Intrinsic Curvature in the Extracellular Matrix of a Simple Multicellular Organism

The extracellular matrix (ECM), a network of proteins and biopolymers secreted by surrounding cells, determines the form and structural stability of many tissues. While its properties played a key role in evolutionary transitions to multicellularity, there is little known about the mechanics of the ECM in such evolutionary contexts. We report the first quantitative study of the sheet-like ECM in simple multicellular eukaryotes, using mechanical shear to break up the spheroidal alga Volvox carteri into smaller sections, which are found to curl spontaneously into a variety of forms, some resembling seed pods of plants, others exhibiting bistability. A model based on thin shell elasticity shows that this phenomenon arises from intrinsic curvature of the ECM, and that Volvox colonies therefore expand in the presence of residual elastic stress over its lifecyle. The implications of this for growth, structural integrity, and aging of the ECM in Volvox and for other organisms in general are discussed.