The reproductive system in males has evolved to support continuous lifelong production of gametes, along with occasional internal insemination involving a high concentration of sperm (more than 60 × 106/mL in 3-5 mL of semen). In adult men, the primary functions of gonadal hormones are to facilitate gametogenesis (spermatogenesis), maintain the male reproductive tract and semen production, and uphold secondary sex characteristics and libido. Unlike the ovaries, the testes are located outside the abdominal cavity within the scrotum. This positioning helps maintain a testicular temperature that is approximately 2°C lower than body temperature, which is crucial for optimal sperm development. The testis is enveloped by a connective tissue capsule and is divided into around 300 lobules by fibrous septa. Each lobule contains two to four loops of seminiferous tubules. These tubules empty into an interconnected network called the rete testis, which is continuous with small ducts known as efferent ductules. The efferent ductules transport sperm out of the testis and into the head of the epididymis located at the superior pole of the testis. From the epididymis, the sperm traverse through its head, body, and tail, eventually entering the vas deferens. Viable sperm can be stored in the tail of the epididymis and vas deferens for several months. The seminiferous tubules create two compartments within each lobule: an intratubular compartment consisting of the avascular seminiferous epithelium and a peritubular compartment composed of neurovascular elements, connective tissue cells, immune cells, and the interstitial cells of Leydig, responsible for testosterone production. The seminiferous tubule is lined by a complex seminiferous epithelium comprising two types of cells: sperm cells at various stages of spermatogenesis and Sertoli cells, which have a supportive role and are in close contact with all sperm cells. Spermatogenesis involves the processes of mitosis and meiosis. Spermatogonia, stem cells found at the basal level of the seminiferous epithelium, divide mitotically to produce daughter spermatogonia through spermatocytogenesis. While some spermatogonia remain as stem cells attached to the basal lamina, the majority enter meiotic division, resulting in the production of haploid spermatozoa upon completion of meiosis. Throughout these divisions, incomplete cytokinesis leads to interconnected daughter cells through a cytoplasmic bridge, contributing to the synchronized development of a clonal population of sperm cells. As the spermatogonia progress into the first meiotic prophase, they migrate away from the basal lamina and become known as primary spermatocytes. During the first meiotic prophase, crucial processes of sexual reproduction, including chromosomal reduplication, synapsis, crossing over, and homologous recombination occur. The completion of the first meiotic division gives rise to secondary spermatocytes, which swiftly complete the second meiotic division within approximately 20 minutes. The initial products of meiosis are haploid spermatids.
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