A s , A pr and A al spermatogonia proliferate during part of the epithelial cycle forming many A al spermatogonia. A s spermatogonia differentiate into A1 spermatogonia. There is no fine regulation of the density of spermatogonial stem cells and consequently, in some areas, many A1 and, in other areas, few A1 spermatogonia are formed. An equal density of spermatocytes is then obtained by the apoptosis of A2, A3 or A4 spermatogonia to remove the surplus cells.
Several mechanisms are available to cope with major or minor shortages in germ cell production.
A pr and A al spermatogonia is extended.