Astrocytes induce oligodendrocyte processes to align with and adhere to axons

In order to study the signals that control the onset of myelination, we coc ultured highly purified postnatal retinal ganglion cells and optic nerve ol igodendrocytes under serum-free conditions that promote their survival for at least a month and found that no myelination occurred. Although the addit ion of optic nerve astrocytes induced the oligodendrocyte processes to alig n with, and adhere to, axons, myelination still did not occur. The effect o f astrocytes was mimicked by removal of polysialic acid from both cell type s using neuroaminidase. These findings provide evidence for a novel role fo r astrocytes in controlling the onset of myelination by promoting adhesion of oligodendrocyte processes to axons. They also suggest that other, as yet unidentified, cell-cell interactions are necessary to induce the myelinati on process itself.