Somites are mesodermal segments, which give rise to the vertebrae and associated muscles. They originate sequentially, in an anterior-to-posterior (head-to-tail) wave, and as they mature, outer somite cells undergo epithelialization to form sharp boundaries between adjacent segments. McMillen et al. used zebrafish embryos to study the involvement of the adherens junction protein cadherin in this process. They found that within each somite, a posterior-to-anterior gradient of stable cadherin-based adhesions is established; thus, cells with large differences in stable cadherin adhesions are juxtaposed at somite boundaries. Analysis of cell mosaics revealed that juxtaposition of cells with different cadherin levels promotes the deposition of fibronectin-based extracellular matrix at boundaries and is sufficient to induce boundary formation. These effects were further associated with a cadherin-mediated increase in cytoskeletal contractility and hence local tissue stiffness at somite boundaries.