"Mechanics and instabilities of healthy and cancerous tissues"
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In this talk, we present some  theoretical and experimental results on  the
growth and  mechanical properties of healthy and cancerous tissues. 

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We first show that because of the coupling between cell division and the local
stress, a tissue can be considered as a visco-elastic liquid with a relaxation
time smaller than the cell division time.  

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We then show recent experiments on spheroid cell aggregates on the effect of
mechanical stress on tissue growth.

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Finally, we discuss situations where a tissue in unstable. 
For a thick epithelial tissue, we propose a hydrodynamic-like instability of the
basement membrane of the tissue driven by cell division.

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We also  discuss the steady state structure of villis which are the protrusions
of the surface of the intestine or the colon. We describe the formation of
villis as a buckling instability of a polar cell monolayer. The polarity of the
layer does not seem to play a role in the intestine where the villis are
arranged in a square array but it is important in the colon where they are
organized in a hexagonal array. Similar instabilities occur as well for
tube-like cellular structures.