Neurogenesis and gliogenesis in the spinal cord of turtles.

A 5-bromo-3'-deoxyuridine (BrdU) pulse administered to juvenile turtles resulted in cell labeling throughout the gray matter (GM) and white matter (WM) of the spinal cord. One and twenty-four hours postinjection, larger densities of BrdU-labeled nuclei (LN) occurred within the GM, with a density peak localized in the central region (CR). Seven days later, density differences between GM and WM disappeared, accompanying a more uniform distribution of LN in the GM (absence of the central peak). Multiple injection experiments also showed similar evolution in the distribution of LN. Morphometric studies revealed that the size of LN had undergone time-related increments: Larger nuclei appeared at protracted fixation time points. Double-labeling experiments indicated that BrdU-labeled cells expressed neuroactive substances, such as gamma-aminobutyric acid (GABA), neuron-specific nuclear protein (NeuN), and the cytoplasmic early postmitotic neuronal marker (TUC-4). Other BrdU-labeled cells expressed the glial-specific protein (GFAP). GABA-BrdU, TUC-4-BrdU, and GFAP-BrdU double-labeled cells were recognized 6 days after the first BrdU injection. NeuN-BrdU double-labeled cells were found at 50 days postinjection. Three-dimensional transmission electron microscopy revealed the presence of synapses and typical kinocilia in putative immature nerve cells. Kinocilia were also found in putative immature glial cells. In consideration of the scattered distribution pattern of BrdU-labeled cells, in animals fixed 1 hour postinjection, the existence of a single proliferating center was discarded. The CR, including the ependymal epithelium, showed the highest density of LN.