Hypomyelination and recovery of the myelin deficit in heterozygous jimpy mice.

The myelin content of the anterior commissure in female carriers of the jimpy gene (X(+)X(jp)) and in normal male mice (X(+)Y) was determined using morphometric methods. Young jimpy heterozygotes (X(+)X(jp)), 42-43 days old, exhibit a 33% reduction in myelin area fraction in the anterior limb of the anterior commissure. At older ages (⩾ 145 days), the myelin area fraction in the heterozygotes is not different from control values in the anterior limb and the posterior limb of the anterior commissure. In comparing the young and old jimpy heterozygotes, a significant increase occurs in the myelin area fraction after 43 days of age. The increase in the myelin area fraction in the anterior limb of the heterozygote may occur in response to the myelin deficit caused by the jimpy mutation and may involve the formation of new myelin. A further study of cellular interactions in the central nervous system of the jimpy heterozygote may provide new insight into mechanisms of myelin formation and remyelination.