Traumatic brain and spinal cord injuries are devastating conditions that can result in long-term disability or death.One promising strategy for the development of effective cell replacement therapies involves the study of regeneration-competent organisms.Among these organisms,teleost fish are distinguished by their exceptional potential to regenerate nervous tissue after injury to the central nervous system (CNS).Here,we investigated the cellular mechanisms that mediate the excellent neuroregenerative potential of young trout Salmo trutta fario,including how this potential relates to their ability to continuously generate large numbers of new neurons in the intact CNS.We examined several molecular factors that are implicated in the control of proliferation and further development of the adult-born cells,including catecholamines,gamma-aminobutyric acid (GABA),gaseous intermediates NO and H2S,transcription factor Pax6,and HuC/D protein in both the intact and injured CNS.The majority of the cells generated in the intact brain are derived from progenitor cells localized in specific proliferation zones,which are often associated with ventricular areas in different parts of the CNS.In certain areas of the trout brain,the progeny of the stem cells/progenitors reside near the same diencephalic and medullary proliferation zones where they were born.However,in areas of secondary neurogenesis,which are located in the external layers of the telencephalon,optic tectum,and cerebellum,the young cells migrate over relatively long distances from the proliferation zone to their target site.Radial glial fibers,identified by tyrosine hydroxylase,GABA,and sometimes Pax6,have been implicated in the guidance of the migrating new cells.Differentiation of these young cells into neurons or glia starts as early as during the migration and further maturation into specific subtypes of neuronal or glial cells becomes evident after arrival at the target site.The processes of differentiation and migration may be regulated by dopamine and GABA,as well as by gaseous intermediates NO and H2S.At the target site,the number of new cells is regulated through apoptotic cell death,which was revealed by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay.In salmon brain,mitotically active progenitor cells in the proliferation zones,identified by immunocytochemical staining for proliferating cell nuclear antigen (PCNA),as well as quiescent progenitor cells,exist in the parenchyma.In the intact brain,there is little mitotic activity among this population of progenitor cells.Similarly,most of the microglia remain in a resting state in the absence of injury.In our experiments,we used mechanical removal of patches of the retina in salmon as an injury model.The effect of such lesions was examined in both the optic nerve and optic tectum,the major projection area of retinal ganglion cells.One characteristic feature of the regenerative response of salmon brain after CNS injury is the rapid onset of cell death.Using the TUNEL assay,the first apoptotic cells can be observed as early as 5 min after application of a stab-wound lesion to the optic nerve.Thirty minutes after injury,the number of apoptotic cells reaches 10.3%.After 2 days post-lesion,the number of TUNEL-positive cells reaches a maximum (38.5%) and starts to decline,returning to background levels approximately 16 days after injury.Further investigation of the morphological appearance of the TUNEL-positive cells using immunolabeling with neurochemical molecular markers (cystathionine beta-synthase,NO synthase,and HuC/D) indicated that the vast majority of these cells undergo apoptosis,as opposed to necrosis.Consequently,the elimination of damaged cells through apoptosis,instead of necrosis,is thought to be a key factor contributing to the enormous regenerative capability of the CNS of teleost fish.Microglia and macrophages have been identified by incorporation of carbocyanine dye Dil within a few days after lesion creation in the mesencephalic tegmentum of salmon brain.The excellent regenerative potential of salmon brain is based not only on its capability to limit the degenerative effects of lesions,but also on its ability to generate new cells that replace those lost to injury.In the cerebellum,optic tectum,and telencephalon,the number of cells undergoing mitosis starts to increase 1 day after lesion formation and peaks approximately 1 week after the injury,compared with controls.After injury to the cerebellum and dorsal telencephalon,the cells generated in response originate from two major sources: PCNA-positive stem cell niches that generate new cells constitutively and PCNA-positive areas in the parenchyma near the injury site,which harbor progenitor cell populations that are quiescent in the intact brain.Many of the new cells generated distal to the lesion site migrate,within 2 weeks after the injury,from the area where they were born to the area of the wound.A common feature of all regeneration-competent CNS systems examined thus far is that cells lost to injury are replaced by new cells that differentiate into various cell types,including neurons.In the dorsal telencephalon,the young cells start to express the neuronal marker protein HuC/D as early as 2 days after the injury.These new neurons emerge both near the area of injury and in regions more distal to the injury site.Over several days after injury,the number of HuC/D-expressing new cells gradually increases,particularly at the injury site.This spatiotemporal pattern suggests that at least some of the young cells acquire characteristic properties of neurons during their migration toward the injury site.The study of regeneration-competent organisms provides us with the unique opportunity to gain a broad biological understanding of tissue repair in the adult CNS.Neuronal regeneration in these systems is intimately linked to adult neurogenesis.Every regeneration-competent organism examined thus far also generates large numbers of new neurons constitutively in many regions of the adult CNS.A broad understanding of the biology directing adult neurogenesis and neuronal regeneration will also facilitate the analysis of the selective pressures that have led to the loss of regenerative potential during the evolution of mammals.The study of regeneration-competent organisms could also help identify the factors required to reactivate this potential.