Imprinting and the evolution of the placenta

Abstract

The evolution of viviparity depends on the evolution of an efficient system of maternal-fetal exchange. Of the three fetal membranes--the amnion, the allantois, and the yolk sac--only the latter two form a placenta. All mammals depend on maternal-fetal exchange and in marsupials and eutherians the chorion fuses with the yolk sac to form a chorio-vitelline placenta or the allantois to form a chorio-allantoic placenta. The definitive mammalian placenta is a highly variable structure and may either be invasive or make only a superficial attachment to the maternal endometrium, but in all cases is critical for the survival of the fetus. Genomic imprinting is a widespread epigenetic phenomenon in ..