Characterization of undifferentiated spermatogonia and the spermatogonial niche in the lambari fish Astyanax altiparanae.

Undifferentiated type A spermatogonia are the foundation of fish spermatogenesis. This cell population includes the spermatogonial stem cell population, which is able to either self-renew or differentiate into cells that will generate the male gamete - the spermatozoa. Spermatogonia stem cells are located in a specific region of the testes known as the spermatogonial niche, which regulates spermatogonial stem cell function. This study characterizes undifferentiated type A spermatogonia and their S-phase label-retaining cell properties in Astyanax altiparanae testes. This is a fish species of substantial commercial, environmental, and academic importance. Two types of undifferentiated spermatogonia have been described in A. altiparanae testis: Aund* and Aund. Among the main differences observed, Aund* spermatogonia have an irregular nuclear envelope, decondensed chromatin, one or two nucleoli, and nuages in the cytoplasm; meanwhile, type Aund have a round nucleus. Aund* is preferentially distributed in areas neighboring the interstitial compartment, whereas Aund is located in the intertubular area. Finally, this study found that undifferentiated type A spermatogonia were able to retain BrdU over a long chase period, suggesting that these cells have a long cell cycle and potential stem cell candidates among them. Based on these findings, undifferentiated type A spermatogonia may be characterized as putative stem cells in A. altiparanae testis. This work will contribute to further studies on the stem cell biology of this promising Neotropical experimental model.