Septate and Scalariform Junctions in Arthropods

Abstract

The structure of invertebrate septate junctions is not exactly the same in the different phyla. It is a peculiarity of the arthropods to possess two types of septate junctions, the distribution of which is dependent on the organs, not on the systematic position. Additionally, the scalariform junctions have only been observed in the arthropods, but are restricted to some (but not all) transporting epithelia. This chapter discusses the functions of septate and scalariform junctions. Among invertebrates, the arthropods have the best-known physiology and a careful examination of the repartition of the junctions and their position and association with other types of junctions or with certain cytoplasmic organelles may throw some light on their functions. A septate junction is constituted by elements of two adjacent cells and appears as a bicellular junction. However, different and very elaborate structures are set up at the line of contact among three cells (abutment of three bicellular junctions); these special features are designated as “tricellular junctions.” Scalariform junctions are characterized by a regular extracellular space of 20 nm wide, crossed by very fine tubular pillars giving in cross section a ladder-like appearance.