Neurogenesis and Neuronal Regeneration in the Adult Reptilian Brain

Abstract

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.

Adult neurogenesis in non‐mammalian vertebrates

“…The formation of new neurons in reptiles is strongly regulated through proliferation within the ventricular zone. The number of dividing cells is photoperiod-dependent, (4) but the molecular mechanisms involved remain unknown. In fishes, a cell-extrinsic cue that may drive adult neurogenesis, is the neuropeptide somatostatin.…”

“…However, the brain territories undergoing adult neurogenesis follow patterns that vary between species and may relate to different functions. (4)(5)(6) Here we review current knowledge of some widely used model systems (see also Fig. 1), although more extensive comparative work is clearly needed to assess the general value of these observations for each taxonomic group.…”

Characterization of adult neurogenesis in lizardPhrynocephalus vlangalii(Agamidae: Reptilia)

“…The present study showed that in reptiles, at least in lizards, lateral ventricular system is responsible for massive proliferative activity. The specialized ependyma known as sulci are exclusively possessed by reptiles, they are remnants of the embryonic germinative tissue and still retain their proliferative potential till adulthood (Font et al 2001). It has been reported that recognized sulcus septomedialis was similar to the neurogenic centers and responsible for adult neurogenesis in birds (Alvarez-Buylla et al 1990).…”

“…We hypothesize that these differences depend on the coordinated interaction between different environmental cues and growth of sensory systems. It is conceivable that corresponding sensory brain regions are closely associated with local adult neurogenesis (Alvarez-Buylla et al 1990;Polenov & Chetverukhin 1993;Zupanc 1999;Font et al 2001;Hou & Wu 2007).…”

Species Selection in Comparative Studies of Aging and Antiaging Research

“…There is some evidence that telomere biology is different in turtles (Girondot and Garcia, 1999), and some results suggest that the brains of turtles have enhanced mechanisms to protect against reactive oxygen species formation and damage (Lutz et al, 2003). Likewise, neurogenesis may be predominant in reptiles (Font et al, 2001). Since other turtles may feature negligible senescence and oocyte regeneration (Finch, 1990), turtles are promising models for antiaging medicine (de Magalhaes, 2004).…”