A quantitative electron microscopic study of myelination in the pyramidal tract of rat.

Abstract

AbstractThe ultrastructure of fibers during myelin formation in the pyramidal tract of rats is described. The distribution of fiber classes based on counts of myelin lamellae was determined for newborn, young and mature rats. In newborn rats (2–12 days), growth of the axon was extremely rapid in fibers undergoing early myelination, resulting in greater variation in the relation between axon circumference and sheath thickness and, also, in the presence of myelin sheaths that were unusually thin in relation to the size of the axons. In young rats (12 days to 8 weeks), the numbers of myelin lamellae present in the sheaths increased in proportion to the increase in axon circumference. In adult rats, the numbers of myelin lamellae present in the sheaths was in linear relation to axon circumference for all sizes of myelinated fibers. Approximately 20% of the fibers were nonmyelinated. The number of glia cells per axon at the onset of myelination was approximately 20% of the adult ratio. During growth, myelination gliosis resulted in a steady increase in the number of glia cells per axon until adult levels were ultimately achieved. Our observations suggest that formation of myelin lamellae by oligodendroglia cells may be controlled by the caliber of the axon.