A comparison of axonal numbers in dorsal roots following spinal cord hemisection in neonate and adult rats.

This study presents myelinated and unmyelinated axon counts from thoracic dorsal roots of rats whose spinal cords were hemisected at birth or at 1 year of age. Axonal numbers from a root on the unoperated side are compared to numbers from the root of the same segment on the operated side of the animal. Counts were made 3 segments cranially and 3 segments caudally from the hemisection. In animals hemisected at birth and sacrificed at 3-8 months, there is a statistically significant increase in unmyelinated axons in roots of the operated as compared to the normal side. We interpret this as sprouting of unmyelinated axons. In animals hemisected at 1 year of age, the statistically significant change was a drop in myelinated axons in roots of the operated side. We interpret this as a loss of myelinated axon cell bodies due to axon section in the dorsal funiculus. Thus axonal sprouting occurs in young rats in our paradigm and a loss of myelinated axons occurs in older animals. We emphasize that different axonal populations respond to hemisection in different ways at different times.