Are BMP-15 and GDF-9 primary determinants of ovulation quota in mammals?

How do mammals control the number of eggs that are ovulated during the estrous and menstrual cycles? Our understanding of this fundamental process has grown in recent years as a result of intense efforts to identify and characterize the genes that control the ovulation quota. An increasing body of evidence shows that two oocyte-specific factors, bone morphogenetic protein-15 (BMP-15) and growth and differentiation factor-9 (GDF-9), play crucial roles in determining folliculogenesis, ovulation rate and litter size in sheep and mice. In this article, we review recent advances on the physiological, cellular and molecular roles of BMP-15 and GDF-9, which, potentially, link these oocyte-secreted factors to the species-specific determination of ovulation quota and litter size in mammals.