Polarization, which confers asymmetry at molecular, cellular and tissue scales, is a fascinating process establishing fundamental features of biological systems. In multicellular organisms, symmetry breaking triggers the specification of embryonic body axes, governing the positioning of subsequent morphogenetic processes. Cells and tissues acquire complex polarity features, which remarkably, are highly precisely positioned within the body axes. How are polarization processes spatially oriented remlains fully enigmatic. During the formation of the nervous system, some crucial processes are polarized. Likewise, the navigation of neuronal projections in the body is a typical polarized process, axons selecting specific pathways to reach their targets. Studies in this field established crucial roles for topographic cues in controlling the polarized growth of neuronal projections. Up to now, my lab has focused on axon guidance mechanisms and while investigating the links between spatial position and neural circuit formation, I became convinced that topographic signalling must be equally required to set other key polarized processes of the developing nervous system. For example in the neuroepithelium, progenitor division is polarized along the apico-basal axis of the neural tube. Likewise in the young post-mitotic neuron, precise coordinates along the body axes define the site where the axon emerges. First, we postulate the existence of a topographic signaling giving to neuronal cells (but this might be a more general case) landmarks of the different embryonic axes so that polarization takes place with appropriate spatial orientation. Second, we make the assumption that this topographic signalling is ensured by cues initially identified for their role during axon navigation. Our goals are to explore these issues, using as a model the sensorimotor circuits, where several processes can be investigated for questioning the interplay between polarity and topography.
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