Mechanics in Morphogenesis: from Physics to Biology and Back

Morphogenesis is a developmental process through which plants and animals acquire their shape and form. While Developmental Biology has identified many of the key genes and cellular mechanisms of morphogenesis, its physical aspects have, until recently, received much less attention. Yet, mechanical interaction between cells is central to forming tissues of a defined shape and its interplay with biological regulation of cellular processes is an essential element of all morphogenetic processes. In this talk I will discuss - and speculate on - how mechanical self-organization on cellular scale acts to convert spatial patterns of developmental gene expression into controlled transformation of tissue shape. I will then focus on tissue mechanics itself arguing that the ability of cells to directly control active cytoskeletal tension -much like muscles! - deprives tissue mechanics of a constitutive relation, pulling the rug under conventional elasticity as a starting point for a theory. Instead, a phenomenological description of tissue mechanics can be built on the solid foundation of force balance in a foam-like network of cytoskeletal "muscles" with their tension under local biological control. The resulting Active Tension Network (ATN) model has neither a constitutive relation nor a reference state, yet it exhibits elasticity and plasticity as emergent behavior, controlled by internally generated tension and external boundary conditions. ATN model links controlled dynamics of local tension on cellular scale to the macroscopic shape of tissue providing a useful theoretical and computational framework in the study of morphogenesis. It also provides an example of how the study of biological phenomena pushes the envelope of physics beyond what we have learned from textbooks.