Structural changes in kittens' ventral and dorsal roots L7 after early postnatal sciatic nerve transection

The number and size distribution of myelinated and unmyelinated axons were studied in spinal roots L7 of 19 kittens, 8 to 200 days after early postnatal left sciatic nerve transection. Ventral and dorsal roots on the side of transection were compared with corresponding contralateral roots. Three normal kittens were used as additional controls. On the control side the proportion of unmyelinated ventral root axons increased from about 15 to 30% between 3 and 7 months postnatally. In the ventral roots on the lesion side there was a loss of myelinated axons of all sizes (total loss 15 to 25%). The loss seemed to be somewhat greater in the gamma population. The number of unmyelinated ventral root axons increased markedly through sprouting. This increase was similar at different root levels. The persistence of such axonal sprouts in the proximal stump after ventral root division in one kitten indicated that they originate proximally in the ventral root or within the central nervous system. The dorsal roots on the lesion side showed a 30% deficit of both myelinated and unmyelinated axons. Signs of axonal sprouting were not observed. Both in ventral and dorsal roots the size spectra of myelinated axons were markedly shifted to the left on the lesion side due to a growth retardation of larger axons. With respect to the unmyelinated axons the size distribution was expanded toward larger sizes in the ventral roots and remained largely unaltered in the dorsal roots.