  Morphological trends in growing colonies of living cells are at the core of
physiological and evolutionary processes. Using active gel equations, which
include cell division, we show that shape changes during the growth can be
regulated by the dynamics of topological defects in the orientation of cells.
The friction between the dividing cells and underlying substrate drives
anisotropic colony shapes toward more isotropic morphologies, by mediating the
number density and velocity of topological defects. We show that the defects
interact with the interface at a specific interaction range, set by the
vorticity length scale of flows within the colony, and that the cells
predominantly reorient parallel to the interface due to division-induced active
stresses.
