Differential regenerative growth of CNS axons into tibial and peroneal nerve grafts in the thalamus of adult rats.

Segments of peripheral nerve were autografted into the thalamus of adult rats. The peroneal nerve was used in one group, the tibial nerve (which has approximately twice the cross-sectional area of the peroneal nerve) in a second group, and two lengths of peroneal nerve side by side in a third group. Between 1 and 4 months later HRP was applied to the distal end of each graft to label neurons which had regenerated their axons into the graft. Serial coronal sections of each brain were reacted to reveal retrogradely transported HRP, and the positions of all labeled neurons were recorded in camera lucida drawings. In all three groups a few labeled neurons resembling thalamocortical projection cells were found in the dorsal thalamus close to the graft tip (mean number, 29 in the single peroneal group; 22 in the tibial group; and 14 in the double-peroneal group). However, neurons in the thalamic reticular nucleus (TRN) regenerated much more successfully into the larger nerve grafts; many more retrogradely labeled cells were found in animals with tibial or double-peroneal nerve grafts (mean number, 1.1 in the single-peroneal group; 272 in the tibial group; and 163 in the double-peroneal group). These neurons were concentrated in the sector of TRN known to project to the part of the dorsal thalamus containing the graft tip. The largest numbers of labeled neurons were found when the graft tip encroached upon the TRN. These results suggest that both graft size and graft position are critical determinants of the extent of axonal regeneration from the TRN. Larger grafts may be more copiously invaded by regenerating axons because such grafts damage larger numbers of TRN axons when implanted and/or because they stimulate regeneration by releasing critical quantities of neurotrophic factors.