Placental function in lizards

Abstract: Frequent evolution of viviparity in squamate reptiles provides excellent models for study of the transition from oviparity to viviparity, and the evolution of complex placentae. Embryonic reptiles have a chorioallantoic placenta at the embryonic pole of the egg, and an omphaloplacenta at the abembryonic pole. The uterus of viviparous reptiles and mammals exhibits common plasma membrane transformation including uterodome formation associated with pregnancy. The transition from oviparity to viviparity requires few changes: (1) reduction in an eggshell, (2) delay of expulsion of oviposition/birth, (3) increased vascularisation of the uterus for gas exchange. We used immunohistochemistry to address the question of eggshell thinning in a range of species, and SEM and TEM to infer the functions of different placental regions in one species with a complex placenta. We conclude that: (1) changes in the calcium channels among species accompany the evolution of viviparity; (2) the omphaloplacenta has a role in nutrient transport via apocrine secretion and histotrophy; (3) the placentome has a role in nutrient transport, but not via apocrine secretion; (4) the paraplacentome of the chorioallantoic placenta has a major role in gas exchange. [Copyright &y& Elsevier]