The effects of low temperature on myelin formation in optic nerves of Xenopus tadpoles.

To test the effect of cold on CNS myelin formation, optic nerves of stages 52-55 Xenopus tadpoles were examined electron microscopically after maintenance at 15, 10, 7 and 4 degrees C for 1-7 days. Nerves from tadpoles maintained at 15 degrees C resembled 22 degrees C (room temperature) controls. After 3 days at 10, 7, or 4 degrees C, tongue processes and perikarya of many myelin forming oligodendrocytes were swollen and filled with vesicular membrane profiles. The number of axonal microtubules was decreased in affected fibers but the lamellar structure of their myelin sheaths remained normal. Astrocytes were hypertrophic and contained large aggregates of filaments. Longer exposure to 10 or 7 degrees C increased the number of affected fibers but the changes were not more severe or associated with degeneration. The delayed onset, lack of progression and reversibility of the changes indicated that cold has a direct metabolic effect on myelin forming oligodendrocytes. Alterations produced by nerve transection or exposure to mitotic inhibitors differed, suggesting that cold induced changes were not due primarily to either axonal degeneration or reduced axonal transport.