other methods, such as gene-modified model, have greatly facilitated SSCs study. SPERMATOGONIAL STEM CELL IDENTIFICATION AND CHARACTERIZATION

As noted above, SSCs comprise a sub-fraction of undifferentiated spermatogonial cells.5 In rodents, there are three types of undifferentiated type A spermatogonial cells, classified according to cell morphological arrangement: As (single), Apr (two paired cells), and Aal (aligned cells of 4, 8, 16 and sometimes even 32 cells). The traditional model considers As cells as SSCs while Apr and Aal cells are progenitors committed to proliferate and eventually produce haploid cells. As cells will either undergo division, producing two daughter As cells, or division, producing Apr cells connected by a cytoplasmic bridge that with further division produce connected Aal cells of 4, 8, 16 and then eventually haploid cells.6 However, this classical model has been challenged in recent years.11 Firstly, not all As cells are actual SSCs and transplantation assays show that only 10 % of total As cells are able to populate the recipient testis, indicating that only a small number of As are actual stem cells.10 Additionally, Apr and Aal cells are not always transit-amplifying cells. In fact, they may act as colony-forming cells during tissue regeneration or even in normal situations, and have been proposed as potential stem cells when compared with actual stem cells. Through a tamoxifen-inducible pulse-label system, Nakagawa et al. proved that the number of colonogenic cells are more abundant than the actual stem cells after transplantation or regeneration and they determined that the transit-amplifying is the main contributor to the extra colony-forming cells.12 Furthermore, cells expressing c‑Kit, th