The retina of sharks as a model for studying radial glia development and cell positioning during the neurogenesis of the central nervous system

Résumé

During neurogenesis, neural stem cells (NSCs) gradually change in potential to generate progenitors (neuroepithelial cells and radial glia; RG) that in turn generate different types of neurons and glial cells at different times and locations. In mammals, adult NSCs are generated from a population of radial glial cells (RGCs) present in the embryo and give rise to distinct types of neurons and glia. Contrasting with the situation in mammals, RGCs are widely maintained in the adult CNS of anamniote vertebrates, where they contribute to CNS regeneration. However, cellular hierarchies involving different types of progenitors remain unsolved. The retina of fishes appears as a good model to approach long-lasting constitutive neurogenesis and regenerative neurogenesis because of the presence of high rates of cell proliferation in the adult (from the ciliary marginal zone and from adult RG) and because of the occurrence of reparation processes involving RGCs (Müller cells). Cartilaginous fishes represent an ancient radiation of vertebrates, currently considered the sister group of bony fishes, which makes them an essential comparative reference that may provide insight into the extent of shared and/or divergent characteristics of adult neurogenesis among vertebrates. Among cartilaginous fishes, the lesser spotted dogfish, Scyliorhinus canicula, offers a great potential as a model species because of its phylogenetic position, and because of the relative size of the retina and the peace with which it grows, which provide a particularly good spatial and temporal frame to solve some of the main questions raised in the field. S. canicula as a model, analyzing the distribution pattern of the microtubule associated protein (MAP) doublecortin (DCX), which is involved in regulating cytoskeleton components during migration processes. This thesis has reported for the first time the distribution of DCX throughout the lifespan of a fish by using immunohistochemistry. We also have immunohistochemically identified different types of progenitors by using proliferation (PCNA), migration (DCX), differentiation (HuC/D, Pax6) and radial glial (GFAP, GS) cell markers. Results obtained in S. caniculawere compared with that reported previously in zebrafish and mouse, representative species that have been previously used to study neurogenesis. This will provide insight into the extent of shared and/or divergent characteristics of neurogenesis among vertebrate taxa.