The male reproductive system has evolved for continuous lifelong gametogenesis coupled with occasional internal insemination with a high density of sperm (>60 × 106/ mL in 3–5 mL of semen). In adult men the basic roles of gonadal hormones are (1) support of gametogenesis (spermatogenesis), (2) maintenance of the male reproductive tract and production of semen, and (3) maintenance of secondary sex characteristics and libido. There is no overall cyclicity of this activity in men.
The Testis
Unlike the ovaries, the testes reside outside the abdominal cavity in the scrotum (
Fig. 44.1). This location maintains testicular temperature at about 2°C lower than body temperature, which is crucial for optimal sperm development. The human testis is covered by a connective tissue capsule and divided into about 300 lobules by fibrous septa (
Fig.
44.2 ). Within each lobule are two to four loops of seminiferous tubules. Each loop empties into an anastomosing network of tubules called the rete testis. The rete testis is continuous with small ducts, the efferent ductules, that lead the sperm out of the testis into the head of the epididymis on the superior pole of the testis (see
Fig. 44.2 ). Once in the epididymis, the sperm pass from the head, to the body, to the tail of the epididymis and then to the vas (ductus) deferens. Viable sperm can be stored in the tail of the epididymis and vas deferens for several months.
• Fig. 44.1 Anatomy of the male reproductive system. (Modified from Drake RL et al. Gray’s Anatomy for Students. Philadelphia: Churchill Livingstone; 2005.)
The presence of the seminiferous tubules creates two compartments within each lobule: an intratubular compartment, which is composed of the avascular seminiferous epithelium of the seminiferous tubule, and a peritubular compartment, which is composed of neurovascular elements, connective tissue cells, immune cells, and the interstitial cells of Leydig, whose main function is to produce testosterone (
Fig. 44.3).
The seminiferous tubule is lined by a complex seminiferous epithelium composed of two cell types: sperm cells in various stages of spermatogenesis and the Sertoli cell, which is a “nurse cell” in intimate contact with all sperm cells (Fig. 44.4
Spermatogenesis involves the processes of mitosis and meiosis. Stem cells called spermatogonia reside at the basal level of the seminiferous epithelium (see
Fig. 44.4,
SA and SB). Spermatogonia divide mitotically to generate daughter spermatogonia (spermatocytogenesis). One or more spermatogonia remain within the stem cell population, firmly adherent to the basal lamina. However, the majority of these daughter spermatogonia enter meiotic division, which results in haploid spermatozoa on completion of meiosis. These divisions are accompanied by incomplete cytokinesis such that all daughter cells remain interconnected by a cytoplasmic bridge. This configuration contributes to the synchrony of development of a clonal population of sperm cells. Spermatogonia migrate apically away from the basal lamina as they enter the first meiotic prophase. At this time they are called primary spermatocytes (see
Fig. 44.4 , S1). During the first meiotic prophase the hallmark processes of sexual reproduction involving chromosomal reduplication, synapsis, crossing over, and homologous recombination take place. Completion of the first meiotic division gives rise to secondary spermatocytes, which quickly (i.e., within 20 minutes) complete the second meiotic division. The initial products of meiosis are haploid spermatids (see