Growth and development beyond the blastocyst require a direct functional relationship between the trophoblast and the maternal blood vascular system in eutherians. Blastocysts may obtain nutrients from uterine gland secretions, but continued growth of the embryo requires an intimate interchange relationship between the embryo and uterus. The areas of close association for physiological interchange between maternal and conceptus blood constitute vascular placentation. In the majority of mammals, the definitive placenta is the chorioallan-toic placenta.

The placenta, when compared to other organs of the body, differs in several respects. The placenta is formed as a result of interaction between fetal and maternal tissues within the pregnant uterus. It is of embryonic origin, but is situated outside the body of the embryo, to which it is connected by a cord of blood vessels. It is a disposable organ with a delimited lifespan, and is not innervated. It exhibits a wide variety of structural modifications such that major differences in anatomy may be found even in closely related species. Functional differences are also evident and add to an already complex situation.

Several different classification systems have been developed, based on anatomical and morphological characteristics, to differentiate types of placentae. These classifications provide only very general ideas as to differences in placental function. Two of the more commonly used systems, the first based on the number of membrane layers present and a second based on placental shape, are summarized. For more detailed and complete information, the reader is referred to the reading list at the end of this article, in particular to the classic work by Mossman.

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