During development, tissues must orient their growth as the embryo is shaped by mechanical forces exerted by the cells that compose it. One way to orient growth is to control the orientation of cell divisions but many parameters have been shown to influence this.
In isolated cells, Hertwig first showed in 1893 that cells from early embryos divide along their long axis. Recently, these rules were further refined using microfabricated chambers to precisely control cell shape. However, by following division orientation in cells adhering to micropatterned substrates, more recent studies identified additional roles for both the geometrical arrangement of cell-substrate adhesions and extrinsic mechanical forces. Consistent with this, both adhesive and mechanical cues have been reported to guide division orientation in vivo and in epithelial monolayers in developing embryos.
Despite these recent advances, the respective roles of cell shape and mechanical tension in guiding division orientation in even simple tissues, such as cell sheets, remain poorly defined.
“The challenge in resolving these issues is that measuring stresses within living tissues can be complex. There’s a tendency to assume that cell shape reflects the stress field applied to it” says Tom Wyatt, lead author of the study.
The team’s solution was to use a newly developed device in which extensions could be directly applied to monolayers (or cell sheets) and the stresses within the tissue probed using cell ablation with pulsed UV lasers.
When a cell within a tissue is destroyed using a pulsed UV laser, the material around it relaxes and the direction of relaxation indicates the principal axis of the stress field. When combined with high magnification microscopy and instruments allowing precise manipulation of monolayer length at the tissue-scale, UV laser ablation revealed that cell shape and stress field were not always aligned.
With this realisation, the team examined the orientation of division of cells within tissues subjected to uniaxial stretch. They focused their attention on cells whose long axis deviated significantly from the axis of stretch application. These cells systematically divided along their long axis rather than in the direction of principal stress, indicating that cell shape dominated over mechanical tension as shown in the figure.
As a consequence of this result, the growth of tissues in response to externally applied stretch can be explained as a natural consequence of the ability of cells to divide along their long axis and the tendency of uniaxial stretch to align long axes in the direction of extension in cellularised materials.
Dr Guillaume Charras, co-senior author on this research, said: “The result of this research is remarkable because it allows us to explain both oriented growth of tissues and the return to isotropic growth once tissue stress has been dissipated by successive oriented divisions without recourse to complex biochemical signalling cascades. It emerges as a simple consequence of the mechanics of cellularised materials and division of cells along their long axis”.