Structure of Necturus gallbladder epithelium during transport at low external osmolarities

Gallbladders transport isotonically over a wide range of osmolarities. This ability has been assumed to depend on the geometry of the lateral intercellular spaces. We report that this geometry in the Necturus gallbladder varies extensively with the external osmolarity and depends in vitro on the integrity of the subepithelial tissues. The structure of the living epithelium was studied by Nomarski light microscopy while ultrastructural effects were revealed by electron microscopy. The short-term effects (less than 60 min) of low external osmolarities were: 1) the cells became bell-shaped with an increased cell height measured centrally, 2) lateral intercellular spaces lost their convoluted character; and 3) numerous membrane-bound cavities appeared in the cells. Furthermore, long-term exposure to the low external osmolarities caused an uneven density of epithelial cells. With subepithelial tissues intact, blistering of the epithelium cell layer was evident. Qualitative electron-microscopic data indicate that the membrane of the cavities was recruited from the basolateral cell membrane. This agrees well with light-microscopic observation that the cavities were initiated as invaginations of this cell membrane.