Abstract
This latest lecture deals with tissue flow and extension, ubiquitously encountered during morphogenesis. Following a historical perspective of embryonic development, we present the phenomenon of convergence-extension, whereby a tissue changes geometry without changing its surface. We first discuss the cellular basis of this process, and in particular the role of cellular intercalation, detailing its mechanical characteristics, the role of protrusive and contractile forces and adhesion. On a more tissue-specific scale, we then highlight the importance of tissue interactions at tissue boundaries and the integrated nature of the process. Boundary geometry plays a decisive role. Tissue also elongates as a result of oriented cell division. The lecture discusses the importance of this process and the interactions between cell division and intercalation.
How are these mechanical cellular processes oriented ? The lecture answers this question by examining the fascinating problem of planar polarization in cell mechanics. Planar polarity couples the global polarity of the embryo (e.g. anterior-posterior and dorso-ventral axes) with the local polarity of the cell.
At the end of this lecture, we present a general synthesis for this year. We propose a renewed vision of morphogenetic information based on three modules in permanent interaction across different scales : biochemical, mechanical and geometrical information. We emphasize the great relevance of a self-organizing model of this information flow, without rejecting the usefulness of a " programmatic " vision.
