The specialization of septate junctions in regions of tricellular junctions: I. Smooth septate junctions (=Continuous junctions)

Abstract

Some regions from the midgut of a crustacean and an insect were comparatively studied, both in sections of lanthanum-impregnated tissues and on freeze-fracture replicas. The epithelial cells are apically joined by zonular smooth septate junctions which are actually composed of alternating parts built and shared in common by either two cells bicellular junctions) or three cells tricellular junctions). The bicellular junctions are composed of strands which include a smooth extracellular septum stretched between two intramembrane particle rows. The strands run nearly or preferentially parallel to the apical surface, except near the cell corners where they become parallel to the axes of the tricellular junctions. This change in strand orientation characterizes the marginal regions of the bicellular junctions, which moreover are bordered by a limiting strand. At tricellular junctions each cell corner membrane displays a series of regularly spaced doublets of juncture particles, somewhat larger than the strand particles. The doublets are apparently the intramembrane components of junctional units (partitions), the chief part of which is an extracellular diaphragm showing a central vesicular part and a thinner periphery. The diaphragms compartmentalize the extracellular space and are linked to the limiting strands of the bicellular junctions abutting onto a tricellular junction.