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Internal stresses show elastic behaviour of a dynamic cell sheet

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Living tissues are intelligent materials that can change their mechanical properties while they develop. In spite of extensive studies, we are only just beginning to understand these dynamic properties and their role in tissue development. The simple bauplan of Volvox embryos is uniquely suited for studies on morphogenesis. V. globator embryos consist of a spherical cellular monolayer which undergoes a process of inversion to expose the flagella [1-3]. During this process the posterior hemisphere shrinks while its positive Gaussian curvature increases. But are both, the decrease in surface area and the increase in curvature the result of active cellular changes? What kind of forces the cells generate can be inferred from the internal stresses of the dynamic cell sheet. Another question is whether the tissue yields or adapts to the internal stresses over time, a behaviour that has been observed in numerous biological tissues. These questions were addressed through a combination of laser cutting experiments and theoretical modelling. The results suggest that the shrinking of the posterior hemisphere is caused by active tissue contraction which results in a passive change in curvature. Furthermore, the dynamic cell sheet remains under stress throughout inversion which indicates the lack of creep-behaviour and is consistent with a purely elastic nature of the cell sheet.

[1] Höhn S and Hallmann A. BMC Biology 9, 89 (2011).

[2] Höhn S, Honerkamp-Smith AR, Haas PA, Khuc Trong P, and Goldstein RE. Physical Review Letters 114, 178101 (2015).

[3] Haas PA, Höhn S, Honerkamp-Smith AR, Kirkegaard JB, and Goldstein RE. PLOS Biology 16, e2005536 (2018).

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