In this work microstructure evolution in a columnar polycrystal of pure aluminum is studied using a microstructure sensitive crystal plasticity finite element model (CPFEM). In the model, based upon the kinematics of crystal deformation and dislocation interaction laws, dislocation generation and annihilation are modeled. Dislocation densities evolve in the form of closed loops and are tracked as state variables, leading to spatially inhomogeneous dislocation densities that show patterning in the dislocation structures. The hardening law is based on the strength of junctions between dislocations on specific slip systems. The CPFEM model is able to show the anisotropic hardening behavior of aluminum single crystals. The measures of accumulated plastic strain in the experiment and the simulation are compared with varying degrees of success.