Fine structure and tracer permeability of the cuticle and cell junctions of the rectal chloride epithelia of Aeshna cyanea larvae; a comparative freeze-fracture and thin-section study

Three types of cell junctions are identified in the rectal chloride epithelia: Zonulae adhaerentes, pleated septate junctions and gap junctions. All three occur in characteristic arrangements between the apico-lateral cell membranes whereas only gap junctions are scattered between the basolateral cell membranes. The cuticle overlying the chloride epithelia differs from the cuticle of the adjacent respiratory epithelia by the occurrence of epicuticular depressions with a diameter of about 200 nm and a frequency of 2 to 3 per µm² Permeability with colloidal and ionic lanthanum ingested with the medium into the rectal lumen by the living animals showed that the tracer was able to penetrate exclusively the cuticle possessing epicuticular depressions over the chloride epithelia. Therefore the depressions seem to represent hydrophilic sites favoring cuticular permeability over the ion transporting epithelia. The tracer was also present in the pleated septate junctions, but always restricted to the apical three quarters of their length regardless of the duration of exposure (3 h to 8 d). This finding suggests that this type of cell junction may seal the intercellular channels.