Mechanism and regulation of calcium transport by the chick embryonic chorioallantoic membrane.

The chorioallantoic membrane (CAM) is the tissue responsible for translocating eggshell calcium into the circulation of the developing chick embryo. Calcium transport by the CAM is highly specific and is expressed as a function of development. We review here first our recent studies on three previously identified, putative components of the CAM calcium transport pathway: a calcium-binding protein (CaBP), a Ca2+-ATPase, and carbonic anhydrase. Using in vivo and in vitro procedures to measure CAM calcium uptake, we have shown that all three components are indeed functionally involved. Based on these results, an "endocytosis mechanism" is postulated for CAM calcium transport. In this mechanistic model, the CaBP serves as the cell surface "calcium-binding receptor" and the Ca2+-ATPase pumps calcium into endosomes. In addition, we summarize here results showing that the expression of calcium transport activity and of functional CaBP in the CAM both require the proximity of the eggshell, the transport substrate, and depend on proper embryonic vitamin K metabolism. We have also initiated a study of the molecular aspects of CaBP expression by cloning a cDNA to CaBP mRNA. Finally, our recent data show that vitamin D may be involved in chick embryonic calcium metabolism by regulating the mobilization of yolk calcium.