title: Questions about spermatogonia posed and answered since 2000.
abstract: 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  those areas that border on interstitial tissue and, in particular, to areas  containing venules and arterioles, suggesting a specific relationship of this  localization with a possible SSC niche. Finally, gene expression studies are  showing how both extrinsic factors produced by Sertoli cells and intrinsic  factors that are products of the germ cells act either to maintain progenitor  cells or to promote differentiation and the commitment to meiosis. Taken  together, this new knowledge adds to our understanding of the balance between  2 opposing forces: one promoting the undifferentiated state and the other  promoting the commitment to meiosis and differentiation that is essential for  spermatogenesis to proceed.
authors.affiliation: Department of Endocrinology and Metabolism, Faculty of Science, University of Utrecht, Utrecht, The Netherlands.
authors.forename: Dirk G
authors.initials: DG
authors.lastname: de Rooij
authors.forename: Michael D
authors.initials: MD
authors.lastname: Griswold
mesh_terms: D000818:Animals; D002454:Cell Differentiation / Q000502:physiology*; D002999:Clone Cells / Q000502:physiology; D004847:Epithelial Cells; D008297:Male; D051379:Mice; D012671:Seminiferous Tubules / Q000166:cytology; D013091:Spermatogenesis / Q000502:physiology*; D013093:Spermatogonia / Q000378:metabolism / Q000502:physiology*; D013234:Stem Cells / Q000502:physiology*