Questions About Spermatogonia Posed and Answered Since 2000

This review focuses on 3 important advances in our understanding of rodent spermatogonial stem cells (SSC) that have emerged since 2000: the identity of SSC, the existence of a SSC niche, and gene expression in spermatogonia. It is now apparent that the original scheme, in which the A(single) (A(s)) spermatogonia are the only stem cells, may be too simple. Rather, separation of pairs of A(paired) (A(pr)) Spermatogonia into singles might also play a role in the steady state situation. However, evidence that in the normal epithelium fragmentation of chains of A(aligned) (A(al)) spermatogonia into smaller clones also plays a role is not yet conclusive. New evidence presented during the last decade indicates that the A(s), A(pr), and A(al) (A(s,pr,al)) spermatogonia are not localized at random over the tubule basal lamina, as originally assumed, but are restricted to