Adult neurogenesis in primate and rodent spinal cord: comparing a cervical dorsal rhizotomy with a dorsal column transection.

Neurogenesis has not been shown in the primate spinal cord and the conditions for its induction following spinal injury are not known. In the first part of this study, we report neurogenesis in the cervical spinal dorsal horn in adult monkeys 6-8 weeks after receiving a well-defined cervical dorsal rhizotomy (DRL). 5-bromo-2-deoxyuridine (BrdU) was administered 2-4 weeks following the lesion. Cells colabeled with BrdU and five different neuronal markers were observed in the peri-lesion dorsal horn 4-5 weeks after BrdU injection. Those colabeled with BrdU and neuron-specific nuclear protein, and BrdU and glial fibrillary acidic protein were quantified in the dorsal horn peri-lesion region, and the ipsi- and contralateral sides were compared. A significantly greater number of BrdU/neuron-specific nuclear protein- and BrdU/glial fibrillary acidic protein-colabeled cells were found on the lesion side (P<0.01). These findings led us to hypothesize that neurogenesis can occur within the spinal cord following injury, when the injury does not involve direct trauma to the cord and glial scar formation. This was tested in rats. Neurogenesis and astrocytic proliferation were compared between animals receiving a DRL and those receiving a dorsal column lesion. In DRL rats, neurogenesis was observed in the peri-lesion dorsal horn. In dorsal column lesion rats, no neurogenesis was observed but astrocytic activation was intense. The rat data support our hypothesis and findings in the monkey, and show that the response is not primate specific. The possibility that new neurons contribute to recovery following DRL now needs further investigation.