  We formulate an active-gel theory for multicellular migration in the
extra-cellular matrix (ECM). The cells are modeled as an active, polar solvent,
and the ECM as a viscoelastic solid. Our theory enables to analyze the dynamic
reciprocity between the migrating cells and their environment in terms of
distinct relative forces and alignment mechanisms. We analyze the linear
stability of polar cells migrating homogeneously in the ECM. Our theory
predicts that, as a consequence of cell-matrix alignment, contractile cells
migrate homogeneously for small wave vectors, while sufficiently extensile
cells migrate in domains. Homogeneous cell migration of both extensile and
contractile cells may be unstable for larger wave vectors, due to active forces
and the alignment of cells with their concentration gradient. These mechanisms
are stabilized by cellular alignment to the migration flow and matrix
stiffness. They are expected to be suppressed entirely for rigid matrices with
elastic moduli of order 10 kPa. Our theory should be useful in analyzing
multicellular migration and ECM patterning at the mesoscopic scale.
