Schwann cell myelination of the myelin deficient rat spinal cord following X-irradiation

The myelin‐deficient (md) rat is an X‐linked myelin mutant that has an abnormality of oligodendrocytes and a severe paucity of myelin throughout the CNS. This lack of myelin makes it an ideal model in which to study the cellular interactions that occur when “foreign” myelinating cells are induced in the milieu of this nonmyelinated CNS. In this study, Schwann cells were induced in the lumbosacral spinal cord by exposing it to radiation, a technique demonstrated repeatedly in other nonmutant strains of rats. Md rats and their age‐matched littermates were irradiated (3,000 to 4,000 R) at 3 days of age and perfused 16–22 days later after pulse labeling with tritiated thymidine. In the md rat, Schwann cell invasion progressed from the area of the spinal cord–nerve root junction and extended into the dorsal columns and adjacent gray matter. Autoradiographic evidence revealed that many of these cells incorporated 3H‐thymidine, indicating that they were undergoing proliferation. Ultrastructural observations showed that there was an integration of these intraspinal Schwann cells with the cells normally occurring in this environment, i.e., oligodendrocytes and astrocytes. The extent of migration and division of Schwann cells, as well as their interactions with glial cells, were similar to those seen in the nonmutant irradiated littermates. These studies provide conclusive evidence that md rat axons are normal with respect to their ability to provide trophic and mitogenic signals to myelinating cells.