Physical forces during collective cell migration

Fundamental biological processes including morphogenesis, tissue repair and tumour metastasis require collective cell motions1–3, and to drive these motions cells exert traction forces on their surroundings4. Current understanding emphasizes that these traction forces arise mainly in ‘leader cells’ at the front edge of the advancing cell sheet5–9. Our data are contrary to that assumption and show for the first time by direct measurement that traction forces driving collective cell migration arise predominately many cell rows behind the leading front edge and extend across enormous distances. Traction fluctuations are anomalous, moreover, exhibiting broad nonGaussian distributions characterized by exponential tails10–12. Taken together, these unexpected findings demonstrate that although the leader cell may have a pivotal role in local cell guidance, physical forces that it generates are but a small part of a global tug-of-war involving cells well back from the leading edge. The single adherent cell moves by the action of two synchronized cycles, one involving extension and contraction of its cytoskeleton and the other involving formation and detachment of its adhesions13,14. Although this complex process remains a matter of intense research14–16, it is now well established that a fundamental aspect of the motility mechanism is the transmission of contractile forces to the surrounding matrix at the cell’s leading and trailing edges17,18. In contrast with the case of migration of the single cell studied in isolation14–16, the case of collective migration of cells within a contiguous cell sheet has more physiological relevance but is substantially less well understood19. Within an advancing epithelial cell sheet, for example, each individual cell is physically constrained by its neighbours, and cell–cell signalling through biochemical and biophysical pathways may influence the collective motion of the group20,21. Do leader cells at the advancing front edge of the sheet exert physical forces locally that are transmitted rearward, from cell-to-cell, and thus act to pull along those cells in the ranks behind5,6,8,9? Or instead is each individual cell in the sheet mechanically self-propelled21? Or does cell proliferation expand the cell colony and thereby push the advancing front forward? Or is the correct answer none of the above? For more than a century these fundamental questions have been debated intensively5,22 and, using a variety ofmethods in vivo23, in vitro4,9,24 and in silico21, much conflicting evidence has accumulated. This conflicting evidence has been in most cases indirect or inferential, however, because within the cell sheet the physical forces themselves have remained largely inaccessible to direct experimental observation. Here, we report by direct measurement the first explicit maps of those physical forces and their distribution. To do this within an advancing cell sheet, we used Fourier-transform traction microscopy together with a balance of forces that is