This fifth lecture addresses the second central theme developed this year, namely that of cell tension. Cellular tension is indeed the main driving force behind tissue plasticity, whether in tissues in which non-cohesive cells migrate, or in more cohesive tissues such as epithelia, in which adherent cell interfaces are constantly remodeled.
We start from the concept of surface tension as a physical parameter that determines certain cell shapes or configurations according to static models of a thermodynamic nature. It then looks at cell shape as a stationary or stable solution of a dynamic system. In this context, the respective contributions of membrane tension and cortical tension are presented. The aim of the lecture is to understand the origin of membrane tension, and to present both quantitative methods for measuring it and physical models for defining its components. Particular emphasis is placed on tension in the plane of the membrane stricto sensu, and tension due to adhesion between cortical actin and the plasma membrane.
Secondly, the lecture explores the implications of a fluid, inelastic membrane model, in which tension is generally uniform across the cell surface. Membrane tension depends, on the one hand, on dynamic interactions with actin, and on the other, on dynamic membrane reservoirs, notably in the form of invaginations. We show how reciprocal interactions between membrane and actin ensure the homeostasis of membrane tension despite any perturbations a cell may experience. Furthermore, the study of cell migration provides an exemplary illustration of how membrane tension can be a controlling parameter of cell physiology, coupling and coordinating distant parts of the cell and determining both its shape and mobility.
