Reptilian Placentation: Structural Diversity and Terminology

Review of the literature on reptilian placental terminology reveals that confusion associated with the assignment of terms to different placental organs is unwarranted. Reptilian placentae can be assigned easily to one of four distinct structural types defined by the extraembryonic membranes that are involved in placental formation. The terms allantoplacenta and placenta have consistently been applied to the organ formed by the chorioallantoic membrane and adjacent uterine epithelium. Some confusion, however, has been associated with the assignment of terms to regions of the yolk sac that participate in placentation. We recognize two distinct types of yolk sac placentation: 1) the placenta, defined as the apposition of the vascularized trilaminar omphalopleure (ectoderm, mesoderm, endoderm) and the uterine epithelium; and 2) the omphaloplacenta, which consists of the non-vascular omphalopleure (ectoderm, endoderm) of the isolated yolk mass and associated structures, in apposition to the uterine epithelium. The omphalallantoic placenta, constituting the fourth structural category, forms as the outer allantoic membrane becomes apposed to the inner margin of the omphaloplacenta. Based upon these definitions, both chorioallantoic and choriovitelline placentation occur among reptiles, marsupials and eutherians, whereas omphaloplacentation and amphalallantoic placentation are unique to squlamates.