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 mul of 1.0%) lysolecithin into the dorsal columns of adult rats of th ree age groups-juvenile, young and old. In the juvenile group (2 month s) all axons were remyelinated by either oligodendrocytes or Schwann c ells. In the young rats (5 months) nearly all axons were remyelinated. However, in the oldest age group ( > 12 months) many axons remained d emyelinated and there was a decrease in both oligodendrocyte and Schwa nn cell remyelination. Also, myelin sheaths formed by oligodendrocytes in the old rats were thinner than those found in the young rats; evid ence of impaired Schwann cell remyelination was also seen. The appeara nce of the lesions in the old animals was variable and many contained myelin debris despite the presence of macrophages within the demyelina tion area. Although some astrocytes were present in the lesion, many o f the demyelinated axons were not separated by astrocyte processes. It is suggested that the failure of remyelination in the old rats follow ing lysolecithin-induced demyelination may be related to sluggish resp onses of astrocytes and/or macrophages to demyelination. However, a la ck of recruitable myelin-forming cells in old animals cannot be exclud ed.