Honors Program

University Honors

Date of Award


Thesis Professor(s)

James R. Stewart

Thesis Professor Department

Biological Sciences

Thesis Reader(s)

Tom W. Ecay, Rebecca A. Pyles


Viviparity (live birth) and placentation have evolved in more than 100 lineages of squamate reptiles. However, highly placentotrophic species in which embryos receive the majority of nutrients for development via maternal transport across a placenta are rare. Pseudemoia pagenstecheri is a viviparous Australian scincid lizard with extensive placental transfer of nutrients. For example, 90% of neonate calcium is received via placental transfer. This species has a regionally differentiated chorioallantoic placenta distinguished by an elliptical-shaped region, the placentome. The placentome is characterized by hypertrophied uterine and embryonic epithelial cells supported by dense vascular networks. The remainder of the chorioallantoic placenta is also highly vascularized but epithelia are thin. A yolk sac placenta with hypertrophied epithelia is located in the abembryonic hemisphere of the egg. We used immunohistochemistry and immunoblotting to test the hypothesis that the placenta has regional functional specializations for calcium transport. Calcium uptake by extraembryonic membranes of squamates is correlated with expression of the intracellular calcium binding protein, calbindin-D28K. Immunohistochemistry was used to localize calbindin-D28K expressing cells. Immunoblotting for calbindin-D28K and the plasma membrane calcium ATPase (PMCA, an additional marker for active calcium transport) was used to assess changes in protein expression levels through development. We found support for our hypothesis because calbindin-D28K was expressed in the embryonic epithelium of the yolk sac placenta and in the chorionic epithelium of the placentome, but not in the remainder of the chorioallantoic placenta. In addition, calbindin-D28K was expressed the chorioallantoic membrane in all embryonic stages studied, which encompassed both early and late development. Immunoblotting data show that calbindin-D28K expression was detectable at low levels in early stages of development and increased significantly prior to birth, when embryonic calcium demand peaks. Expression of PMCA also increases significantly throughout development, though less dramatically. Expression of calbindin-D28K and PMCA protein by the chorioallantoic placenta parallels the accrual of calcium in the embryo. These data suggest that placental calcium secretion occurs over an extended interval of gestation, with increasing activity as embryonic demand escalates in late development. In conclusion, our results support our original hypothesis that regional structural differentiation in the placenta reflects functional specializations for calcium transport to the embryo during development.

Document Type

Honors Thesis - Withheld

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