Abstract
This lecture and the following one deal with morphogenesis on a larger scale, that of whole tissues and organs. The aim is to illustrate how tissue morphogenetic processes proceed jointly from a flow of information, as in a deterministic program, and in a self-organizing manner.
We begin with a broad class of deformations : curvatures, folds and invaginations.
The lecture is divided into two main parts. The first is devoted to folding self-organized by mechanical instabilities arising from the differential growth of mechanically connected tissues. Examples include the convolutions of the vertebrate cortex (gyrification), the formation of intestinal loops and the emergence of instestinal villi (villification). Elastic buckling models give a remarkable account of the structures observed. The positioning and orientation of the folds are also determined by the overall geometry of the tissue, as this dictates the stress field in the tissue.
Secondly , the lecture develops the formation of invaginations in vertebrate and invertebrate embryos during the so-called " gastrulation " process. In this case, there is a single invagination at a specific site of an embryo. The general principle is one of hierarchical control by a genetic program that defines the tissue territory to be invaginated, its geometry, and locally directs the amplitude and orientation of contractile invagination forces at the cellular level. Nevertheless, we show that invagination in an embryo can be genetically initiated and then mechanically self-sustained by feedback, as in a self-organizing system. Thus, even in an embryo, program and self-organization constitute two joint modalities of morphogenetic information flow.
