Failure of remyelination in areas of demyelination produced in the spinal cord of old rats.

The extent of remyelination was determined one month after injecting 1 microliter of 1.0% lysolecithin into the dorsal columns of adult rats of three age groups--juvenile, young and old. In the juvenile group (2 months) all axons were remyelinated by either oligodendrocytes or Schwann cells. In the young rats (5 months) nearly all axons were remyelinated. However, in the oldest age group (> 12 months) many axons remained demyelinated and there was a decrease in both oligodendrocyte and Schwann cell remyelination. Also, myelin sheaths formed by oligodendrocytes in the old rats were thinner than those found in the young rats; evidence of impaired Schwann cell remyelination was also seen. The appearance of the lesions in the old animals was variable and many contained myelin debris despite the presence of macrophages within the demyelination area. Although some astrocytes were present in the lesion, many of the demyelinated axons were not separated by astrocyte processes. It is suggested that the failure of remyelination in the old rats following lysolecithin-induced demyelination may be related to sluggish responses of astrocytes and/or macrophages to demyelination. However, a lack of recruitable myelin-forming cells in old animals cannot be excluded.