Acid-base homeostasis in the chicken embryo. A proposed role for the anion exchanger (AE1) and carbonic anhydrase (CAII) of the chorioallantoic membrane.

The chorioallantoic membrane (CAM) is an extraembryonic structure with multiple roles in morphogenetic events. The CAM serves as extraembryonic respiratory organ and is the site of transport of either Ca2+ from the eggshell, and water and electrolytes from the allantoic cavity (Narbaitz et al., 1995, J.Anat. 186: 245-252). It is thought to participate also in preventing metabolic acidosis in the developing embryo, by providing an extra “non respiratory” bicarbonate supplement. In view to elucidate molecular components or mechanisms by which the CAM contributes to maintenance of acid-base balance, antibodies specific for the chicken AE1 anion exchanger have been used to determine the expression of this electroneutral transporter in the chick CAM during development. Immunohistochemical analyses demonstrated that the AE1 accumulates in the basolateral membrane of a subset of cells in both the chorionic and the allantoic epithelial layers. Double immunostaining indicated that the AE1-positive cells were also positive for the carbonic anhydrase isoform, CAII, which serves as marker for the villus cavity (VC) cells of the chorion and the mitochondria-rich cells of the allantois (Gabrielli et al. 2000, J.Anat. 198: 229-238). These results represent the first demonstration of AE1 expression in the chick CAM. The colocalization of AE1 and carbonic anhydrase in the chorionic VC cells suggests a role for this cell type either in regional acidification, allowing for mineral calcite dissolution and Ca2+ mobilization, and in bicarbonate ion reabsorption for maintaining acid-base balance in the embryo during development.