Electron microscopic studies of ion- and H2O-transporting epithelial cells in the horizontal ampulla of the pigeon.

Earlier morphological studies of the epithelial structure in the semicircular canals of mammals have focused on the sensory cells of the crista ampullaris. This report draws attention to the fact that there exist at least seven further cell types in the horizontal ampulla walls of pigeon with various functions; the role of ion- and H2O-transporting epithelial cells is dealt with here in detail. While the dark cells appear to play a decisive role in the regulation of ionic composition, the cells in the planum semilunatum may transport H2O and assist in the regulation of endolymph volume. In addition, protein-secreting structures are located in the apical region of the cells of the planum semilunatum. The question whether the proteins are dispersed in the endolymph or contribute to cupula formation remains unclear. The morphology and possible functions of these two cell types are discussed on the basis of electron microscopic results.