Spermatogenesis

Initially, the primordial sex cells (PGC) lie close together in the coelomic epithelium. They move into the gonadal cords, where they divide and give rise to type A1 spermatogonia. These cells are smaller than the PGC and are in direct contact with the external basal lamina of the testicular cords. They lie between the Sertoli cells, which provide them with nutrients. During puberty, they divide to form further A1 spermatogonia as well as paler cells, the type A2 spermatogonia. A1 spermatogonia are therefore stem cells, which are able to regenerate or produce a new cell type. A2 spermatogonia give rise to A3, which in turn develop into A4 spermatogonia. It is likely that A2, A3 and A4 spermatogonia are also stem cells. A4 spermatogonia can be renewed, they can die off (apoptosis), or they can develop further to type B spermatogonia. Type B spermatogonia are the precursors of spermatocytes. They divide for one last time through mitosis to form primary spermatocytes. The primary spermatocytes enter the 1st maturation division (meiosis I) (Meiosis), and produce secondary spermatocytes that accomplish the 2nd maturation division (meiosis II). The resulting haploid cells are called spermatides.