  Cell spreading requires a major reorganisation of the actin cytoskeleton,
from a cortical structure to a lamellipodium where filaments are mostly
parallel to the substrate. We propose a model inspired by the physics of
nematic liquid crystals and fluid membranes, in which the coupling between
actin mechanics, filaments orientation, and the local curvature of the cell
membrane naturally yields the collective reorientation of actin filaments at
the highly curved edge of a spreading cell. Filament orientation increases the
traction force exerted by the frictional flow of polymerising actin on the
substrate, creating a positive feedback loop between edge curvature, filament
alignment, and traction force that promotes cell spreading. We establish the
condition under which this feedback leads to a full wetting transition, which
we interpret as the initiation of a lamellipodium, and we uncover the existence
of bi-stability between partial and full spreading which could trigger
spontaneous cell polarization and lead to migration.
