Sustained hypomyelination and high serum thyroid hormone in aged black tremor hamster

Oligodendrocytes and myelin in the corpus callosum of black tremor and normal hamsters aged over 1.5 years were ultrastructurally examined to determine the myelination index (ratio of myelin thickness/diameter of axon), percentage of naked axons, and proportions of oligodendroglial subtypes (light, medium and dark). The mutant hamsters were remarkably hypomyelinated, with a low myelination index and a high proportion of naked axons, and high proportions of the dark subtypes. Serum concentrations of thyroid hormones (T3 and T4) in 6-week-old mutant hamsters were 2-fold (T3) to 3-fold (T4) higher than those of age-matched normal animals. However, in the aged animals (over 1.5 years old) only T4 levels of the mutant hamsters were higher in the mutant than normal hamsters. The black tremor hamsters were hypomyelinated throughout their life and high serum level of thyroid hormones might have played a role in the hypomyelination.