Neurogenesis In The Lamprey CNS Following Spinal Cord Transection

After spinal cord transection, lampreys recover functionally and axons regenerate. It is not known whether this is accompanied by neurogenesis. Previous studies suggested a baseline level of non-neuronal cell proliferation in the spinal cord and rhombencephalon (where most supraspinal projecting neurons are located). To determine whether cell proliferation increases after injury and whether this includes neurogenesis, larval lampreys were spinally transected and injected with BrdU at 0-3 weeks post-transection. Labeled cells were counted in the lesion site, within 0.5 mm rostral and caudal to the lesion, and in the rhombencephalon. One group of animals was processed in the winter, and a second group was processed in the summer. The number of labeled cells was greater in winter than in summer. The lesion site had the most BrdU labeling at all times, correlating with an increase in the number of cells. In the adjacent spinal cord the percentage of BrdU labeling was higher in the ependymal than in non-ependymal regions. This was also true in the rhombencephalon but only in summer. In winter, BrdU labeling was seen primarily in the subventricular and peripheral zones. Some BrdU-labeled cells were also double-labeled by antibodies to glial-specific (anti-keratin) as well as to neuron-specific (anti-Hu) antigens, indicating that both gliogenesis and neurogenesis occurred after spinal cord transection. However, the new neurons were restricted to the ependymal zone, were never labeled by anti-neurofilament antibodies, and never migrated away from the ependyma, even at 5 weeks after BrdU injection. They would appear to be CSF-contacting neurons.