spermatogenezis

Spermatogenezis, or spermatogenesis, is the process by which male germ cells develop into mature spermatozoa. It takes place in the seminiferous tubules of the testes and is continuous from puberty onward in most mammals.

The process consists of three overlapping phases: mitotic proliferation of spermatogonia, meiotic divisions that reduce chromosome number, and spermiogenesis, the morphogenetic transformation of round spermatids into elongated spermatozoa.

Spermatogonia include undifferentiated stem cells (Type Ad and Ap) and type B cells that enter meiosis to become primary spermatocytes. Through mitosis they replenish the stem cell pool and supply cells for differentiation.

Meiosis I yields two secondary spermatocytes; meiosis II yields four haploid spermatids. Crossing over occurs, producing genetic diversity.

Spermiogenesis converts round spermatids into spermatozoa, including chromatin condensation, acrosome formation, flagellum development, and formation of the mitochondrial sheath. Excess cytoplasm is shed as residual bodies and phagocytosed by Sertoli cells.

Hormonal regulation: the hypothalamic-pituitary-gonadal axis controls spermatogenesis through GnRH, FSH, LH, and testosterone. Sertoli cells provide structural and nutritional support; the process takes about 64 days in humans, and millions of sperm are produced daily. Sperm mature and gain motility in the epididymis.

Clinical relevance: disruptions can cause infertility; factors include genetic abnormalities (AZF regions on the Y chromosome), environmental toxins, chemotherapy, varicocele, and hormonal imbalances.