Relation of axon membrane to myelin membrane in sciatic nerve during development: comparison of morphological and chemical parameters.

The relationship of axon circumference to myelin area on cross-section both for myelinated and total fibres has been estimated at a mid-point in the sciatic nerve of two rabbits, one newborn and the other 20 weeks old. The slopes of the regression lines relating myelin area to axon circumference for myelinated fibres were 0.72 (standard error 0.03) with a correlation coefficient of 0.81 in the newborn nerve and 2.96 (standard error 0.05) with correlation coefficient of 0.96 in the 20-week nerve. Using the above parameters, and assuming constant thickness of axolemma and constant packing density of myelin, a correlation was made of the change in relationship of axolemma volume of myelinated fibres to myelin volume in the nerve between the two ages with the changes in content of two membrane lipids, ganglioside and cerebroside. This revealed a constant relationship of morphological to chemical parameters at the two ages although the proportion of unmyelinated fibres was much greater in the newborn. If cerebroside is regarded as a marker for myelin membrane, this observation suggests an association of gangliosides with myelination in peripheral nerve.