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Although it has been known for decades that the tight junctions of fluid-transporting epithelia are leaky to ions, it has not been possible to determine directly whether significant transjunctional water movement also occurs. An optical microscopic technique was developed for the direct visualization of the flow velocity profiles within the lateral intercellular spaces of a fluid-absorptive, cultured renal epithelium (MDCK) and used to determine the velocity of the fluid flow across the tight junction. The flow velocity within the lateral intercellular spaces fell to near zero adjacent to the tight junction, showing that significant transjunctional flow did not occur, even when transepithelial fluid movement was augmented by imposition of osmotic gradients.

Apolipoprotein J (apoJ) is an ubiquitous protein thought to play roles in lipid transport, sperm maturation, regulation of complement, apoptosis and membrane recycling. In situ hybridization analysis was conducted to localize the sites of apoJ synthesis. The results showed that apoJ synthesis is restricted to epithelial and other secretory cells in diverse tissues. It is proposed that apoJ functions to stabilize cell membranes at different fluid-tissue interfaces.