Spermatogenesis

Spermatogenesis is the process by which diploid spermatogonia, present in the wall of the seminiferous tubule from before puberty, are transformed into haploid, motile spermatozoa; it begins at puberty under hormonal stimulation and continues, essentially without interruption, throughout adult male life. The process is conventionally divided into three phases: a multiplicative phase, a meiotic phase, and a transformation phase (spermiogenesis).

In the multiplicative phase, spermatogonia lying against the basement membrane of the seminiferous tubule divide repeatedly by mitosis; some daughter cells remain spermatogonia, replenishing the stem-cell population for future cycles, while others enlarge and commit to becoming primary spermatocytes, the largest germ cells in the tubule, which then enter the meiotic phase. Each primary spermatocyte, still diploid, completes the first meiotic division (a reductional division, separating homologous chromosomes) to yield two haploid secondary spermatocytes; each secondary spermatocyte then rapidly completes the second meiotic division (an equational division, separating sister chromatids) to yield two haploid spermatids. A single primary spermatocyte therefore ultimately gives rise to four haploid spermatids.

These spermatids are initially rounded, non-motile cells bearing little outward resemblance to a mature sperm, and must undergo an extensive process of structural remodelling called spermiogenesis to become functional spermatozoa. During spermiogenesis, the Golgi apparatus condenses to form a cap-like acrosome over the front of a greatly condensed, compact nucleus, forming the head of the sperm; the acrosome contains hydrolytic enzymes essential for penetrating the egg's coverings at fertilisation. Behind the head, mitochondria arrange themselves into a helical sheath around the base of a newly formed flagellum (tail), forming the middle piece that will power the sperm's motility, while most of the surplus cytoplasm is shed. The mature sperm that results is a streamlined cell with a head (nucleus plus acrosome), a middle piece (mitochondrial sheath) and a long tail. Once fully formed, sperm are released from their attachment to the Sertoli cells into the lumen of the seminiferous tubule, a step called spermiation, from where they pass into the epididymis for final maturation and storage. …