The cerebral cortex is a highly complex, layered, grey matter structure which plays major roles in processes such as consciousness, memory, and perception. In humans, it is the cortex which bestows us with cognitive abilities unique to our species, in part due to the massive expansion of cortical progenitors during embryonic development. The mature cortex of mammals is composed of a highly heterogenous population of cells, however only the glutamatergic projection neurons are generated within the embryonic neocortex itself. These cells are formed from apical and basal cortical progenitors in the developing dorsal telencephalon, and in the mouse cortical neurogenesis begins at approximately embryonic day 10.5 (E10.5).
Regulation of the differentiation of progenitors into neurons is a tightly regulated process, under the control of a strict interplay between signalling molecules and transcription factors. The zinc-finger transcription factor Gli3 plays established roles in regulating the development of the dorsal telencephalon. Through the evaluation of a number of mutants, Gli3 has been shown to function in patterning, the production and fate of cortical neurons, lamination, and axon tract formation, amongst other things. It is shown here that Gli3 is highly expressed in Pax6+ apical progenitors in the dorsal telencephalon, the largest population of progenitors between E11.5 and E12.5. The aim of this thesis was to evaluate how Gli3 regulates the proliferation and differentiation of cortical progenitors into neurons during these time points in the mouse.
Embryos of the Gli3Xt/Pdn mutant, which exhibit a reduction in Gli3, were used. At E11.5, the proportions of apical, basal, and early-born cortical neurons were altered in the dorsal telencephalon of the mutant. Microarray analysis revealed an up-regulation of Cdk6 in Gli3Xt/Pdn embryos compared to control. Cdk6 is required for the transition from G1-phase into S-phase during the cell cycle, and shortening of G1-phase has been shown to correlate with progenitors self-renewing instead of differentiating. In agreement with the function of Cdk6, G1-phase and the length of the total cell cycle were shortened in the E11.5 Gli3Xt/Pdn mutant. At E12.5, the length of G1-phase and total cell cycle length were also shortened, and the proportion of apical and basal progenitors and early-born neurons were altered. It appears as though there is a delay in apical progenitor differentiation into basal progenitors and
neurons in the Gli3Xt/Pdn embryo, which correlates with the shortening of G1-phase observed. Interestingly, the length of S-phase was also decreased in the mutant at both E11.5 and E12.5, despite no obvious candidates indicative of S-phase regulation being identified in the microarray screen. Taken together, the evidence demonstrates that Gli3 plays a key role in regulating the differentiation of cortical progenitors at the correct time, likely in part due to regulation of the cell cycle, in order to regulate growth of the cerebral cortex
