Reproduction Abstracts

Continual spermatogenesis relies on the actions of an undifferentiated spermatogonial population that is comprised of stem cell and progenitor fractions. Self-renewal by spermatogonial stem cells (SSCs) maintains a foundational pool from which progenitor spermatogonia arise that transiently amplify in number before committing to a pathway of terminal differentiation. At present, molecular features that distinguish the stem cell state in mammalian spermatogonia are undefined. In previous studies, we identified the transcriptional repressor inhibitor of DNA binding 4 (ID4) as a putative SSC-specific marker. To examine this population in more detail, we have generated a novel transgenic mouse line in which ID4 expressing cells are marked by GFP. We found that ID4-GFP+ cells exist primarily as a subset of the type A-single pool and frequency is greatest in neonatal development then decreases in proportion during establishment of the spermatogenic lineage, eventually comprising ∼2% of the undifferentiated spermatogonial population in adulthood. Based on functional transplantation analyses with isolated ID4-GFP+ and ID4-GFP− spermatogonial fractions, we discovered that most, if not all, SSCs reside in the ID4-GFP+ spermatogonial population and the ID4-GFP− spermatogonia represent the progenitor spermatogonial population. RNA-sequencing analysis revealed that 11 and 25 genes are expressed differentially between ID4-GFP+/stem cell and ID4-GFP−/progenitor fractions respectively. Collectively, these findings provide the first definitive evidence that stem cells exist as a rare subset of the A-single pool and reveal transcriptome features distinguishing stem cell and progenitor states within the mammalian male germline. This research was supported by grant HD061665 awarded to J.M.O. from the National Institutes of Health.