title: Tissue cohesion and the mechanics of cell rearrangement.
abstract: Morphogenetic processes often involve the rapid rearrangement of cells held together by mutual adhesion. The dynamic nature of this adhesion endows tissues with liquid-like properties, such that large-scale shape changes appear as tissue flows. Generally, the resistance to flow (tissue viscosity) is expected to depend on the cohesion of a tissue (how strongly its cells adhere to each other), but the exact relationship between these parameters is not known. Here, we analyse the link between cohesion and viscosity to uncover basic mechanical principles of cell rearrangement. We show that for vertebrate and invertebrate tissues, viscosity varies in proportion to cohesion over a 200-fold range of values. We demonstrate that this proportionality is predicted by a cell-based model of tissue viscosity. To do so, we analyse cell adhesion in Xenopus embryonic tissues and determine a number of parameters, including tissue surface tension (as a measure of cohesion), cell contact fluctuation and cortical tension. In the tissues studied, the ratio of surface tension to viscosity, which has the dimension of a velocity, is 1.8 µm/min. This characteristic velocity reflects the rate of cell-cell boundary contraction during rearrangement, and sets a limit to rearrangement rates. Moreover, we propose that, in these tissues, cell movement is maximally efficient. Our approach to cell rearrangement mechanics links adhesion to the resistance of a tissue to plastic deformation, identifies the characteristic velocity of the process, and provides a basis for the comparison of tissues with mechanical properties that may vary by orders of magnitude.
authors.affiliation: Department of Cell and Systems Biology, University of Toronto, 25 Harbord Street, Toronto, Ontario, Canada M5S 3G5.
authors.forename: Robert
authors.initials: R
authors.lastname: David
authors.affiliation: Department of Cell and Systems Biology, University of Toronto, 25 Harbord Street, Toronto, Ontario, Canada M5S 3G5.
authors.forename: Olivia
authors.initials: O
authors.lastname: Luu
authors.affiliation: Department of Cell and Systems Biology, University of Toronto, 25 Harbord Street, Toronto, Ontario, Canada M5S 3G5.
authors.forename: Erich W
authors.initials: EW
authors.lastname: Damm
authors.affiliation: Department of Cell and Systems Biology, University of Toronto, 25 Harbord Street, Toronto, Ontario, Canada M5S 3G5.
authors.forename: Jason W H
authors.initials: JW
authors.lastname: Wen
authors.affiliation: Department of Cell and Systems Biology, University of Toronto, 25 Harbord Street, Toronto, Ontario, Canada M5S 3G5.
authors.forename: Martina
authors.initials: M
authors.lastname: Nagel
authors.affiliation: Department of Cell and Systems Biology, University of Toronto, 25 Harbord Street, Toronto, Ontario, Canada M5S 3G5 r.winklbauer@utoronto.ca.
authors.forename: Rudolf
authors.initials: R
authors.lastname: Winklbauer
mesh_terms: D000818:Animals; D002448:Cell Adhesion / Q000502:physiology*; D002465:Cell Movement / Q000502:physiology*; D004625:Embryo, Nonmammalian / Q000166:cytology*; D005362:Ficoll; D007091:Image Processing, Computer-Assisted; D018613:Microscopy, Confocal; D008954:Models, Biological; D009024:Morphogenesis / Q000502:physiology*; D013500:Surface Tension; D014783:Viscosity; D014981:Xenopus / Q000196:embryology*
chemical_list: D005362:Ficoll
keywords: Cell adhesion; Cell rearrangement; Tissue surface tension; Tissue viscosity