Primate Spermatogonial Stem Cells and Their Niche: A Single-Cell View

Antineoplastic treatments improve the life expectancy of pediatric cancer patients, but may result in male infertility by ablating spermatogonial stem cells (SSCs). Cryopreservation of testicular tissue obtained before sterilizing treatment is a promising fertility preservation approach, but its clinical application is hindered by the lack of exclusive biomarkers to identify human SSCs and limited size of testicular biopsies. To address this, single-cell RNA-seq was performed on testis cells from immature baboons and macaques and compared to published prepubertal human and mouse testis cell data. The cross-species analysis revealed cell types analogous to human SSCs in baboons and macaques, which differed from mouse SSCs. The correlation of molecular definitions of human spermatogonia with histological definitions of Adark/Apale spermatogonia indicated that all human SSCs are Adark. Furthermore, differences in testicular somatic cell populations between primates and rodents were found, including presence of peripheral glial cells and monocytes in humans. In contrast to mice, primate Sertoli cells segregated into distinct clusters enriched for either ribosomal or mitochondrial gene expression did not include a proliferative subpopulation. Additionally, sets of ligands produced in the somatic cell populations with receptor expression in human SSCs were examined. Cross-species comparison revealed seven ligand-receptor pairs conserved in primates but distinct from mice, including components of activin signaling (INHA-ACVR1B, INHBA-ACVR1B) and signaling to the receptor SDC2 (MMP2-SDC2 FN1-SDC2 and TGFB1-SDC2). Overall, improved identification of prepubertal human SSCs and unique components of their niche may be leveraged for advancing propagation of these cells in vitro for clinical purposes.