Abstract
Non-apical neuronal progenitors divide at a distance from the ventricle and are thought to be responsible for the expansion of the cerebral cortex during mammalian evolution. Non-apical progenitors are also present in non-cortical regions and non-mammalian species, but their function in these regions is less well understood. Non-apical progenitors have not been widely studied in zebrafish. The aim of this thesis is to characterize non-apical neuronal progenitors during zebrafish embryonic development and investigate the mechanisms regulating their generation and distribution.We have identified a varied population of non-apical divisions using immunohistochemical analysis of mitotic cells throughout early zebrafish development. In the spinal cord, non-apical divisions appear to be highly spatially restricted and express either vsx1 or olig2. In the hindbrain, these non-apical divisions are seen in different spatial compartments and the majority express vsx1. We have also observed a small number of non-apical divisions in the telencephalon, the analogous structure to the mammalian cortex, although coexpression with vsx1 in this region is rare. Therefore, similar to mammalian brains, our data shows that diverse subpopulations of non-apical neuronal progenitors are generated during zebrafish embryonic development. Our data also suggest that non-apical progenitors share features of their differentiation with neurons; they express the early neuronal marker HuC/D, they require aPKC to differentiate (Alexandre et al. 2010) and are regulated by Notch signalling. These findings might provide insights into the evolution of non-apical progenitors in the zebrafish neural tube as well as mammalian species.
To further understand the development of vsx1 non-apical progenitors I carried out a time-lapse analysis of neuronal differentiation in the Tg(vsx1:GFP) line. This revealed that initial vsx1 progenitors appear in a long-distance spacing pattern and subsequent vsx1 progenitors differentiate in the intervening space. This pattern forms independently of mesoderm-derived signals. By quantifying the spatiotemporal dynamics of this pattern as it forms, I found that two progenitors are unlikely to differentiate close in time and space. High-resolution imaging of non-apical progenitors during the differentiation process revealed that they extend transient processes along the basal surface (basal arms) that retract before division. We propose that these transient basal arms deliver long-distance inhibitory signals to self-organise the differentiation of vsx1 progenitors, similar to a mechanism known to pattern neuronal cells in Drosophila (Cohen et al. 2010; De Joussineau et al. 2003). I have tested and refined this hypothesis by generating a simple model using quantitative data regarding the pattern formation and the dynamics of basal arms. These data provide the first detailed insight into spatiotemporal dynamics of neurogenesis in the zebrafish spinal cord.
| Date of Award | 2016 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Corinne Houart (Supervisor), Jon Clarke (Supervisor) & Paula Ale De Paiva Alexandre (Supervisor) |