An examination of the extent of adult neurogenesis in the carpet shark (Cephaloscyllium isabellum)

Abstract

Following Altman’s and Kirsche’s challenge to the dogma that no new neurons could be produced during adulthood in the 1960’s, adult neurogenesis was shown in most vertebrate lineages. From a phylogenetic point of view, adult neurogenesis is not an uncommon event, having been demonstrated in reptiles and birds, amphibians, bony fi shes and mammals. At present, however, adult neurogenesis has not been examined in cartilagineous fi shes, the stem line of vertebrates. Sharks are an ideal group in which to study the extent of adult neurogenesis for several reasons: (i) they exhibit continuous body growth throughout life; (ii) in the stingray the number of peripheral axons and neurons continues to increase into adult life; and (iii) in adult gray reef sharks the number of inner ear hair cells also continues to increase. We have begun to evaluate the extent of adult neurogenesis in the carpet shark (Cephaloscyllium isabellum). A specimen of C isabellum was injected i.p. with 230 mg/kg of BrdU, anaesthetised and perfused after 2 hrs. The brain was cryoprotected and cut at 40 μm, and processed following standard immunocytochemical techniques. BrdU was found in a small number of nuclei in close proximity to the ventricular surface, in a similar position than occasional cells labelled with an antibody aginst β-tubuline (III). Some BrdU labelled nuclei were also found throughout the brain that were not stained with our neuronal marker. These preliminary data suggest that adult neurogenesis occurs in sharks and that like in bony fi shes, but unlike birds and mammals, it may also occur in non-telencephalic areas. If widespread adult neurogenesis can be unequivocally demonstrated in sharks, it would indicate that it represents the primitive condition. This therefore raises the question of what modifi cations in brain evolution of modern vertebrate lineages led to the restriction of this ability to specifi c forebrain areas.