  General structures of the charged lepton and the neutrino mixing matrices
leading to tri-bimaximal leptonic mixing are determined. These are then
integrated into an SO(10) model within which detailed fits to fermion masses
and mixing angles are given. It is shown that one can obtain excellent fits to
all the fermion masses and quark mixing angles keeping tri-bimaximal leptonic
mixing intact. Different perturbations to the basic structure are considered
and those which can or which cannot account for the recent T2K and MINOS
results on the reactor mixing angle $\theta_{13}^l$ are identified.
