ECAY, T.W.; STEWART, J.R.*; BLACKBURN, D.G.; East TN State Univ; East TN State Univ; Trinity College: Timing and Mechanism of Calcium mobilization During Embryonic Development of the Corn Snake

Patterns of embryonic calcium mobilization vary among oviparous amniotes, yet the mechanism of calcium transport has been studied only in domestic chickens, which rely heavily on eggshell calcium. Mobilization of calcium from the yolk begins early in embryonic chickens and is facilitated by the calcium binding protein calbindin-D28K in endodermal cells of the yolk splanchnopleure. In contrast, calcium transport from the eggshell begins midway in incubation and is mediated by the chorioallantoic membrane, which does not express calbindin-D28K. We studied calcium mobilization by embryos of the corn snake and confirmed that, as in other squamates and in contrast to birds, yolk is the primary source of calcium. Calcium withdrawal from the yolk begins early in incubation, but calcium recovery from the eggshell occurs during late embryonic stages. We also analyzed developmental expression of calbindin-D28K in the extraembryonic membranes of corn snakes by immunoblotting to test the hypothesis that the mechanism of calcium transport of corn snakes is similar to birds. High calbindin-D28K expression was detected in samples of corn snake yolk splanchnopleure and chorioallantoic membrane during late embryonic stages when calcium removal from the yolk and the eggshell also was highest. We conclude that the timing and mechanism of calcium transport from the yolk sac of the corn snake is similar to birds, but that both the timing and mechanism of calcium transport by the chorioallantoic membrane differs. Further, the concurrent timing of eggshell calcium loss and expression of calbindin-D28K in the chorioallantoic membrane suggests that recovery of eggshell calcium in the corn snake is regulated developmentally. (Supported by a grant from the ETSU Research Development Committee.)