The spermatogonial transplantation experiment can be used as an unequivocal detection assay of spermatogenic stem cells (SSCs) in both a qualitative and quantitative manner, based on their regenerative capacity. In this study, the proliferative patterns and kinetics of donor-derived GFRα1-positive spermatogonia containing potential SSCs were examined during early colonization following spermatogonial transplantation.Donor-derived GFRα1-positive cells frequently formed several aggregates of A(al(aligned)) /morula-like structures in a single spermatogenic cell patch before and on day 14 post-transplant, indicating a possible involvement in the formation of a stable spermatogenic colony at 21 days post-transplant. The appearance of these A(al) /morula-like aggregates is positively correlated with regional, high-level expression of immunoreactive GDNF signals, a ligand for GFRα1, associated with colony expansion.These data raise the hypothesis that regional GDNF signals regulate the balance between donor-derived A(al) -like cell aggregates and their differentiation in each small patch, which subsequently leads to further selection of survival colonies at later stages.
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