PLEATED SEPTATE JUNCTIONS IN LEECH PHOTORECEPTORS - ULTRASTRUCTURE, ARRANGEMENT OF SEPTA, GATE AND FENCE FUNCTIONS

The leech photoreceptor forms a unicellular epithelium: every cell sur rounds an extracellular ''vacuole'' that is connected to the remaining extracellular space via narrow clefts containing pleated septate junc tions. We analyzed the complete structural layout of all septa within the junctional complex in elastic brightfield stereo electron microgra phs of semithin serial sections from photoreceptors infiltrated with c olloidal lanthanum. The septa form tortuous interseptal corridors that are spatially continuous, and open ended basally and apically. Indivi dual septa seem to be impermeable to lanthanum; interseptal corridors form the only diffusional pathway for this ion. The junctions form no diffusion barrier for the electron-dense tracer Ba2+, but they hinder the diffusion of various hydrophilic fluorescent dyes as demonstrated by confocal laser scanning microscopy (CLSM) of live cells. Even those dyes that penetrate gap junctions do not diffuse beyond the septate j unctions. The aqueous diffusion pathway within the septal corridors is , therefore, less permeable than the gap-junctional pore. Our morpholo gical results combined with published electrophysiological data sugges t that the septa themselves are not completely tight for small physiol ogically relevant ions. We also examined, by CLSM, whether the septate junctions create a permeability barrier for the lateral diffusion of fluorescent lipophilic dyes incorporated into the peripheral membrane domain. AFC(16), claimed to remain in the outer membrane leaflet, does not diffuse beyond the junctional region, whereas DiIC(16), claimed t o flip-flop, does. Thus, pleated sepatate junctions, like vertebrate t ight junctions, contribute to the maintenance of cell polarity.