Novel arthropod cell junctions with restrictive intercellular ?linkers?

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Abstract
The peripheral glial cells that surround the components of the avascular CNS in certain groups of primitive arthropods are characterized by unusual intercellular junctions. In the centipedes and millipedes (Myriapoda), these glial cells are associated by interconnecting filamentous ‘linkers’ which produce a reduction, but not an occlusion, of the intercellular cleft; these are interposed between conventional gap junctions. In replicas, the freeze-cleave images are of loosely aggregated gap junctional connection plaques, fracturing on to the extracellular membrane half leaflet (E face), together with linear alignments of intramembranous particles (IMPs) and furrows; complementary P face ridges also occur. Exogenous tracers appear unable to penetrate beyond these junction-rich glial clefts, possibly by binding to the ‘linkers’ or extracellular matrix between them. Peripheral glial cells in the cerebral ganglion of the horseshoe crab,Limulus, are also characterized by linear IMP arrays; in this case they primarily exhibit E face grooves and complementary ridges of P face IMPs, which also do not produce complete membrane fusion. These, too, are intimately associated with gap junctional plaques of E face particles or P face pits. These intramembranous particle rows are novel structural modifications, called here ‘linker’ junctions, and are quite distinct from conventional tight or septate junctions found between the outer glial cells in more highly evolved arthropods such as the insects and arachnids. They seem to represent a new category of intercellular junction.