Activin A Accelerates the Progression of Fetal Oocytes Throughout Meiosis and Early Oogenesis in the Mouse

Activins can exert several roles in ovary development. However, little is known about their involvement in early mammalian oogenesis. In this study, we reported that activin receptors (including ActRIA, ActRIB, ActRIIA, and ActRIIB) are expressed throughout the development of the mouse ovaries from 12.5 days postcoitum (dpc) to 21 days postparturition (dpp). Moreover, we found that in vitro, the addition of activin A (ActA) to the culture medium of 12.5dpc ovarian tissues accelerated the progression of oocytes throughout meiotic prophase I stages. This result was reproduced in vivo following administration of ActA to pregnant mice. The in vitro effect of ActA was associated with increased expression of premeiotic and meiotic genes (including Dazl, Spo11, Stra8, Scp3, and Rec8) in the ovarian tissues. Mechanistically, ActA-dependent SMAD3 signaling modulated the expression of members of the retinoic acid (RA) system, including the RA degradation CYP26B1 enzyme and the RA receptors. Finally, ActA promoted the survival and growth of fetal and early postnatal oocytes and primordial follicle assembly both in vitro and in vivo. In conclusion, the present study identifies new roles of ActA in early oogenesis and suggested that ActA and RA might cooperate in promoting meiosis in female germ cells.