Placental Structure and Classification

The placentas of all eutherian (placental) mammals provide common structural and functional features, but there are striking differences among species in gross and microscopic structure of the placenta. Two characteristics are particularly divergent and form bases for classification of placental types:

1. The gross shape of the placenta and the distribution of contact sites between fetal membranes and endometrium.
2. The number of layers of tissue between maternal and fetal vascular systems.

Differences in these two properties allow classification of placentas into several fundamental types.

Classification Based on Placental Shape and Contact Points

Examination of placentae from different species reveals striking differences in their shape and the area of contact between fetal and maternal tissue:

• Diffuse: Almost the entire surface of the allantochorion is involved in formation of the placenta. Seen in horses and pigs.
• Cotyledonary: Multiple, discrete areas of attachment called cotyledons are formed by interaction of patches of allantochorion with endometrium. The fetal portions of this type of placenta are called cotyledons, the maternal contact sites (caruncles), and the cotyledon-caruncle complex a placentome. This type of placentation is observed in ruminants.
• Zonary: The placenta takes the form of a complete or incomplete band of tissue surrounding the fetus. Seen in carnivores like dogs and cats, seals, bears, and elephants.
• Discoid: A single placenta is formed and is discoid in shape. Seen in primates and rodents.

Classification Based on Layers Between Fetal and Maternal Blood

Just prior to formation of the placenta, there are a total of six layers of tissue separating maternal and fetal blood. There are three layers of fetal extraembryonic membranes in the chorioallantoic placenta of all mammals, all of which are components of the mature placenta:

1. Endothelium lining allantoic capillaries
2. Connective tissue in the form of chorioallantoic mesoderm
3. Chorionic epithelium, the outermost layer of fetal membranes derived from trophoblast

There are also three layers on the maternal side, but the number of these layers which are retained - that is, not destroyed in the process of placentation - varies greatly among species. The three potential maternal layers in a placenta are:

1. Endothelium lining endometrial blood vessels
2. Connective tissue of the endometrium
3. Endometrial epithelial cells

One classification scheme for placentas is based on which maternal layers are retained in the placenta, which of course is the same as stating which maternal tissue is in contact with chorionic epithelium of the fetus. Each of the possibilities is observed in some group of mammals.

In humans, fetal chorionic epithelium is bathed in maternal blood because chorionic villi have eroded through maternal endothelium. In contrast, the chorionic epithelium of horse and pig fetuses remains separated from maternal blood by 3 layers of tissue. One might thus be tempted to consider that exchange across the equine placenta is much less efficient that across the human placenta. In a sense this is true, but other features of placental structure make up for the extra layers in the diffusion barrier; it has been well stated that "The newborn foal provides a strong testimonial to the efficiency of the epitheliochorial placenta."

Summary of Species Differences in Placental Architecture

The placental mammals have evolved a variety of placental types which can be broadly classified using the nomenclature described above. Not all combinations of those classification schemes are seen or are likely to ever be seen - for instance, no mammal is known to have a diffuse, endotheliochorial, or a hemoendothelial placenta. Placental types for "familiar" mammals are summarized below, with supplemental information provided for a variety of "non-familiar" species.

Resources

Comparative Placentation is an excellent and comprehensive site for obtaining information on placental structure for a large number of domestic and wild animals