Minimal concentrations of retinoic acid induce Stimulated by Retinoic Acid 8 and promote entry into meiosis in isolated pregonadal and gonadal mouse primordial germ cells Running title: Retinoic acid and meiosis in primordial germ cells

In the present study, we demonstrate that minimal concentrations of retinoic acid (RA) (≤1 nM), equivalent to the quantity contaminating serum-containing culture media, are sufficient to promote meiotic entry and progression through meiotic prophase I (MPI) stages in isolated 12.5 days postcoitum (dpc) XX and XY mouse primordial germ cells (PGCs) in culture. Similar very low RA concentrations were found to up-regulate or induce stimulated by retinoic acid 8 (Stra8) in such cells, both at mRNA and protein level. In preleptotene/leptotene germ cells, STRA8 was localized in nuclear dots that disappeared at later MPI stages. Besides Stra8, other meiotic genes such as Dmc1 and Rec8 appeared stimulated by RA directly in PGCs with similar concentration-dependent trends. Finally, we found that RA induced Stra8, Sycp3, Dmc1 and Rec8 transcripts promoting meiotic entry in culture also in pregonadal 10.5 dpc PGCs of both sexes. When cultured isolated from somatic cells, such PGCs, however, were unable to progress through MPI stages while they, after entering meiosis, progressed through MPI when cultured within aorta/gonad/mesonephros tissues. We conclude that, beside RA, germ cell intrinsic factors and other exogenous signals from the surrounding somatic cells are probably necessary for meiotic entry and progression in mouse PGCs. Summary sentence: Minimal retinoic acid concentrations promote entry into meiosis in pregonadal and gonadal PGCs.