Age-related development of a refractive index plateau in the human lens: evidence for a distinct nucleus.

The human lens comprises two distinct regions in which the refractive index changes at different rates. The periphery contains a rapidly increasing refractive index gradient, which becomes steeper with age. The inner region contains a shallow gradient, which flattens with age, due to formation of a central plateau, of RI = 1.418, which reaches a maximum size of 7.0 x 3.05 mm around age 60 years. Formation of the plateau can be attributed to compression of fibre cells generated in prenatal life. Present in prenatal but not in postnatal fibre cells, gamma-crystallin may play a role in limiting nuclear cell compression.