THE SHELL-FORMING MANTLE EPITHELIUM OF BIOMPHALARIA-GLABRATA (PULMONATA) - ULTRASTRUCTURE, PERMEABILITY AND CYTOCHEMISTRY

In the freshwater snail Biomphalaria glabrata, the outer mantle epithelium and the transitional epithelium from the outer mantle epithelium to the belt are characterized by apical vesicles of different electron density and vacuoles including lipid droplets and fibrillar structures. Wide intercellular spaces predominate in the transitional epithelium. In addition to belt desmosomes, freeze fracture studies detect septate junctions as apical intercellular junctions. The permeability of the septate junctions is tested by injecting anaesthetized snails with solutions containing peroxidase and lanthanum-nitrate. The septate junctions appear to be impermeable for the protein and inhibit permeation of the ion. Alkaline phosphatase is detected at the light microscope level in the transitional and outer mantle epithelium. At the electron microscope level localization of alkaline phosphatase is restricted to the apical and basal cell membranes of the same epithelia. An ATPase with low affinity to Ca2+ and Mg2+ ions is observed in the lumen of vacuoles in the proximal belt, the transitional and the outer mantle epithelium. The possible role of the vacuoles as a site of synthesis or modification of the intercrystalline matrix is discussed. A high affinity Ca2+/Mg2+ ATPase is detected with the help of Electron Spectroscopic Imaging in the cell membranes and in membranes of the rER. A model for solute transport through the epithelia to the extrapallial space is proposed.