Spermatogenesis

1. Mitotic phase (spermatocytogenesis). Spermatogonial stem cells on the basement membrane divide to renew themselves (type A dark — reserve; type A pale — renewing) and to produce type B spermatogonia, which divide and differentiate into primary spermatocytes (diploid, 2n, with duplicated chromatids — 4C). Daughter cells remain connected by intercellular bridges, so clones develop synchronously.

2. Meiotic phase. Primary spermatocytes move through the blood–testis barrier into the adluminal compartment and enter a long prophase I (≈3 weeks, the stage with the largest cells), during which homologous chromosomes pair and cross over. Meiosis I produces two secondary spermatocytes (haploid, n, 2C), which are short-lived and quickly complete meiosis II to form four haploid spermatids (n, 1C). Half carry an X and half a Y chromosome.

3. Spermiogenesis transforms round spermatids into spermatozoa without further division: the Golgi phase (formation of the acrosomal vesicle), cap phase (acrosome spreads over the nucleus), acrosome phase (nuclear condensation and elongation; histones replaced by protamines; the flagellum grows from the distal centriole; mitochondria gather in the midpiece) and maturation phase (excess cytoplasm discarded as a residual body). 4. Spermiation releases the spermatozoa into the lumen.