s / Placenta 36 (2015) A1eA60 A2 IFPASA. FETAL MEMBRANES e A FOCUS FOR THE FUTURE OR A LEGACY FROM THE PAST? A.M. Carter. University of Southern Denmark, Denmark The fetal membranes of amniotes are allantois, amnion, chorion and yolk sac. The manner of their development and contribution to placentation varies greatly acrossmammals. This is true also of the cavities they enclose, which are the allantoic sac, amniotic sac and exocoelom. In human development, there is an allantoic stalk, but no allantoic sac; the yolk sac is a short-lived structure and the exocoelom is obliterated in the second trimester by expansion of the amnion. The mouse is commonly used as a model of human placentation. Here, too, there is no allantoic sac and the exocoelom does not persist to term. In contrast, the inverted vascular yolk sac supports the embryo until the chorioallantoic placenta is formed and it acts as an accessory placenta through term. It has a string of important functions yet frequently is discarded or ignored either through ignorance or because it has no obvious equivalent in human pregnancy. In ruminants, which are important to veterinary research and as models in fetal physiology, there is yet another pattern that includes transient yolk sac placentation and a persistent allantoic sac. Embryologists of the late 19C, like Hubrecht and Selenka, uncovered the role of fetal membranes in early development and placentation. They feature prominently in the mid-20C accounts of Amoroso and

are the amnion, chorion, allantois, and yolk sac. The amnion and the chorion are the chorioamniotic membranes that make up the amniotic sac, which surrounds The fetal membranes are the four extraembryonic membranes associated with the developing embryo and fetus in humans and other mammals. They are the amnion, chorion, allantois, and yolk sac. The amnion and the chorion are the chorioamniotic membranes that make up the amniotic sac, which surrounds and protects the embryo. The fetal membranes are four of six accessory organs developed by the conceptu The fetal membranes are the four extraembryonic membranes associated with the developing embryo and fetus in humans and other mammals. They are the amnion, chorion, allantois, and yolk sac. The amnion and the chorion are the chorioamniotic membranes that make up the amniotic sac, which surrounds and protects the embryo. The fetal membranes are four of six accessory organs developed by the conceptus that are not part of the embryo itself; the other two are the placenta and the umbilical cord.

first form. It fills with amniotic fluid, which causes the amnion to expand and become the amniotic sac that provides a protective environment for the developing The amnion (pl.: amnions or amnia) is a membrane that closely covers human and various other embryos when they first form. It fills with amniotic fluid, which causes the amnion to expand and become the amniotic sac that provides a protective environment for the developing embryo. The amnion, along with the chorion, the yolk sac and the allantois protect the embryo. In birds, reptiles and monotreme The amnion (pl.: amnions or amnia) is a membrane that closely covers human and various other embryos when they first form. It fills with amniotic fluid, which causes the amnion to expand and become the amniotic sac that provides a protective environment for the developing embryo. The amnion, along with the chorion, the yolk sac and the allantois protect the embryo. In birds, reptiles and monotremes, the protective sac is enclosed in a shell. In marsupials and placental mammals, it is enclosed in a uterus. The amnion is a feature of the vertebrate clade Amniota, which includes reptiles, birds, and mammals. Amphibians and fish lack the amnion and thus are anamniotes (non-amniotes). The amnion stems from the extra-embryonic somatic mesoderm on the outer side and the extra-embryonic ectoderm or trophoblast on the inner side. In the…