The placenta is unique among mammalian organs for a number of reasons. Placentae of different species exhibit a broad diversity in structure that includes differences in cellular organization, shape, and pattern of distribution over the uterine endometrium. Placentae have been classified into three main types based on the number of layers separating maternal and fetal blood. Alternatively, placentae have been classified into four main types that relate to the degree to which the placenta is localized into a single discrete organ. Placentae differ greatly among species, yet all have the same task in maintaining and supporting embryonic and fetal growth, and appear to be equally successful in doing so.

from about 60 to 100, with about equal distribution in both the gravid and nongravid horn. In contrast, typical numbers of placentomes in deer, moose, and reindeer are 4 to 8, usually with equal numbers in each uterine horn.

Three basic shapes of placentomes have been described flat, convex, and concave. Flat placentomes are characteristic of pronghorn, duikers, gnu, and certain Cervidae such as the American elk, and may contain straight, slightly branched, or complexly branched villi. Convex placentomes are the most common and are characteristic of bovidae, giraffes, and many cervids. Concave placentomes are characteristic of sheep and goats. Convex and concave placentomes contain complexly branched or treelike villi. Cotyledonary placen-tomes generally have marginal folds of chorioallantois

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