Abstract

Cell sheet folding is an essential process during multi-cellular development, examples including gastrulation, neurulation and organogenesis. In spite of significant progress in understanding the molecular components that drive local cellular changes it is still poorly understood how these translate into global deformations of cell sheets. The so called inversion process in the multicellular green micro-alga Volvox is a powerful emerging model for studying the biomechanical mechanisms underlying cell sheet deformations. During inversion the spherical Volvox embryos undergo invagination and eventually turn themselves inside-out. Different types of cell sheet deformations have evolved in different Volvox species and are associated with different environmental conditions as well as physical constraints. A combination of light sheet fluorescence microscopy and mathematical modelling revealed that equatorial invagination during type B inversion has to be complemented by active contraction and expansion in the posterior and anterior hemisphere, respectively. While inversion is achieved by a single wave of cell-wedging in some species, in others the anterior hemisphere is pulled over an inflection point by contraction rather than bending of the cell sheet. This suggests a transition towards a higher complexity of embryonic development within the family Volvocaceae. Ongoing laser ablation experiments will elucidate how spatial and temporal differences in tensile forces relate to differences in global deformations. These comparative studies shed light on the interaction of mechanical constraints and environmental cues in the context of evolving morphogenesis.