A quantitative morphological study of the recovery of cat lingual nerves after transection or crushing

The morphological changes were examined proximal and distal to crush and transection injuries of the lingual/chorda tympani nerve. Under general anaesthesia the nerve was transected unilaterally in 6 adult cats and crushed with watchmakers forceps in 6 others. After 12 wk, again under general anaesthesia, the injured and contralateral (control) nerves were removed, fixed and embedded for histological examination. Sections were cut from sites proximal and distal to the injury and from a site equivalent to that of the injury on the control side. Using systematic randomised sampling techniques the number of nonmyelinated axons and the number and size of myelinated axons in each nerve at each location was estimated. In addition, the mean number of nonmyelinated axons in each Schwann cell unit was determined. The only significant difference between control and injured nerves proximal to either injury was a reduction in the number of myelinated axons in the chorda tympani after transection, and an increase in their mean size. This indicates a selective loss of smaller fibres and is consistent with the poor recovery of gustatory and thermosensitive fibres previously reported (Robinson, 1989). Distal to both types of injury there was an increase in the number of fascicles. The mean number of myelinated axons was reduced distal to a crush injury but unchanged distal to transection. The number of nonmyelinated axons distal to a transection injury was 5 times control counts and after a crush injury double. These findings suggest that sprouting persists 12 wk after both injuries but is much greater after transection.