STUDIES ON AN E P I T H E L I A L (GLAND) CELL JUNCTION I. Modifications of Surface Membrane Permeability

Membrane permeability of an epithelial cell junction (Drosophila salivary gland) was examined with intraccllular microclectrodes and with fluorescent tracers. In contrast to the non-junctional cell membrane surface, which has a low permeability to ions (I0 -4 mho/cm2), the junctional membranc surface is highly pcrmcable. In fact, it introduces no substantial restriction to ion flow beyond that in the cytoplasm; the resistance through a chain of cells (150 ~ cm) is only slightly greater than in extrudcd cytoplasm (100 ~ cm). Thc diffusion resistance along thc intcrccllular spacc to the cxtcrior, on thc other hand, is very high. Here, there exists an ion barrier of, at least, 10 4 ~ cm 2. As a result, small ions and fluorescein move rather freely from one cell to the next, but do not leak appreciably through the intercellular space to the exterior. The organ here, rather than the single cell, appears to be the unit of ion environment. The possible underlying structural aspects are discussed.