IFPA Senior Award Lecture: Mammalian fetal membranes.

Background: Fetal membrane development varies greatly across mammals with significant implications for models of human placentation.

Method: Therefore the major patterns of fetal membrane development are reviewed with special focus on functions of the inverted yolk sac in murine rodents.

Findings: In most mammals, yolk sac and chorion form a choriovitelline placenta to support the early embryo, although this soon is supplanted by a chorioallantoic placenta. Human and haplorrhine primates follow a second pattern where precocious development of the extraembryonic mesoderm leads to formation of a secondary yolk sac within the exocoelom. In rodents there is an inverted visceral yolk sac that encloses the embryo and amnion and functions as an accessory to the chorioallantoic placenta through term. Where present, the inverted yolk sac performs a number of functions that in human are assumed by the syncytiotrophoblast of the chorioallantoic placenta. These include transfer of passive immunity, iron, cobalamin and lipoprotein; protein and lipid synthesis; haematopoiesis; and germ cell storage. Most mammals have a large, fluid-filled allantoic cavity. This is not the case in human and haplorrhine primates where there is an allantoic stalk but no allantoic cavity. Some rodents have a small allantoic cavity, but the mouse and other murine rodents do not. The evolution of amnion, yolk sac and allantois is explored.

Conclusions: Fetal membranes deserve close attention. In particular, the mouse model is incomplete unless the yolk sac is studied along with the chorioallantoic placenta.