embryos turn inside-out*

Living tissues are intelligent materials that can change their mechanical properties while they develop. In spite of extensive studies in multiple model organisms we are only just beginning to understand these dynamic properties and their role in tissue development. Although many tissues are known to exhibit visco-elastic properties, it is unclear which properties dominate three-dimensional shape changes of cellular monolayers, such as epithelia.
The embryonic inversion process in the micro-algal order Volvocales is uniquely suited for studies on epithelial morphogenesis. Volvocalean embryos consist of cup-shaped or spherical cellular monolayers which invert their curvature in order to expose their flagella. Volvox globator exhibits one of the most striking processes of cell sheet folding: Through inwards folding at the equator of the initially spherical cell sheet adopts a mushroom shape and eventually turns itself entirely inside-out through an anterior opening [1]. These global deformations are driven by several waves of active cell shape changes [2, 3]. A combination of advanced imaging and computational analyses is used to explore the role of tissue contractility during invagination. The associated internal stresses as well as the elastic properties of the dynamic cell sheet are determined through laser ablation experiments.

[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).