[The jimpy mouse].

Ultrastructural study of the Jimpy mice shows lack of myelin in the central nervous system: low density of glial cells, defect of maturation of oligodendrocytes and deficiency in axonal growth. The lack of myelin is more proeminent in late myelinating structures: corpus callosum and corticospinal fasciculus. Failure in myelin is rather the consequence of an early interruption of myelination as that of demyelination (a destruction of the sheats).