Stark, K.; Hatkevich, T.; Miao, E. A.; Souma, T.; Capel, B.

In mammals, a small population of spermatogonial stem cells (SSCs) is established shortly after birth. These cells self-renew and produce sperm for the entirety of a male's reproductive lifespan, passing the genome on to the next generation. Thus, establishment of a population of SSCs with high genomic integrity is essential. SSCs are derived from a much larger precursor population of male germ cells (MGCs) that differentiate during fetal life. During the last third of gestation, MGCs undergo a prolonged period of G0 cell cycle arrest during which they sustain high levels of transcription and acquire epigenetic programming for SSC fate. Although these differentiation steps can cause cellular and genomic damage, it has been unclear whether selection for germ cell quality occurs during G0 arrest since no classic markers of cell death have been detected. In this study, we utilize a mouse model to characterize a population of MGCs that begin to accumulate markers if cell death, such as AnnexinV (AnV) and propidium iodide (PI), at E16.5. The AnV- and PI-positive MGC population is characterized by low expression of the RNA-binding protein, Dead End 1 (DND1), and exhibit dsDNA breaks and mitochondrial dysfunction. Interestingly, we do not see evidence of an active cell death cascade until the time of birth, where we see phosphorylation of MLKL, a hallmark of a necroptotic cell death mechanism. Based on these findings, we propose that variable cellular health is an important basis for selection of the SSC precursors.