Calcium mobilization from the avian eggshell during embryonic development

The avian eggshell acts as the major source of calcium for embryogenesis in bird. Using scanning electron microscopy, the inner eggshell surface was analysed before and after embryonic development and its accompanying calcium removal. This was done in eggs from two bird species with different growth rates and modes of development, the precocial Japanese quail Coturnix japonica and the altricial starling Sturnus vulgaris. Next, enzyme histochemistry was used to localize carbonic anhydrase in the extra-embryonic chorioallantoic membrane to provide support for the hypothesis that calcium may be released from the eggshell by means of acidification through the action of carbonic anhydrase. However, neither in the precocial quail nor in the altricial starling could any staining for carbonic anhydrase be detected. These results lead us to the conclusion that the role of carbonic anhydrase in embryonic calcium mobilization from the avian eggshell remains unclear.