BLACKBURN*, D. G.; JOHNSON, A. R.; PETZOLD, J. L.; Trinity College, Hartford, CT: Placental ultrastructure in the brown snake Storeria dekayi

Placental membranes in viviparous snakes function in gas exchange, as well as in the transfer of water and nutrients to the developing embryos. Using light and transmission electron microscopy, we examined placental morphology in Storeria dekayi as part of a research program focusing on viviparous thamnophine snakes. By mid-gestation, brown snakes exhibit two distinct placental organs, formed from the chorioallantois and the omphalallantoic membrane. In the chorioallantoic placenta, the uterine and chorionic epithelia are very thin, offering little barrier to transplacental gas diffusion. In the omphalallantoic placenta, the uterine epithelium consists of enlarged cells with secretory droplets and short microvilli. Epithelium of the omphalopleure contains cells with elongated microvilli and abundant mitochondria, and other cells with large cytoplasmic granules and short microvilli. The isolated yolk mass undergoes depletion, leading to vascularization of the omphalopleure by the allantois. Morphological evidence suggests that the chorioallantoic placenta functions in respiration, whereas the omphalallantoic placenta is specialized for maternal � fetal nutrient transfer. The placental membranes of S. dekayi exhibit features not previously observed among viviparous snakes. Our studies will contribute to ongoing reconstructions of placental evolution in the thamnophine clade.