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The postnatal differentiation of rat optic nerve fibres was examined by transmission electron microscopy. The results show that many early developing axons contain clusters of vesiculotubular profiles prior to myelination. At places vesicular elements appear to fuse with the axolemma, and, in addition, some axons exhibit deep axolemmal invaginations and axoplasmic lamellated bodies. It is suggested that these features might reflect axolemmal remodelling, possibly involving axoglial signalling and/or functional differentiation of the axolemma. The size distribution of unmyelinated optic nerve axons changes little during development. Ensheathment of larger axons commences 6 days postnatally. The subsequent formation of compact myelin sheaths is accompanied by an increase in axonal diameter. The early sheaths are a few microns long and separated by long bare axon segments. In optic nerves from 10-12-day-old rat pups, a few sheaths consisting of about five layers border primitive asymmetric nodes with a patchy axolemmal undercoating. Extensions from one of the terminating sheaths are often associated with undercoated patches of axolemma. Relatively differentiated nodes of Ranvier first appear 14-16 days after birth. The continued nodal maturation involves establishment of a regular nodal geometry, increasing distinctness of the axolemmal undercoating, and formation of perinodal astrocytic processes embedded in an extracellular node gap substance. The results are compared with available data on the conduction properties of rat optic nerve fibres during development.

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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.