Abstract
Carbonic anhydrase activity was demonstrated in a distinct population of mucosal cells of whole stretched bladder of turtle and toad. The apical portions of the cells containing the enzyme were discretely stained as noncontiguous polygonal forms and allowed an estimation of the luminal surface representation of this epithelium. The slight luminal surface (0.8% or less) occupied by cells with carbonic anhydrase activity in the toad bladder contrasted with a greater representation in turtle bladder (6.4-12.2%) and correlated with the relative capacity of these tissues to acidify. After completion of the enzyme staining reaction the tissues of the turtle bladder were directly processed for electron microscopic observations without osmium tetroxide postfixation. The deposition of the electron-opaque cobalt sulfide compound was selective and was found only in cells with luminal representation and small apical granules, the recently described "third cell type"; cobalt sulfide was mainly present in the cytoplasm, an observation consonant with the known localization of carbonic anhydrase primarily in supernatant fractions.