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Axons in the Chick Embryo Follow Soft Pathways Through Developing Somite Segments.

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Schaeffer, Julia 
Weber, Isabell P 
Thompson, Amelia J 
Keynes, Roger J 


During patterning of the peripheral nervous system, motor axons grow sequentially out of the neural tube in a segmented fashion to ensure functional integration of the motor roots between the surrounding cartilage and bones of the developing vertebrae. This segmented outgrowth is regulated by the intrinsic properties of each segment (somite) adjacent to the neural tube, and in particular by chemical repulsive guidance cues expressed in the posterior half. Yet, knockout models for such repulsive cues still display initial segmentation of outgrowing motor axons, suggesting the existence of additional, yet unknown regulatory mechanisms of axon growth segmentation. As neuronal growth is not only regulated by chemical but also by mechanical signals, we here characterized the mechanical environment of outgrowing motor axons. Using atomic force microscopy-based indentation measurements on chick embryo somite strips, we identified stiffness gradients in each segment, which precedes motor axon growth. Axon growth was restricted to the anterior, softer tissue, which showed lower cell body densities than the repulsive stiffer posterior parts at later stages. As tissue stiffness is known to regulate axon growth during development, our results suggest that motor axons also respond to periodic stiffness gradients imposed by the intrinsic mechanical properties of somites.



AFM, axon pathfinding, nervous system development, somite polarity, spinal motor axons, stiffness patterns, tissue stiffness

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Front Cell Dev Biol

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Frontiers Media SA
National Institute of Child Health and Human Development (R21HD080585)
European Research Council (772426)
Wellcome Trust (099743/Z/12/Z)
JS was supported by the International Spinal Research Trust (Nathalie Rose Barr Studentship, ref. RG71958), the Rosetrees Trust (ref. M317), and the Cambridge Philosophical Society (studentship to JS), and acknowledges funding from Fondation pour la Recherche Médicale (FRM) (postdoctoral fellowship SPF201909009106). IW was supported by an EMBO Long-Term Fellowship (ALTF 1263-2015; European Commission FP7, Marie Curie Actions, LTFCOFUND 2013, GA-2013-609409), AT by the Wellcome Trust (grant 099743/Z/12/Z to AT), the Cambridge Philosophical Society and the Cambridge Trusts (studentships to AT), and KF was supported by the European Research Council (Consolidator Grant 772426), the Alexander von Humboldt Foundation (Alexander von Humboldt Professorship), and the Eunice Kennedy Shriver National Institute Of Child Health and Human Development of the National Institutes of Health under Award Number R21HD080585. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.