Toward a physics of biological tissues

This lecture presents a review of the recent results obtained in our theory group on tissue mechanics and growth. In the first part, we present our general description of the mechanics and growth of tissues, introducing the notion of homeostatic pressure and showing that over times longer than the cell division time, an tissue has a liquid-like behavior and flows. We then illustrate these concepts by discussing 3 types of tissues: multicellular spheroids, the intestinal epithelium and tissue monolayers for which we can compare our theoretical results to experiments. Multicellular spheroids are cell aggregates, which grow to a finite size of the order of a fraction of a millimeter. We discuss the spontaneous flow of cells inside the spheroid and the response of the spheroid to an external pressure. The intestine is the fastest renewing organ. Its surface is a monolayered epithelium. We explain the structure of crypts and villis of the intestinal surface by a buckling instability and discuss the role of cell differentiation.In a monolayer, elongated cells can have a nematic orientational order. We discuss the structure of the defects of the nematic order and a sponta-neous instability that could play a role in cancerous tissues.