Spermatocytogenesis is the male form of gametocytogenesis and involves stem cells dividing to replace themselves and to produce a population of cells destined to become mature sperm. The stem cells involved are called spermatogonia and are a specific type of stem cell known as gametogonia. Three functionally separate spermatogonial cell types are recognised on the basis of the appearance of the nuclei: type A dark spermatogonia (Ad), type A pale spermatogonia (Ap), and type B spermatogonia (B).