Some observations on early myelination in the human spinal cord. Light and electron microscope study.

Segments of cervical spinal cord from a 23-week-old human foetus have been examined by light and electron microscopy. Myelinated fibres were found in the dorsal, ventral and peripheral lateral tracts, while the lateral corticospinal tract was completely unmyelinated. Myelin sheaths appeared to be formed by spiral wrapping of elongated mesaxons which originated from the apposition of the plasma membranes of oligodendrocytes. Preparations stained with Sudan red and Sudan black revealed the occurrence of lipid inclusions in the interfascicular glia. The topographical relation and the ultrastructural features of these inclusions are described. The possible significance of the inclusion bodies is discussed.