Changes to the uterine epithelium during the reproductive cycle of two viviparous lizard species ( N iveoscincus spp.)

We investigated morphological differences in uterine epithelia of the reproductive cycle between two closely related viviparous skinks, N iveoscincus metallicus (lecithotrophic) and N iveoscincus ocellatu s (placentotrophic), which have similar placental complexity but different degrees of placentotrophy. Scanning ( SEM ) and transmission electron microscopy ( TEM ) revealed that the uterine surface of non‐reproductive females of both species is mainly covered by ciliated cells. As vitellogenesis begins, the uterine epithelium consists of ciliated and non‐ciliated cells under a thin glycocalyx. Microvilli are greatly reduced at mid‐pregnancy, and the uterus differentiates into two structurally distinct regions: the chorioallantoic and the omphaloplacenta. At late stages of pregnancy, the uterine epithelium of chorioallantoic placenta in both species is further ridged, forming a knobbly uterine surface. The ultrastructural evidence between N . metallicus and N . ocellatus cannot strictly account for the distinct differences in their placentotrophy; as yet unexplored molecular nutrient transport mechanisms that are not reflected in uterine ultrastructure must play significant roles in nutrient transportation. Characteristics consistent with a plasma membrane transformation were confirmed in both species.