Direct communication between axons and sheath glial cells in crayfish

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Abstract
Evidence for the transfer of molecules as large as proteins from glial or Schwann cells to axons frequently has been reported. This phenomenon is well supported in crayfish and squid but there is doubt about its existence in vertebrates. In invertebrates most of the data support the transfer from glial cells to axons, but little is known about movement of molecules in the opposite direction. Recently data was reported on the passage of a fluorescent dye (Lucifer yellow CH, MW 476) from a crayfish median giant axon to all glial cells to the adaxonal layer. Little is known of the mechanism by which molecules are exchanged between axons and glial cells, but the bidirectionality of the phenomenon in crayfish axons suggests that channels or pores for direct intercellular communication may exist. In most cells, direct communication is provided by gap junctions, but these structures have never been seen between axons and glial cells of crayfish. Regions of plasma membrane specialization have been reported, but they are structurally different from gap junctions. Pore structures observed between 51 axons and glial cells of 6 crayfish spinal cords, which may represent the means of communication between these cells are reported.