Séminaires MSC
"Matière et Systèmes Complexes"

Lundi 5 juillet 2010 à 11h30
Bâtiment Condorcet, 4ème étage, salle 454 A.

Lev Beloussov
(Laboratory of Developmental Biophysics, Moscow State University)

Interplay of active and passive mechanical forces in the development of organisms

By several fundamental criteria, the development of organisms should be regarded as a process of self-organization rather than a mere unfolding of any pre-established program. Therefore, it should be based upon non-linear feedbacks between its dynamic components, and the main question is what might be the nature of these feedbacks. We suggest that the leading role in the developmental feedbacks is played by interactions between mechanical forces (or, more precisely stresses, that is, mutually balanced forces) applied to a given part of embryo from outside (passive stresses) and those generated within a given part itself (active stresses). More specifically, we ascribe to embryonic tissues a tendency to response to any stress shifts (let they be produced by another part of the same embryo, or imposed by experimenter) by the active restoration of the initial stress value followed by a certain overshoot (hyper-restoration, or HR hypothesis). We present several examples belonging to cellular and supracellular levels which confirm this idea and describe some standard morphogenetic feedbacks derived from HR hypothesis. We show that several prolonged and complicated developmental processes can be represented as the chains of such mutually coupled feedbacks. There are good evidences to suggest that in some cases at least the spatial patterns of gene expression also obey this rule. In the broader aspect, HR responses can be regarded as the natural consequences of the dynamics described by well known Van der Pol equations. These equations parameters, and hence the overshoot responses themselves can be opened for genetic and epigenetic regulation.