The corpus callosum during postnatal undernourishment and recovery: a morphometric analysis of myelin and axon relationships.

This study was designed to compare morphometric relationships between myelin lamellae and axons in undernourished and well nourished developing rats, and in rats nutritionally rehabilitated for two weeks. Although sampling techniques employed in this study were not specifically designed to compare numbers of myelinated fibers in test and control populations, we did observe a trend indicating a reduction in the numbers of myelinated fibers. The mean numbers of myelin lamellae, from an average of all myelinated axons, were not different in control and test population. However, regression analysis of axon sizes by numbers of myelin lamellae revealed significant differences from the normal in 21-day-old undernourished rats. For callosal axons of any size, there were too few myelin lamellae in the undernourished rats. A partial recovery was observed in relatively small fibers by 35 days of age, but no recovery was observed in larger sized fibers. Comparison of the frequency distribution of axon circumferences of myelinated fibers revealed an increase in average axonal caliber. Computation shows that although mean numbers of lamellae were not altered by undernourishment, the axons themselves are increased in size by about 10%. This unexpected result indicates that the relationship normally governing the numbers of myelin lamellae is altered by postnatal nutritional deprivation, and that the relatively larger axon calibers do not produce in the ensheathing oligodendroglia any compensatory increase in the layers of myelin.