The actin cytoskeleton controls many cellular functions, including shape, mechanical properties, or motility. It 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 understood. I will describe our unpublished research about self-organization and polymorphism using an active gel model accounting for nematic order. Combining linear stability analysis and  nonlinear finite element simulations, we establish the conditions for nematic bundle formation and how active gel parameters control the architecture, orientation, connectivity and dynamics of self-organized patterns of nematic bundles. I will discuss the connection between our active gel model and discrete network models, as well as the interaction between active nematic structures and cell shape. Finally, I will discuss how cytoskeletal dynamics within cells give rise to emergent mechanical behaviors in cell monolayers.