A morphometric comparison of central and peripheral hypomyelination induced by postnatal undernourishment of rats.

Developing Long-Evans rats were undernourished to produce a body-weight deficit of 39% at the age of weaning. Well-nourished litter mates were used as controls. Morphometric analyses were made of pyramidal tracts and posterior tibial nerves of each animal. In measuring pyramidal tract, we observed that axonal circumferences of myelinated fibers were smaller in the undernourished rats and that the number of myelin lamellae per axon appeared reduced by a small amount. The most striking observation in the undernourished rats compared to the controls was that the proportion of myelinated fibers was decreased by 40% at 20 days of age. Axon circumferences of nonmyelinated axons were not measured. Similar decreases were observed in myelinated axon circumference and myelin lamellae of posterior tibial nerves of undernourished rats. Because of sampling problems, we did not attempt to compare the proportion of myelinated and nonmyelinated fibers in posterior tibial nerves of undernourished and well-nourished rats. These morphometric data, particularly the reduction of myelinated fibers, are consistent with biochemical studies of brain hypomyelination in undernourished rats. The data indicate that the mechanism of hypomyelination in nutritionally deprived rats involves a failure of the "trigger" by which myelin-forming cells begin to sheath axons in myelin.