Efficiency of spermatogenesis in animals and humans

Spermatogenesis is a lengthy process of life involving cell division and cell differentiation by which spermatozoa are produced in seminiferous tubules. Efficiency of spermatogenesis is the estimated number of spermatozoa produced per day per gram of testicular parenchyma. This measure is not influenced by species differences in testicular size, but it is influenced by species differences in the numerical density of germ cells and in the life span of these cells. Activity of spermatogonia, spermatocytes, and spermatids partition spermatogenesis into three major divisions: spermatocytogenesis, meiosis, and spermiogenesis, respectively. The spermatogenic cycle or cycle of the seminiferous epithelium is superimposed on these three major divisions of spermatogenesis. Spermatogenesis and germ cell degeneration can be quantified from numbers of germ cells in various steps of development throughout spermatogenesis, and these quantitative measures are related to number of spermatozoa in the ejaculate. Germ cell degeneration occurs throughout spermatogenesis; however, the greatest impact occurs during spermatocytogenesis and meiosis. Efficiency of spermatogenesis is influenced by the amount of germ cell degeneration, pubertal development, season of the year, and aging of the animal. The number of Sertoli cells is related to the efficiency of spermatogenesis. Hormones, growth factors, and local factors are involved in the regulation of spermatogenesis, including its efficiency. Future studies may employ in vitro methods, emergent molecular biological technologies, and unique animal models to obtain basic information needed to better understand and manipulate growth and control the efficiency of spermatogenesis.