Sperm Physiology and Assessment of Spermatogenesis Kinetics In Vivo

Abstract

Adult men produce millions of spermatozoa daily, dependent on adequate testicular environment and hormonal control. Spermatogenesis and steroidogenesis are regulated by a master switch (gonadotropin-releasing hormone [GnRH] pulse generator) that controls two separate and independent feedback systems, namely, the androgen production (luteinizing hormone[LH]-testosterone) and sperm production (follicle-stimulating hormone [FSH]-inhibin). The pituitary gonadotropins, LH and FSH, which are secreted in response to hypothalamic GnRH, stimulate the testis. Their action on germ cell development is affected by androgen and FSH receptors on Leydig and Sertoli cells, respectively. While FSH acts directly on the germinative epithelium, LH stimulates secretion of testosterone by the Leydig cells. Testosterone stimulates sperm production and virilization, and also feeds back the hypothalamus and pituitary to regulate GnRH secretion. FSH stimulates Sertoli cells to support spermatogenesis and to secrete inhibin B that negatively feedback FSH secretion. When spermatozoa leave the testis, they are neither fully motile nor able to recognize or fertilize oocytes. Human spermatozoa must migrate through the epididymis and undergo a specific maturation process in order to become a functional gamete. The epididymis is a dynamic organ which promotes sperm maturation under the influence of androgens. It also provides a place for sperm storage and plays a role in the transport of the spermatozoa from the testis to the ejaculatory duct. In addition, the epididymis protects the male gametes from harmful substances and reabsorbs both fluids and products of sperm breakdown, thus enabling the sperm to fertilize the ovum and to contribute to the formation of a healthy embryo.