  The actin cytoskeleton is remarkably adaptable and multifunctional. It often
organizes into nematic bundles such as contractile rings or stress fibers.
However, how a uniform and isotropic actin gel self-organizes into dense
nematic bundles is not fully understood. Here, using an active gel model
accounting for nematic order and density variations, we identify an active
patterning mechanism leading to localized dense nematic structures. Linear
stability analysis and nonlinear finite element simulations establish the
conditions for nematic bundle self-assembly and how active gel parameters
control the architecture, orientation, connectivity and dynamics of
self-organized patterns. Finally, we substantiate with discrete network
simulations the main requirements for nematic bundle formation according to our
theory, namely increased active tension perpendicular to the nematic direction
and generalized active forces conjugate to nematic order. Our work portrays
actin gels a reconfigurable active materials with a spontaneous tendency to
develop patterns of dense nematic bundles.
