Meeting Abstract
The eggshell is an important source of calcium for embryos of oviparous reptiles. Transport of calcium from the eggshell is a property of the chorioallantoic membrane. The chorionic epithelium faces the eggshell and is the primary tissue implicated in calcium mobilization and transport to the inner allantoic vascular system. Previous immunoblotting studies demonstrated an increase in expression of the calcium binding protein, calbindin-D28K, and the enzyme, carbonic anhydrase II, in the chorioallantoic membrane of corn snake embryos coincident with calcium uptake from the eggshell. Calbindin-D28K is a marker for calcium transport in squamate chorioallantoic membranes and carbonic anhydrase II is implicated in producing acidic environmental conditions conducive to solubilizing calcium from the inner aspect of the eggshell. We used immunohistochemistry to test the hypotheses that (1) cells expressing these two proteins are located in the chorionic epithelium and (2) the two proteins are expressed in different cells. Our results demonstrate that there are two cell types in the outer layer of the chorionic epithelium, one of which expresses calbindin-D28K and the other carbonic anhydrase II. Differentiation of chorionic epithelial cells resulting in functional specialization of individual cells for either calcium mobilization or for calcium transport also occurs in embryonic chickens. The existence of a similar phenomenon in corn snakes suggests that these functional attributes are likely to be widespread among oviparous reptiles.
