PT  - JOURNAL ARTICLE
AU  - “Forces in tissues” workshop participants
TI  - Measuring forces and stresses <em>in situ</em> in living tissues
AID  - 10.1101/016394
DP  - 2015 Jan 01
TA  - bioRxiv
PG  - 016394
4099  - http://biorxiv.org/content/early/2015/05/22/016394.short
4100  - http://biorxiv.org/content/early/2015/05/22/016394.full
AB  - Development, homeostasis and regeneration of tissues result from the interaction of genetics and mechanics. Kinematics and rheology are two main classes of measurements respectively providing deformations and mechanical properties of a material. They are now applied to living tissues and have contributed to the better understanding of their mechanics. Due to the complexity of living tissues, however, a third class of mechanical measurements, that of in situ forces and stresses, appears to be increasingly important to elaborate realistic models of tissue mechanics. We review here several emerging techniques of this class, their fields of applications, their advantages and limitations, and their validations. We argue that they will strongly impact on our understanding of developmental biology in the near future.Deformationrelative change in size of an object. In one dimension, it is a dimensionless number: the fraction of change in the object length, whether positive (elongation) or negative (contraction). In two or three dimensions, elongation in one direction can coexist with contraction in another direction, so that the deformation is described by amplitudes in different directions (see “dilation” and “shear”). Also called “strain”.Deformation ratedeformation per unit time, expressed as the inverse of a time. In one dimension, it can be for instance a change of a few percent per hour. In two or three dimensions, it is more complex (see “deformation”), and is equal to the symmetrical part of the velocity gradient.Dilationcomponent of the deformation, which is isotropic, corresponding to a change of size.Dynamicsdescription of a material evolution in reference to the forces which cause it.Elastic deformationreversible part of the deformation.Elastic modulussee “stiffness”.Elasticitycapacity of a material to deform and store energy in response to an applied stress, and reversibly return to its initial rest shape after stress is removed.Kinematicsvisual description of motion, without reference to forceLiquid-like behaviourbehaviour of a material with a non-defined rest state, which can be continuously deformed by a very small shear stress.Plastic deformationirreversible part of the deformation.Plastic materialmaterial that undergoes irreversible deformation in response to an applied stress.Plasticitycapacity of a material to be sculpted, i.e. acquire a new rest shape under an applied stress, and keep it when the stress is removed.Purely elastic materialMaterial that responses immediately and elastically to applied stress.Purely viscous materialmaterial that flows in response to a shear stress and does not recover its initial shape after stress is removed.Rheologyscience of flow and deformation of materials.Shearcomponent of the deformation that is purely anisotropic, that corresponds to a change of shape.Solid-like behaviourbehaviour of a material with a defined rest state.Stiffnessquantity which measures the resistance of an elastic solid in response to an applied stress. Also called “elastic modulus” or “rigidity”.Strainsee “deformation”.Stresshere mechanical stress, i.e. forces internal to the tissue, exerted by a tissue region onto a neighbour tissue region. It is a more coarse-grained notion than force, and is expressed as a force per unit size of tissue boundary.Tensionforce internal to an object, tending to decrease its size. The “surface tension” of an aggregate or a piece of 3D tissue tends to decrease its surface area, and is expressed in Newtons per meter. A one-dimensional object like a cell-cell adherens junction has a “line tension”, tending to decrease its length, and expressed in Newtons.Viscosity(1) tendency of a material to resist gradual deformation and flow in response to an applied stress; (2) quantity which measures this property (see “viscous modulus”).Viscous modulusquantity which measures the resistance of a viscous liquid in response to an applied stress. Usually called “viscosity” for short.Yieldhere, defines the transition between the elastic behaviour and the plastic behaviour.