Changing relation between onset of myelination and axon diameter range in developing feline white matter

The size spectra of unmyelinated, ensheathed and initially myelinating CNS axons were examined by electron microscopy in the spinal cord ventral funiculus and the corpus callosum of the cat during development. The first myelin sheaths appeared 4 weeks before and 3 weeks after birth in the spinal and callosal areas, respectively. De novo myelination had largely ceased by 4 months in the ventral funiculus and by 7 months in the corpus callosum. The result show that the diameter ranges, within which spinal and callosal axons undergo primary ensheathment and initial myelination, are markedly different if similar levels of myelination are compared. In both areas, these diameter ranges shift towards smaller sizes with development. However, spinal and callosal axons, which undergo primary ensheathment and initial myelination simultaneously, present comparable diameter ranges. The findings support the view that other factors than the absolute physical size of the axon trigger initiation of CNS myelination. In this respect the developmental stage of the animal appears to play an important role.