Axonal underdevelopment from axotomy in kittens.

Permanent axotomy, by hind limb amputation in kittens, is found by morphometric evaluation at 9 months after amputation to result in failure of ventral and dorsal root myelinated fibers (MFs) to attain adult calibers. The positions of the median diameter and of diameter peaks were unequivocally displaced to smaller diameters, but more so for dorsal than for ventral roots. When fiber spectra of amputated roots were compared with those of kittens at the time of amputation, the median diameter, the position of peaks of diameter histograms, and the diameters of largest MFs were clearly at larger diameters, an indication that axonal development had continued (especially for motor fibers) for some time after amputation. These studies have shown that whereas in adult cats, permanent axotomy results in axonal atrophy, in kittens axonal development continues, but stops short of reaching adult values. As axonal caliber presumably ultimately relates to perikaryal synthesis of protein, one must infer either that the signal to decrease protein synthesis is slow in arriving at the perikarya or that the decreased axonal flow of materials resulting from such a signal is slow and thus allows continued axonal growth for a period after amputation.