Epitheliochorial

Epitheliochorial placentation is a form of placentation in which the fetal chorion comes into contact with the maternal uterine epithelium, and the maternal epithelium remains intact. In this arrangement the maternal epithelium, stroma, and endothelium lie between the maternal blood and the fetal circulatory system, and the fetal side consists of the chorionic epithelium (trophoblast) with its underlying mesoderm and endothelium. Overall, there are six tissue layers separating maternal and fetal blood.

This placentation is considered non-invasive because there is no destruction or deep invasion of maternal tissues by fetal trophoblast. The exchange between maternal and fetal blood occurs across a relatively thicker barrier and relies primarily on diffusion and other transport mechanisms rather than direct cellular invasion.

Occurrence and variation across species: epitheliochorial placentation is characteristic of several domestic mammals, most notably in cattle, sheep, goats, and pigs. In these species, the placental organization can differ in its overall shape and attachment pattern, with cotyledonary placentation common in ruminants and diffuse placentation typical of pigs. The type is contrasted with endotheliochorial and hemochorial placentation, where the maternal epithelium is breached or maternal blood comes into direct contact with fetal tissues, respectively.

Functional implications: the thicker placental barrier in epitheliochorial placentation generally results in less rapid or efficient transfer of certain nutrients and gases compared with more invasive placentation types. Nonetheless, it supports successful gestation in the species that possess it and reflects a long-term evolutionary strategy balancing placental exposure to the maternal immune system with fetal resource needs.