Senile cortical and nuclear cataracts are formed through different mechanisms.

During cortical cataract formation, Na+ in the lens increases, the protein content decreases and localized hydration changes are found in the cortex. These and other changes are consistent with alterations in the mechanisms responsible for maintaining the osmotic equilibrium of the lens. It is likely that the capsule and the fibre cell membranes have become leaky.

Nuclear cataract formation is accompanied by a large and progressive insolubilization of proteins in the lens nucleus and by the formation of coloured protein aggregates which are held together by disulphide bonds and other covalent crosslinks. All of the changes observed appear to be the result of extensive oxidation due to failure of the normal protective systems.