The fate of prelabeled Clarke's column neurons after axotomy

We studied Clarke's Column of the L-1 spinal cord segment of young adult female rats after first prelabeling its neurons by the intracerebellar injection of Fluoro-Gold or true blue and subsequently axotomizing the labeled cells by a complete spinal cord transection at T-9. In control rats, the number of labeled neurons at 1, 5, 10, and 20 weeks showed a progressive decrease, probably due to leakage of dye from the cells. A much greater loss of labeled neurons was found in T-9 spinal cord-transected rats than in their matched controls. At 5 weeks after transection, loss of large neurons was somewhat offset by an increase in small neurons; neuron shrinkage was a likely cause of this increase, because small, very intensely labeled neurons were found in transected rats but not in control rats. By 10 and 20 weeks post-transection, the number of all prelabeled neurons in transected rats had sharply decreased. In transected rats, but not in controls, very significant increases in labeled astroglia and microglia and other labeled small cells were found at 5 weeks. At 10 weeks, the identifiable labeled astroglia had decreased but marked increases in microglia and other labeled small cells persisted. We conclude that, following a complete T-9 spinal cord transection, axotomized Clarke's column neurons first shrink in size and then die. Labeled reactive astrocytes, which are most evident 5 weeks after injury, probably indicate phagocytosis of axotomized neurons.