Comparative Studies of Structure and Function in Mammalian Placentas with Special Reference to Maternal-Fetal Transfer of Iron

Abstract

The mammalian chorioallantoic placenta is an extremely complex and structurally diverse organ. To better understand the placenta a number of classification schemes have been devised. Classification on the basis of gross form or shape recognizes four major categories—diffuse, cotyledonary, zonary and discoid. Another important classification scheme is based on the number of tissue layers separating the maternal and fetal vascular systems in the placenta. On this basis, three major types of placentas are recognized—epitheliochorial, endotheliochorial and hemochorial. In many species, pathways in addition to the chorioallantoic placenta exist for maternal-fetal exchange. Some species have a yolk sac placenta functioning concurrently with the chorioallantoic placenta whereas in some other species exchange may occur across a chorioamnion. Some species have accessory or paraplacental structures which function in nutrient exchange; these include hemophagous regions and areolae. In the final sections, I review how these different structural specializations come into play to accomplish a particular function, namely that of iron transfer to the fetus. There are at least four different mechanisms by which different species accomplish iron transfer to the fetus. These include 1) absorption of maternal transferrin-bound iron by the placenta; 2) absorption of iron by the yolk sac; 3) phagocytosis of maternal erythrocytes and 4) absorption of iron-rich uterine secretions by accessory placental structures.