STEWART, J.R.*; BRASCH, K.R.: Placental Ultrastructure of a Predominantly Lecithotrophic Squamate Reptile

Virginia striatula is a viviparous snake with a complex pattern of embryonic nutrition. Nutrients for embryonic development are provided by large yolked eggs, supplemented by placental transfer. Placentation in this species is surprisingly elaborate for a predominantly lecithotrophic squamate reptile. The embryonic–maternal interface consists of three structurally distinct areas, an omphalallantoic placenta and a regionally diversified chorioallantoic placenta. The chorioallantoic placenta over the embryonic hemisphere (para-mesometrial region) of the egg, features close apposition of embryonic and uterine blood vessels because of the attenuate form of the interceding epithelial cells. The periphery of the chorioallantoic placenta, adjacent to the omphalallantoic placenta, is characterized by a simple cuboidal uterine epithelium apposed to a stratified cuboidal chorionic epithelium. The morphology of the omphalallantoic placenta is similar to the peripheral chorioallantoic placenta, except that the height of uterine epithelial cells is greater and allantoic blood vessels are not associated with the embryonic epithelium. The functional capabilities of the three placental regions are not known, but structural characteristics suggest that the omphalallantoic placenta and peripheral zone of the chorioallantoic placenta are sites of nutritional provision via histotrophy. The para-mesometrial region of the chorioallantoic placenta is also nutritive, in addition to functioning as the primary embryonic respiratory system. The structure of the chorioallantoic placenta of V. striatula is a new placental morphotype for squamate reptiles that provides new insight into the evolution of reptilian placentation.