The Fetus Depends on the Placenta for Exchange of Gas, Nutrients, and Metabolic Byproducts

From conception until birth, the embryo and fetus depend on the mother for a supply of oxygen and nutrients and for removal of carbon dioxide and other metabolic byproducts. The embryo exchanges these substances by diffusion through the uterine fluids.

The gross appearance of the placenta of different species varies widely. In horses and pigs the placenta is diffuse and covers most of the uterine epithelium. In ruminants the placenta has rows of discrete circular-to-oval cotyledons

that are attached to approximately IOO highly vascularized caruncles in the uterine epithelium. In dogs the placenta is zonary, forming a circular band around the allantochorion of the puppy. Table 51-I lists types of placentation for different species.

In addition to differing in the amount of uterine surface to which they are attached, placentas also differ in the number of layers of cells that separate the maternal and fetal blood (see Table 51 -1). In horses, pigs, sheep, and cows the fetal chorion is applied to the maternal uterine epithelium (epitheliochorial placentation), whereas in cats and dogs the chorion is applied to the endothelium of maternal vessels (endotheliochorial placentation); in rodents and most primates the chorion invades the uterine mucosa and erodes the maternal capil­laries, so it becomes bathed by maternal blood (hemochorial placentation).

Table 51-1

Mare

Fetus

Placentation in Domestic Mammals