Proliferation, migration and differentiation in juvenile and adult Xenopus laevis brains.

In contrast to mammals, the brain of adult non-mammalian vertebrates exhibits a higher proliferative and/or neurogenic activity. To provide new models on this issue, we have examined origin, distribution and fate of proliferating cells in the entire brain of juvenile and adult Xenopus laevis. Using immunohistochemistry for the Proliferation Cell Nuclear Antigen (PCNA), and/or the thymidine analog, 5-Bromo-2' deoxyUridine (BrdU), the labeled cells are located in ventricular zones of the olfactory bulbs, cerebral hemispheres, preoptic region, ventral hypothalamus and cerebellum. Qualitatively, the highest level of proliferative cells was found in the telencephalic ventricles. By using in situ hybridization/immunocytochemistry double-labeling techniques, we demonstrate for the first time in post-metamorphic frog brain that the proliferative cells are localized in very close vivinity to the radial glial cells, progenitor cells that we have also identified in the ventricular layer using classical molecular markers (BLBP, Vimentin). In addition, after long post-BrdU administration survival times ranging between 14 and 28days, BrdU labeling combined with immunohistochemistry for markers of cell migration (DoubleCortin) or radial glial cells (BLBP), reveals that the proliferative cells are able to migrate from the ventricular zone into the brain parenchyma, most likely by migrating along the radial processes. Finally, at survival time of 28days and by using a combination of BrdU labeling and in situ hybridization for markers of differentiation states (Neuro-β-tubulin, Proteolipid Protein), we demonstrate that newborn cells can differentiate in large portion into either neurons or oligodendrocytes.