Relation between axons and oligodendroglial cells during initial myelination. I. The glial unit.

The morphology of oligodendroglial-axon units was examined by electron microscopy during ensheathment and initial myelination in developing feline spinal cord and corpus callosum white matter. In addition to a qualitative examination of single sections from many stages of development, a morphological analysis of spinal cord and corpus callosum units was made on the basis of serial sections from a few stages. The results show that myelination commences around embryonic/fetal day 40 and the 20th postnatal day in the spinal cord and corpus callosum areas, respectively. In both areas immature glial cells, lacking the cytological features of typical oligodendrocytes, initially associate with several axons and provide them with cytoplasmic sheaths. Serial section analysis of units, which have begun formation of compact myelin, indicates that individual cells are associated with single myelin sheaths in the spinal cord area, in a way principally similar to the Schwann cell-myelin units in developing peripheral nerves. This suggests the possibility that early spinal cord oligodendrocytes might shift from a polyaxonal to a monoaxonal association after initial ensheathment and before formation of compact myelin. In the corpus callosum area the examined serially-sectioned cells were found to be connected to several myelin sheaths through long thin processes. The myelin sheaths related to one cell are relatively uniform in terms of number of myelin lamellae and axon diameter, but the clockwise/counter-clockwise course of the myelin spiral varies randomly. Units containing both homogeneously uncompacted (cytoplasmic) and fully compacted (myelin) sheaths have not been found. In both areas the ensheathing cells achieve an oligodendrocyte-like cytology during formation of the first layers of compact myelin. These observations support the view that oligodendrocytes are structurally heterogeneous: those myelinating prospective large axons seems to differ from those myelinating axons destined to remain small. The possible functional and pathophysiological implications of this heterogeneity remain to be elucidated.