REGENERATIVE AXONAL SPROUTING IN THE CAT TROCHLEAR NERVE

Following peripheral trochlear nerve axotomy in the cat, the normal nu mber of myelinated axons is restored despite significant motor neuron death, suggesting regulation of the number of myelinated axons in the regenerated nerve. In this study we used light and electron microscopy to examine the production and maintenance of axonal sprouts at differ ent locations in the nerve and at different postoperative intervals. D espite proliferative sprouting and an overproduction of nonmyelinated axons in the regenerating trochlear nerve, the number of myelinated ax ons was strictly regulated. Only similar to 1,000 regenerated axons we re eventually remyelinated, but many nonmyelinated axons were still pr esent 6-8 months postaxotomy. Regenerated axons were remyelinated in a proximal-to-distal direction between 3 and 4 weeks postaxotomy. We al so examined the maturation of regenerated myelinated axons by measurin g axon diameter and myelin index (an expression of myelin thickness). Mean myelinated axon diameter remained significantly below normal in l ong-term regenerated nerves. Mean myelin index was not different from normal at 4 weeks postaxotomy but was significantly decreased at long postoperative intervals, reflecting a slightly thicker myelin sheath r elative to the axon diameter. This relative increase in mean myelin th ickness could serve to restore normal conduction velocity despite the decrease in mean axon diameter. We suggest that the regulation of the number of myelinated axons at the normal number despite cell death and the increase in mean myelin thickness may both be compensatory mechan isms that function to restore preoperative conditions and maximize fun ctional recovery. (C) 1995 Wiley-Liss, Inc.