Distribution of the spermatogonial subpopulations during the stages of the epithelial cycle

The maintenance of seminiferous epithelium function hinges on the biological activity of spermatogonial stem cells (SSCs), they play a pivotal role in maintaining spermatogenesis, the process by which spermatozoa are produced, and ensuring continuous sperm production throughout a male's reproductive life. Despite their critical importance, our understanding of SSC biology, particularly their spatial distribution within the testicular microenvironment, remains limited, prompting investigations into their precise localization and the existence of specialized microenvironments known as spermatogonial stem cell niches. In rodents, research has revealed that undifferentiated spermatogonia, including SSCs, exhibit non-random distribution within the seminiferous tubules, preferring areas adjacent to interstitial regions rather than tubule-tubule contact regions at specific stages of the seminiferous epithelium cycle (SEC). Here, we present a pioneering investigation into the topological localization of undifferentiated spermatogonia (undiff-SPG) in primates, specifically Macaca Fascicularis, using immunofluorescence (IF) and immunohistochemistry (IHC) analyses with specific markers. Remarkably, our examination of undiff-SPG subsets' relative positioning along the basal membrane in relation to interstitial tissue uncovers significant distinctions. While PIWIL4+ and NANOS3+ undiff-SPG exhibit random distribution along the basal compartment across all SEC stages, GFRA1+ undiff-SPG display stage-dependent localization patterns. During the first half of the cycle, GFRA1+ undiff-SPG preferentially inhabit regions adjacent to the interstitium, while in the second half, they predominantly occupy tubule-tubule contact regions. This spatial organization appears to be coordinated with the stages of the SEC, suggesting dynamic regulation of SSC behavior throughout the process of sperm production. Our study contributes to the growing body of literature aimed at deciphering the complexities of SSC biology and the regulation of spermatogenesis in mammalian species, with implications for understanding male fertility, reproductive health, and the development of novel therapeutic interventions targeting SSCs.