Anatomy & Physiology of Male Reproduction

Compared to other species, human males have relatively poor sperm producing capacity and human testicular function is very sensitive to a wide variety of environmental insults. This may be related to the human (upright) posture and hydrostatic pressure on venous testicular outflow, or other unknown factors, but it is necessary for clinicians to be aware of the high incidence of subfertility in men. Perhaps it is a reflection of the incredible ability of humans to adapt the environment to promote their own survival or the expectation that fertility should be nearly spontaneous, but many human couples seek evaluation for infertility.

The human male reproductive system includes the hypothalamic-pituitary-testis axis as well as the epididymis, vas deferens, seminal vesicles, prostate and urethra. Production of spermatozoa requires approximately 3 months from the initial mitotic divisions through the myriad changes readying sperm for ejaculation and fertilization. Highlights of this transformation include (1) the unique environment created within the testis for spermatogenesis to occur; (2) preservation of a set of stem cells relatively resistant to external injury and able to produce rapidly proliferating germ cells destined to become spermatozoa; (3) meiosis, that results in formation of the haploid gamete; and (4) the dramatic differentiation of the prospective gamete in a form that is specialized to transport chromosomal material in a structure ideally suited for transit of the female reproductive tract. The spermatozoon resulting from this complex process assumes its final shape and size in the testis. In the normal state, it also acquires the ability to fertilize as well as a capacity for motility in the epididymis. Unfortunately, the mechanisms by which the epididymis exerts these changes on the traversing spermatozoon and the actions of the human reproductive tract after relief of chronic obstruction remain largely unknown.