Three-dimensional electron microscopic studies of the transitional oligodendrocyte associated with the initial stage of myelination in developing rat hippocampal fimbria.
We identified the transitional oligodendrocyte and their processes of rat hippocampal fimbria associated with the initial stage of myelination in both the morphological and functional classifications by means of three-dimensional ultrastructural analysis. Transitional oligodendrocytes appeared around P7, and their cell bodies were morphologically an intermediate form between the light and medium oligodendrocytes described by Mori and Leblond [J. Comp. Neurol. 139 (1970) 1]. Three phenotypes of the transitional oligodendrocytic processes were recognized. Spiral wrapping processes were ensheathing processes, club-like processes were nonensheathing processes, and sheet-like processes were possibly the transmuting form between the nonensheathing and ensheathing processes. Club-like processes were the major part of the nonensheathing processes, and most likely function as sensors to perceive axon maturation and find target axons. Multivesicular bodies that appeared to be associated with the initial ensheathment were observed in the transitional oligodendrocytic processes, suggesting that their roles are crucial in myelinogenesis.