Lack of glia-axon transfer of proteins in the normal optic system of goldfish

Abstract

An ultrastructural autoradiographic study of the goldfish optic tectum was carried out to determine whether proteins synthesized in glial cells are transferred into adjacent optic axons. Goldfish were injected intracranially over the optic tecta with [ 3H]leucine and fixed by perfusion 30 min, 4 and 24 h later. All unincorporated precursors were removed by repeated washings with fixatives, and tissue slices from the optic tecta were embedded and processed for electron microscopy autoradiography (EMA). The densities of the silver grains were determined over optic axons and their myelin sheaths. The densities over the axons were lower than those over the myelin sheaths at all time intervals. The density of the intraaxonal grains, in absolute terms as well as relative to that of the myelin, was highest in the 4 h experiment. Analysis of the distribution of the grain densities over the myelin sheath and over concentric axonal compartments was carried out at this time interval to determine whether the grain density over the axon represented intraaxonal [ 3H]proteins or was only the result of grains ‘scattered’ from [ 3H]proteins located in the surrounding myelin sheaths. When the experimental distribution of the grain densities over the axon was compared with the theoretical distribution expected over the axon if only the myelin sheaths were labeled, no significant difference was found. This indicates that the silver grains present in the axons were scattered from the adjacent myelin sheath and did not represent intraaxonal radioactivity. It is therefore concluded that in our system there is not a quantitatively significant transfer of proteins between glial cells and adjacent axons.