Fundamentals of cataract development

The lens is a highly specialized tissue in the vertebrate eye, whose growth characteristics and metabolic turn‐over are designed for longevity in respect to its optical function. It grows throughout life and does neither shed any of its cells, nor excrete any degraded proteins. The lens manages to maintain an optical and a biochemical gradient based on the high protein content (crystallins) and slow metabolic turn‐over in the superficial fibre cells. It is well equipped with anti‐oxidative defence mechanisms, can seal off groups of damaged fibre cells and changes its transmission properties during ageing. The 3 main ageing characteristics in the lens are a tissue layer specific increase in light scattering, a yellowish discoloration of the proteins and a loss of accommodative capacity. Cataract development can be triggered by a wide selection of factors including genetic predisposition, certain diseases, optical and ionizing radiation, chemical compounds, drugs, nutritional and environmental factors, as well as trauma. Patho‐physiological mechanisms involved are DNA damage in epithelial cells, protein glycosylation and aggregation and molecular cross linking in lens fibre cells, metabolic breakdown of selected groups of fibre cells or of a whole layer, protein degradation and finally globular degeneration and liquefaction. Ageing in this context is the time line which allows an increasing number of noxious factors to interact in the lens at decreasing metabolic capacities. While the sensitivity of the lens to oxidative damage increases during ageing, its sensitivity to low dose ionizing radiation decreases. The function of molecular receptors for acetylcholine, steroids, the sigma receptor and other receptors found in the lens is still unknown, but might explain the specific reaction to certain drugs.