Eutheria: Epitheliochorial Placentation Pig and Horse

In this form of placentation the uterine epithelium and trophectoderm come into contact, the fetal and maternal microvilli interdigitate and placental development essentially involves a vast increase in area with no loss of any layers between the fetal and maternal bloodstreams. The exclusively cellular layers may become exceedingly attenuated, reducing the diffusion distance as pregnancy proceeds, but all persist to term (Amoroso 1952; Steven 1975a, 1983; Ramsey 1982; Mossman 1987).

The uterine glands are numerous and actively secreting through pregnancy (Bazer and First 1983), and there are fetal expansions over the gland mouths, forming areolae consisting of cells specialized for histotrophic absorption. The domestic pig placenta will be described as the basic type, and variations on that pattern considered for the horse, American mole (Scalopus) and the primates (lemurs and Galago).