Spermatogonia are male gonadal stem cells arising from pluripotent gonocytes during fetal and early post-natal development and are the basis for spermatogenesis after puberty. Failure to establish the spermatogonial pool can lead to testicular cancer and/or infertility and spermatogonial stem cells allow recovery of fertility after gonadotoxic damage. In this project, we have generated single cell RNA-sequencing data of marmoset testes at different post-natal developmental time points to identify the cell-cell interactions leading to transcriptional changes triggering male germline differentiation. The direct impact of irradiation regimes, which are applied for treatment of boys with e.g. leukaemia, will be assessed using in vitro 3D models. Thereby this project will provide insight into the molecular regulation of early germ cell dynamics and the impact caused by gonadotoxic treatments to prospectively develop strategies to protect spermatogonia.