Meeting Abstract
The eggshell is a calcium storage depot accessed by oviparous reptile embryos. Eggshell calcium is mobilized into embryonic tissues of turtles and squamates late in development. In contrast, bird embryos mobilize calcium earlier in development and deposit it in both yolk and embryonic tissues. The chorioallantoic membrane, which lies adjacent to the eggshell, is responsible for solubilizing and transporting calcium. We have shown that squamate chorioallantois expresses the epithelial calcium transport marker protein calbindin-D28K late in development coincident with calcium transport. Studies by others show that chick chorioallantois does not express calbindin-D28K at any stage of development. These results suggest that squamate chorioallantois calcium transport is similar to mechanisms of intestinal or renal epithelia but that birds express a different (perhaps unique) chorioallantois calcium transport mechanism. We used immunoblotting for calbindin-D28K in the turtle (Trachemys script) chorioallantois to test the hypothesis that similarity in the pattern of calcium mobilization between turtles and squamates reflects functional similarity in the mechanism of calcium mobilization. In a developmental series of tissue samples from six egg clutches, we find little evidence for calbindin-D28K expression in the chorioallantois. However, yolk splanchnopleure expression of calbindin-D28K follows a similar developmental pattern observed in both squamates and birds. We conclude that the mechanism of calcium mobilization from yolk is conserved across reptilian lineages, but that specializations for calcium transport by the chorioallantoic membrane are independent of the ontogenetic pattern of calcium mobilization and may reflect phylogenetic patterns.
