Self-organization and cellular clustering without chimiotaxis : Instabilities, order and disorder associated with density

Self-organization and clustering are ubiquitous phenomena in living systems. They are observed at all scales, from bird flocks to bacterial colonies, including cell cytoskeleton self-organization. There is a wide range of such kind of self-organized biological processes: bacterial colonies, tissue formation, embryogenesis and tumor growth, to cite only a few examples.From a Physics point of view, these systems can be categorized as self-propelled particles that do not conserved momentum and are mostly out of equilibrium. In these systems, the absence of conservation laws allows for the emergence of a large variety of self-organized structures, which can be studied by stability analysis of the corresponding macroscopic system descriptions.In this chapter, we are interested in the study of mechanisms associated with collective motion and cluster formation of cellular colonies. We aim at showing that intercellular communication by chemotaxis, which is usually identified as the base process leading to the formation of any complex structure, is not required to observe self-organization and clustering. We present several models all showing that cell movement regulated by the local cell density is an alternative to chemotaxis in order to observe cell clustering.