Direct communication between axons and sheath glial cells in crayfish

Abstract

Evidence for the transfer of molecules as large as proteins from glial or Schwann cells to axons has frequently been reported^1–8. This phenomenon is well supported in crayfish^3,4,8 and squid^5–7 but there is some doubt about its existence in vertebrates⁹. 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. However, Grossfeld et al.¹⁰ recently reported data on the passage of a fluorescent dye (Lucifer yellow CH, molecular weight (MW) 476) from a crayfish median giant axon to all glial cells of 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 junctions¹¹ but these structures have never been seen between axons and glial cells of crayfish^12–14. Indeed, regions of plasma membrane specialization have been reported¹³, but they are structurally different from gap junctions. I describe here pore structures, observed between 51 axons and glial cells of six crayfish spinal cords, which may represent the means of communication between these cells.