Radial glia and neurogenesis in the embryonic zebrafish hindbrain
This thesis is an investigation of the behaviour of radial progenitors in the hindbrain of the teleost fish Danio rerio, the zebrafish, during embryonic neurogenesis. These progenitors have many of the characteristics of radial glial cells described in other systems. Recent studies from several laboratories worldwide have shown that radial glia play an important role in the patterning and neurogenesis of the mammalian forebrain. However, despite the enormous interest in the subject, investigations into neurogenic radial glia have to date been largely limited to the mouse telencephalon. No similar study has yet investigated embryonic neurogenic radial glia in other vertebrates, and very few have been carried out in brain regions other than the forebrain. In this study I used the zebrafish as a model system to identify and characterise radial glia and progenitors in the hindbrain, and examine the role of these cells in neurogenesis in this brain region. Radial progenitor cells in the hindbrain ventricular zone (VZ) were found to express glial fibrillary acidic protein (GFAP) during embryonic development, and some of these cells also expressed markers of proliferation such as PH3. Fatemapping these radial glia-like cells using single-cell labelling techniques, I demonstrated that the major neurogenic event in the zebrafish hindbrain after 48hpf is direct differentiation of progenitors into neurons, without cell division. Very few cell divisions were recorded, and those few that were observed were without exception symmetric, neuron-pair terminal mitoses. In contrast to previous studies of mammalian neurogenesis, no asymmetric cell divisions were observed in my experiments. I then examined the behaviour of VZ progenitors as a population, by taking advantage of the Tg(zFoxD3:GFP) stable transgenic line. In this line, a subpopulation of hindbrain VZ progenitors expresses GFP during development, and I recorded timelapse movies of hindbrain neurogenesis, in transgenic embryos, between 34hpf and 50hpf This revealed the radial migration of GFP-expressing newborn neurons from VZ into the mantle layer, and how these cells utilise multiple GFAP-expressing radial processes as a substrate for this migration. These timelapse videos are the first description of radial migration in a vertebrate brain in vivo.