Studies on the equine placenta II. Ultrastructure of the placental barrier.

In early pregnancy the equine placenta consists of a simple apposition of fetal and maternal epithelia, but it becomes more complex with the formation of microcotyledons between 75 and 100 days of gestation. Although the placental barrier maintains an epitheliochorial arrangement throughout the course of pregnancy, a thinning of the maternal epithelium and a progressive indentation of the chorionic epithelium by fetal capillaries shortens the length of the diffusion pathway and reduces the amount of placental tissue between fetal and maternal bloodstreams. These structural modifications may reflect the changing requirements of the fetus for O2 and other metabolites as gestation proceeds. During the first 200 days of pregnancy there is evidence of intense pinocytotic activity by the cells of the trophoblast. From the 100th day of pregnancy there is a pronounced development of smooth endoplasmic reticulum, while rough endoplasmic reticulum and irregular, dense, membrane-bound bodies are a prominent feature of the paranuclear cytoplasm from Day 200. These changes suggest that the cells of the trophoblast become more highly involved in synthetic processes with increasing gestational age.