If it’s in the arm or leg, the entire limb could swell up. Swelling often occurs at the site of the blood clot in the body. Placental types for 'familiar' mammals are summarized below, with supplemental information provided for a variety of 'non-familiar' species.2. 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. The placental mammals have evolved a variety of placental types which can be broadly classified using the nomenclature described above. Summary of Species Differences in Placental Architecture 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.' One might thus be tempted to consider that exchange across the equine placenta is much less efficient that across the human placenta. In contrast, the chorionic epithelium of horse and pig fetuses remains separated from maternal blood by 3 layers of tissue. In humans, fetal chorionic epithelium is bathed in maternal blood because chorionic villi have eroded through maternal endothelium. Each of the possibilities is observed in some group of mammals.

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. Endothelium lining endometrial blood vessels.The three potential maternal layers in a placenta are: 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. Chorionic epithelium, the outermost layer of fetal membranes derived from trophoblast.Connective tissue in the form of chorioallantoic mesoderm.Endothelium lining allantoic capillaries.There are three layers of fetal extraembryonic membranes in the chorioallantoic placenta of all mammals, all of which are components of the mature placenta: Just prior to formation of the placenta, there are a total of six layers of tissue separating maternal and fetal blood.

Seen in primates and rodents.Ĭlassification Based on Layers Between Fetal and Maternal Blood

Discoid: A single placenta is formed and is discoid in shape.

Seen in carnivores like dogs and cats, seals, bears, and elephants.

Zonary: The placenta takes the form of a complete or incomplete band of tissue surrounding the fetus.

This type of placentation is observed in ruminants. The fetal portions of this type of placenta are called cotyledons, the maternal contact sites (caruncles), and the cotyledon-caruncle complex a placentome.

Cotyledonary: Multiple, discrete areas of attachment called cotyledons are formed by interaction of patches of allantochorion with endometrium.

Diffuse: Almost the entire surface of the allantochorion is involved in formation of the placenta.

The number of layers of tissue between maternal and fetal vascular systems.ĭifferences in these two properties allow classification of placentas into several fundamental types.Ĭlassification Based on Placental Shape and Contact PointsĮxamination of placentae from different species reveals striking differences in their shape and the area of contact between fetal and maternal tissue:.

The gross shape of the placenta and the distribution of contact sites between fetal membranes and endometrium.

Two characteristics are particularly divergent and form bases for classification of placental types: 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.

Placental Structure and Classification Reproduction Index

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