Placentation in the Mexican scincid lizard Plestiodon brevirostris (Squamata: Scincidae)

Viviparity is the reproductive pattern in which females gestate eggs within their reproductive tract to complete their development and give birth to live offspring. Within extant sauropsids, only the Squamata (e.g., snakes, lizards, and amphisbaenians) evolved viviparity, representing 20% of the existing species. The genus Plestiodon is represented by 43 species and is one of the most widely distributed genera of the Scincidae in Mexico. The goal of this research has been to determine the placental morphology and ontogeny during gestation in the lizard Plestiodon brevirostris. Specimens were dissected to obtain the embryonic chambers and the embryos were categorized to carry out the correlation between the development stage and the placenta development. The embryonic chambers were processed using the conventional histological technique for light microscopy. The identified embryonic stages were 4, 29, 34, 36, and 39. A thin eggshell surrounds the egg in early developmental stages; however, this structure is already absent in the embryonic hemisphere during the developmental stage 29. The results revealed that P. brevirostris is a lecithotrophic species, but a close maternal-fetal relationship is established by tissue apposition. Ontogenically, the placental types that form in the embryonic hemisphere are the chorioplacenta, choriovitelline placenta, and chorioallantoic placenta; whereas the omphaloplacenta is formed in the abembryonic hemisphere. The structure of the chorioallantoic placenta in P. brevirostris suggests that it may play a role during gas exchange between the mother and the embryo, due to the characteristics of the epithelia that comprise it. The structure of embryonic and maternal epithelia of the omphaloplacenta suggests a role in the absorption of the eggshell during gestation and possibly in the transport or diffusion of some nutrients. In general, it is evident that ontogeny and placental characteristics of P. brevirostris match those of other species of viviparous lecithotrophic scincids.