Astroglial-axonal interrelationship during regeneration of the optic nerve in goldfish. A freeze-fracture study

The cytoplasmic membranes of astrocytes in the regenerating optic nerve of the goldfish were investigated by means of freeze-fracture technique. In contrast to avian and mammalian astrocytes the astrocytes in the fish white matter do not reveal the characteristic orthogonal particle assemblies. Whereas astrocytes hitherto were believed to be interconnected mainly by gap junctions, astrocytes in the optic nerve of the goldfish are additionally interconnected by prominent desmosomes and large tight junctions. The tight junctional strands are frequently intercalated by gap junctions. Astroglial membrane domains, which are directly juxtaposed to untreated myelinated optic fibers, are longitudinally deformed. The distance of these longitudinal grooves corresponds to the caliber of fibers associated with them. The intramembranous particles in these membranous domains are distributed randomly. During optic nerve regeneration, membranes of astrocytes associated with regenerating, still unmyelinated fibers are either longitudinally deformed or smooth. If deformed according to the axonal course the membrane shows a non-random distribution of intramembranous particles; if smooth, the membrane shows a random distribution of its particles. The non-random particle distribution in the astrocytic P-face is characterized by an impoverishment of particles along the concave groove and an enrichment of particles along the convexe bulge which corresponds to the wedge between two associated axons. This phenomenon is restricted to the regeneration period and disappears some weeks after recovery of vision. Then the astrocytic membranes which are associated to regenerated fibers are undulated nevertheless revealing a random particle distribution. It is suggested, that these findings reflect a glial-neuronal interrelationship characterized by the neuron as inductor and the glial cell as responder to axonal signals. The astroglial response to axonal signals would include the formation of attractive glial channels. The working hypothesis is proposed that the presence of orthogonal particle assemblies could contribute to a disturbed astroglial response in the mammalian central nervous system.