Lack of sprouting and its presence after lesions of the cat spinal cord

Degeneration methods were used to study the dorsal root and descending projections after chronic partial denervation of adult cat spinal cord. Conventional mapping methods were used, supplemented in some cases by densitometric measurements of the amounts of degeneration present. The amount of shrinkage of spinal gray matter in some sections was estimated by planimetric measurement. Two preparations were used: (1) partial unilateral rhizotomy in which all dorsal roots caudal to T4 were cut except L6 (spared root preparation); (2) complete unilateral deafferentation. The projection of L6 dorsal roots was examined in spared root preparations. T13 dorsal root projections were examined in deafferented preparations in which T13 was the lowest remaining root. The projection of descending systems was mapped in spared root and deafferented preparations. The spared root displayed an increased projection in the lateral portion of the dorsal horn, in the zona intermedia, Clarke's nucleus and in the base and reticular zone of nucleus gracilis. The lowest remaining root (T13) increased its projection to laminae VII and VIII and to the base and reticular zone of nucleus gracilis. In all cases, when an increased projection resulted from prior denervation, the increase never exceeded the boundaries of the normal projection. No sprouting was observed in those regions with the strictest topographical organization, the cell nests of nucleus gracilis or lamina IX of the spinal cord, even though these regions were partially denervated by the chronic lesions. Descending projections were increased on the experimental side of deafferented preparations12 but not of spared root preparations, suggesting that the presence of the spared root may prevent sprouting by descending systems. Because measurements of gray matter indicated that maximal sprouting occurred in segments showing least shrinkage (sprouting of L6 spared root into L6 segment), in this case shrinkage cannot account for the increased density of degeneration. These results suggest that certain conditions are important for the regulation of sprouting in the adult CNS. Firstly, sprouting is limited by a requirement for proximity and/or overlap. Secondly, the strictness of topographical localization within a particular region may limit the likelihood of sprouting into that region. Finally, a competitive or hierarchical relationship among the remaining systems may modify the capacity of a particular system to sprout.