Neurogenesis and neuronal regeneration in the adult reptilian brain

Evidence accumulated over the last few decades demonstrates that all reptiles examined thus far continue to add neurons at a high rate and in many regions of the adult brain. This so-called adult neurogenesis has been described in the olfactory bulbs, rostral forebrain, all cortical areas, anterior dorsal ventricular ridge, septum, striatum, nucleus sphericus, and cerebellum. The rate of neuronal production varies greatly among these brain areas. Moreover, striking differences in the rate and distribution of adult neurogenesis have been noted among species. In addition to producing new neurons in the adult brain, lizards, and possibly other reptiles as well, are capable of regenerating large portions of their telencephalon damaged as a result of experimentally-induced injuries, thus exhibiting an enormous potential for neuronal regeneration. Adult neurogenesis and neuronal regeneration take advantage of the same mechanisms that are present during embryonic neurogenesis. New neurons are born in the ependyma lining the ventricles and migrate radially through the brain parenchyma along processes of radial glial cells. Several lines of evidence suggest that radial glial cells also act as stem cells for adult neurogenesis. Once they reach their final destination, the young neurons extend axons that reach appropriate target areas. Tangential migration of neurons alongside the ventricular ependyma has also been reported. Most of these tangentially migrating neurons seem to be destined for the olfactory bulbs and are, thus, part of a system similar to the mammalian rostral migratory stream. The proliferation and recruitment of new neurons appear to result in continuous growth of most areas showing adult neurogenesis. The functional consequences of this continuous generation and integration of new neurons into existing circuits is largely conjectural, but involvement of these phenomena in learning and memory is one likely possibility.