Trans-glial channel-facilitated translocation of tracer protein across ventral nerve root sheaths of crayfish

Abstract

Trans-glial channels, which traverse the multilamellate glial sheath of crayfish nerves, are easily recognized in freeze-fracture preparations. Their structure and position in the glial layers of the sheath strongly supports the suggestion that they serve to facilitate rapid movement of molecules and fluids from outside the sheath to the surface of axons contained within. Segments of ventral ganglion nerve roots, which were ligated at their free ends, were immersed in crayfish Ringer solution containing 10 mg/ml horseradish peroxidase (HRP). Electron microscopic examination of the nerve sheath 30 sec after exposure to peroxidase showed that the protein had passed across the sheath and was present near the axon surface. Reaction product was present in trans-glial channels as well as in extracellular clefts and adaxonal tubular lattices thereby supporting the notion that these structures constitute a specialized conduit traversing the sheath. Often, 'fronts' of reaction product were observed across the sheath from its exterior to the interior reflecting a gradual accumulation of protein in extracellular clefts toward the axon. After 5 min in HRP-Ringer, protein appeared in all channels, extracellular clefts, and tubular lattices. With increased length of exposure of ligated nerve segments to HRP-Ringer, reaction product was found in vesicles in glial cytoplasm adjacent to axons. Results from this study suggest that trans-glial channels constitute an efficient system for rapid solute movement across nerve sheaths and may represent a mechanism whereby ions and nutrients are made available to nerves isolated in an avascular sheath.