Sources and Timing of Calcium Mobilization During Embryonic Development of the Corn Snake, Pantherophis guttatus

Embryos of oviparous Reptilia (=turtles, lepidosaurs, crocodilians and birds) extract calcium for growth and development from reserves in the yolk and eggshell. Yolk provides most of the calcium to embryos of lizards and snakes. In contrast, the eggshell supplies most of the calcium for embryonic development of turtles, crocodilians and birds. The yolk sac and chorioallantoic membrane of birds recover and transport calcium from the yolk and eggshell and homologous membranes of squamates (lizards and snakes) probably transport calcium from these two sources as well. We studied calcium mobilization by embryos of the snake Pantherophis guttatus during the interval of greatest embryonic growth and found that the pattern of calcium transfer was similar to other snakes. Calcium recovery from the yolk is relatively low until the penultimate embryonic stage. Calcium removal from the eggshell begins during the same embryonic stage and total eggshell calcium drops in each of the final 2 weeks prior to hatching. The eggshell supplies 28% of the calcium of hatchlings. The timing of calcium transport from the yolk and eggshell is coincident with the timing of growth of the yolk sac and chorioallantoic membrane and expression of the calcium binding protein, calbindin-D28K, in these tissues as reported in previous studies. In the context of earlier work, our findings suggest that the timing and mechanism of calcium transport from the yolk sac of P. guttatus is similar to birds, but that both the timing and mechanism of calcium transport by the chorioallantoic membrane differs. Based on the coincident timing of eggshell calcium loss and embryonic calcium accumulation, we also conclude that recovery of eggshell calcium in P. guttatus is regulated by the embryo