Difference between epitheliochorial and endotheliochorial placenta
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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:
The gross shape of the placenta and the distribution of contact sites between fetal membranes and endometrium.
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.
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