Changes in nerve cell bodies during the myelination of their axons

Abstract

AbstractChanges in nerve cell perikarya were studied at twelve phases of the myelination period of rat sciatic nerve; data are given on nuclear and nucleolar diameters, the area of cytoplasm, neuronal packing density, length of the nerve, limb and femur, and on the distribution of chromophil substance in the cytoplasm. The study was coordinated with and supplements a previous quantitative electron microscopic study of myelination in rat sciatic nerve.The amounts of cytoplasm in anterior horn motor cells and in large cells of the spinal ganglia increased during myelination, and growth curves for these two cell populations were nearly identical. The increase in cytoplasm was compared with the increase in axoplasm, calculated from the increase in caliber and in fiber length. The volumes of axoplasm and of cytoplasm in the perikarya increased in strict proportion, indicating a coordinated growth of axons and perikarya during the myelination period. Axon growth in turn, occurs in strict coordination with myelin deposition.The growth curve for the small cells in spinal ganglia differed entirely and is consistent with the assumption that nonmyelinated or very thinly myelinated fibers arise from these cells. Changes in the volume of cytoplasm in nerve cells were associated with changes in the nucleo‐cytoplasmic relation, a decrease in neuronal packing density, and a redistribution of RNA in the cells.