Effects of dorsal root transection on morphology and chemical composition of degenerating nerve fibers and reactive astrocytes in the dorsal funiculus.

The morphology and chemical (elemental) composition of the dorsal funiculus of the rat spinal cord were examined 1 and 7 days after unilateral transection (rhizotomy) of the L4 and L5 dorsal roots, using light and electron microscopy as well as X-ray microanalysis. Changes were observed only in the dorsal funiculus on the side of injury and included disintegration of the axonal cytoskeleton, enlargement of axonal mitochondria, and widening of the myelin lamellae of the injured axons. X-ray microanalysis demonstrated a significant increase in intraaxonal sodium at 1 day after injury. This increase was abolished at 7 days, but at this stage there was a significant lowering of potassium in axons and myelin sheaths and of phosphorus in myelin as well as a marked increase in calcium in the axoplasm of the degenerating axons. The nonneuronal cell compartment, largely composed of astrocytes, showed elevated sodium, chlorine, and calcium and lowered potassium levels. The changes in chemical composition paralleled an increase in immunoreactivity for the calcium-binding Mts1 (S100A4) protein, which is exclusively expressed by white matter astrocytes. The influx of calcium is likely to play a crucial role in the loss of axonal integrity after rhizotomy, while the alterations in potassium, and perhaps also phosphorus, may contribute to activation of the nonneuronal cells, including the up-regulation of Mts1 expression in astrocytes.