Cell motility versus tissue motion in embryos: Which cells are really moving?

It is axiomatic that during embryogenesis new cellular patterns form within constantly expanding tissue scaffolds. For example, primordial endothelial cells, in bird and mammalian embryos, assemble early vascular patterns in the context of continuous tissue expansion. Our novel computational time‐lapse software and algorithms are designed to quantify large‐scale tissue drift and deformations, in warm‐blooded embryos. The computer code also enables recording of fluorescent objects, i.e., cells and ECM fibers. Computation of tissue displacement fields using the motion of ECM fibrils as in situ markers for tissue drift — while simultaneously tracking individual cells in the same tissue— allows calculation of active cell‐autonomous motility versus passive convective tissue motion. The data demonstrate that passive tissue motion contributes significantly to apparent embryonic cell trajectories. In other words, tissue drift significantly impacts the de novo assembly of vascular networks. Previous notions regarding “cell migration” in warm‐blooded embryos will have to be revisited in light of these data. The work has profound implications for understanding widely accepted embryonic cell guidance mechanisms, such as cellular chemotaxis in response to gradients of soluble molecules.