Fine structural changes associated with the onset of calcium, sodium and water transport by the chick chorioallantoic membrane.

An attempt is made to correlate structure and transport function in the embryonic chorioallantoic membrane. The fine structure of the endoderm and ectoderm in the membrane was examined with particular attention given to the morphological changes that occur when transport is established,in vivo. Two distinctive cells, a granule-rich cell and a mitochondria-rich cell, appear in the endoderm at the time allantoic fluid sodium, chloride and water reabsorption commences. These are indistinguishable from the cells described in toad bladder epithelium. It is suggested that the granule-rich cell is responsible for bulk water movement and the mitochondria-rich cell is specifically engaged in active sodium transport. In the ectoderm, two distinctive cell types are also found to be associated with the onset of active calcium transport. These are referred to as the capillary-covering cell and the villus-cavity cell. The preponderate capillary-covering cell is most likely responsible for transcellular calcium transport. It is postulated that the function of the villus-cavity cell is to secrete hydrogen ions which are necessary, along with carbonic anhydrase, to mobilize Ca(++) from the insoluble calcium carbonate of the eggshell.