Ultrastructural study on myelination in rat spinal cord during the early postnatal stage

The progress of myelination and the appearance of myelinated fibers in the anterior funiculus of the lumbar spinal cord of newborn rats were examined by electron microscopy. Myelin was seen only in the relatively larger axons on the first postnatal day, and the number of myelinated axons increased in number with age, but no tract-specific development in myelination could be observed in the anterior funiculus. During the early development of the white matter in the spinal cord, active immature oligodendrocytes, whose cytological characteristics differed from those of mature oligodendrocytes, were seen. The cytoplasmic processes of these immature oligodendrocytes possessed electron dense material, which might be contributive to oligodendrocytic phagocytosis. This element might play a significant role in the myelination mechanism.