Strings and topological defects govern ordering kinetics in endothelial cell layers.

Many physiological processes, such as the shear flow alignment of endothelial cells in the vasculature, depend on the transition of cell layers between disordered and ordered phases. Here we demonstrate that such a transition is driven by the non-monotonic evolution of nematic topological defects in a layer of endothelial cells and the emergence of string excitations that bind the defects together. This suggests the existence of an intermediate phase of ordering kinetics in biological matter. We use time-resolved large-scale imaging and physical modelling to analyse the non-monotonic decrease in the number of defect pairs. The interaction of the intrinsic cell layer activity and the alignment field determines the occurrence of defect domains, which defines the nature of the transition. Defect pair annihilation is mediated by string excitations spanning multicellular scales within the cell layer. Our results, therefore, suggest a mechanism by which intermediate ordering and string excitation might contribute to regulating morphogenetic movements and tissue remodelling in vivo.