Role of the septate junction in the regulation of paracellular transepithelial flow.

Abstract

A comparison of the distribution of septate junctions in invertebrate epithelia and tight junctions in vertebrate systems suggests that these structures may be functionally analogous. This proposition is supported by the internal design of each junction which constitutes a serial arrangment of structures crossing the intercellular space between cells to effectively provide resistance to the paracellular flow of water and small molecules. The validity of this analogy was tested by examining whether the osmotic sensitivity of the septate junctions of planarian [Dugesia tigrina] epidermis follow the rather striking pattern observed for the junctions of very tight vertebrate epithelia (e.g., toad urinary bladder). The septate junctions in this system responded in similar fashion to their vertebrate counterparts, blistering with accumulated fluid when the medium outside the epidermis was made hypertonic with small, water-soluble molecules. The 2 types of junction probably are functionally analogous and in each case, this rectified structural response to transepithelial osmotic gradients may be indicative of the role of such structures in the transport function of epithelia.