Fig. 2. A scheme of spermatogonial multiplication and stem cell renewal in rodents. Spermatogonial stem cells (SSCs, purple) can either (A) self renew by forming two new single cells (A-single or A_s spermatogonia) or (B) can differentiate, in which case the daughter cells stay together to form a pair of spermatogonia (A-paired or A_pr spermatogonia; pink) that are linked by a cytoplasmic bridge. The A_pr spermatogonia divide to form a chain of four spermatogonia (A-aligned or A_al; A_al4) that subsequently form a chain of eight spermatogonia (A_al8). After a total of nine to ten divisions, spermatocytes form. Spermatocytes give rise to spermatids following meiotic division, and the spermatids then transform into spermatozoa, which exit the seminiferous tubules through the lumen.