperature. Two systems help maintain the testes at a cooler temperature. One is the pampiniform plexus of blood vessels, which serves as a countercurrent heat exchanger between warm arterial blood reaching the testes and cooler venous blood leaving the testes. The second is the cremasteric muscle, which responds to changes in temperature by moving the testes closer or farther away from the body. Prolonged exposure of the testes to elevated temperature, fever, or thermoregulatory dysfunction can lead to temporary or permanent sterility as a result of a failure of spermatogenesis, whereas steroidogenesis is unaltered.

The testes are encapsulated by a thick fibrous connective tissue layer, the tunica albuginea. Each human testis contains hundreds of tightly packed seminiferous tubules, ranging from 150 to 250 |xm in diameter and from 30 to 70 cm long. The tubules are arranged in lobules, separated by extensions of the tunica albuginea, and open on both ends into the rete testis. Examination of a cross section of a testis reveals distinct morphological compartmentalization. Sperm production is carried out in the avascular seminiferous tubules, whereas testosterone is produced by the Ley-dig cells, which are scattered in a vascular, loose connective tissue between the seminiferous tubules in the interstitial compartment.

Each seminiferous tubule is composed of two somatic cell types (myoid cells and Sertoli cells) and germ cells. The seminiferous tubule is surrounded by a basement membrane (basal lamina) with myoid cells on its perimeter, which define its outer limit. On the inside of the basement membrane are large, irregularly shaped Sertoli cells, which extend from the basement membrane to the lumen (Fig. 37.4). Sertoli cells are attached to one another near their base by tight junctions (Fig. 37.5). The tight junctions divide each tubule into a basal compartment, whose constituents are exposed to circulating agents, and an adluminal compartment, which is isolated from bloodborne elements. The tight junctions limit the transport of fluid and macromole-cules from the interstitial space into the tubular lumen, forming the blood-testis barrier.

Located between the nonproliferating Sertoli cells are germ cells at various stages of division and differentiation. Mitosis of the spermatogonia (diploid progenitors of spermatozoa) occurs in the basal compartment of the seminiferous tubule (see Fig. 37.5). The early meiotic cells (primary spermatocytes) move across the junctional complexes into the adluminal compartment, where they mature into spermatozoa or gametes after meiosis. The adluminal compartment is an immunologically privileged site. Spermatozoa that develop in the adluminal compartment are not recognized as "self" by the immune system. Consequently, males can develop antibodies against their own sperm, resulting in infertility. Sperm antibodies neutralize the ability of sperm to function. Sperm antibodies are often present after vasec-

Adluminal Compartment

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Essentials of Human Physiology

Essentials of Human Physiology

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