1. Placenta : Types

2. Definition:
Placenta is an organ that serves as the interface between fetal and maternal circulations and has endocrine ,immune, nutritive and respiratory functions.

3. Placenta types:
Classification Based on Placental Shape and Contact Points:
Diffuse Placenta: Almost the entire surface of the allantochorion is involved in formation of the placenta. Example: Horse and Pig.
Cotyledon Placenta: 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. Example: Cow, Goat, Giraffe,Deer etc.
Zonary Placenta: 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.
Discoidal Placenta: A single placenta is formed and is discoid in shape. Seen in Primates and rodents.

4. Placenta types:

5. Classification Based on Layers Between Fetal and Maternal Blood
Just prior to formation of the placenta, there are total of six layers of tissue separating maternal and fetal blood. There are three layers of fetal extra embryonic membranes in the chorioallantoic placenta of all mammals, all of which are components of the mature placenta:
Endothelium lining allantoic capillaries
Connective tissue in the form of chorioallantoic mesoderm
Chorionic epithelium, the outermost layer of fetal membranes derived from trophoblast

6. 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:
Endothelium lining endometrial blood vessels
Connective tissue of the endometrium
Endometrial epithelial cells

7. Maternal Layers Retained Endometrial Epithelium
Classification 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.
Type of Placenta
Maternal Layers Retained
Examples
Endometrial Epithelium
Connective Tissue
Uterine Endothelium
Epitheliochorial +
Horses, swine, ruminants
Endotheliochorial -
Dogs, cats
Hemochorial
Humans, rodents