Human reproduction depends on the integrated action of hormones, the nervous system, and the reproductive system. Gametes are the cells involved in sexual reproduction produced by the gonads, and contain only one copy of each chromosome. A failure in the production of healtly sperm in the semen is termed male infertility.

Male gonads are the testes, which produce both sperm (the male mature gamete is spermatozoa) and male sex hormones. The testes are about 1.9 in. (5 cm) in length and 1.2 in. (3 cm) in diameter, and are enclosed in a tough, white fibrous capsule. The testes are complex organs containing a variety of different cell types that are structurally and functionally compartmentalized, thereby properly allowing the production of spermatozoa (spermatogenesis). The testes are microscopically organized into tubules named seminiferous tubules. About 273 yd. (250 m) of tubules are packed into each testis.

Spermatogenesis (sperm...