Myelination of jp,jpmsd, and qk axons by normal glia in vitro: ultrastructural and autoradiographic evidence.

Normal optic nerve glia were 'injected' into hypomyelinated mutant jp,jpmsd, and qk cerebellum by co-culturing explants in direct physical contact. Quantitative light microscopic studies demonstrated that such glial injection significantly increased the number of myelin profiles counted in cultures, suggesting that axons in all 3 mutants can accept myelination from competent glia when they are made available. In each mutant, the observed increase in myelination was independent of the ages of donor optic nerves and recipient cultures, but absolutely required positioning of the optic nerve so that direct contact occurred with the mutant cerebellar explants. The additional myelin found near the zone of fusion with the optic nerve morphologically resembled normal, not mutant myelin. Autoradiographs made after [3H]thymidine-labeled normal optic nerve was injected into jpmsd cultures showed that labeled cells had colonized the nearby mutant tissue. Labeled cells identified as oligodendrocytes by ultrastructural criteria were found adjacent to myelin segments near the fusion zone, but direct continuity between processes of these oligodendrocytes and myelin sheaths was not demonstrated. The astrocytes and phagocytic cells which were also labeled had no obvious relationship to myelinated axons. These results provide experimental evidence that the primary abnormalities produced by the three mutations jp,jpmsd, and qk are inherent in their glial cells, probably although not definitely in the oligodendrocytes.