Sperm Physiology and Assessment of Spermatogenesis Kinetics In Vivo

Abstract

Male fertility in humans depends on the continuous daily production of millions of spermatozoa. Spermatogenesis is a sequence of highly intricate stages by which an undifferentiated diploid spermatogonium matures into a specialized, genetically unique haploid spermatozoon. Spermatozoa leaving the Sertoli cells enter the epididymis for final maturation. This process requires a very elaborated interplay of autocrine, paracrine, and endocrine factors. Classically, the duration of spermatogenesis from the differentiation of pale spermatogonia to the ejaculation of mature spermatozoa has been estimated to be around 3 months. Recent data originated from a novel noninvasive method for direct measurement of human spermatogenesis kinetics in vivo, which applies isotope labeling with enriched heavy water and analysis of DNA isotopic enrichment in ejaculated sperm by gas chromatography/mass spectrometry physiology, indicate that the appearance of new sperm in the ejaculate occur much faster, at a mean time of approximately 2 months. A significant interindividual biological variability also seems to exist as the time lag for the completion of a full spermatogenic cycle and have new sperm in the ejaculate ranges from 42 to 76 days. This new knowledge on in vivo spermatogenesis kinetics is likely to change the time lines proposed for improvement or recovery in countless infertile couples undergoing male infertility treatment worldwide.