SUMMARYThis paper reviews the changes which occur in the human lens in diabetes. They include refractive changes and cat aract and age-related increases in thickness, curvatures, light scattering, autofluorescence and yellowing. The inci dence of cataract is greatly increased over the age of 50 years, slightly more so in women, compared with non diabetics. Experimental models of sugar cataract provide some evidence for the mechanism of the uncommon, but morphologically distinct, juvenile form of human dia betic cataract, where an osmotic mechanism due to sugar alcohol accumulation has been thoroughly studied in dia betic or galactose-fed rats. The discrepancy between the ready accumulation of sugar alcohol in the lens in model systems and the very slow kinetics of aldose reductase (AR) has not been satisfactorily explained and suggests that the mechanism of polyol formation is not yet fully understood in mammalian systems. The activity of AR in the human lens lies mainly in the epithelium and there appears to be a marginal expectation that sufficient sorbi tol accumulates in cortical lens fibres to explain the lens swelling and cataract on an osmotic basis. This is even more so in the cataracts of adult diabetics, which re semble those of age-related non-diabetic cataracts in appearance. The very low levels of sorbitol in adult dia betic lenses make an osmotic mechanism for the increased risk of cataract even less likely. Other mech anisms, including glycation and oxidative stress, are dis cussed. The occurrence of cataract is a predictor for increased mortality in the diabetic.The diabetic lens is larger than normal, disposed to refrac tive change and at increased risk of cataract, sometimes of a specific type. This paper discusses the factors involved.
ANATOMY AND PHYSIOLOGYThe lens is enclosed in a collagenous capsule containing other matrix proteins and proteoglycans. A monolayer of epithelial cells is interposed between the anterior capsule and the main cellular mass of lens fibres. The lens fibres are laid down in a series of onion-skin layers, which arch over the equator to meet their opposite numbers at the lens sutures. The innermost fibres comprising the nucleus of the lens are free of organelles and show limited metabolic activity. The outer fibres comprise the cortex. The most superficial fibres of the cortex are nucleated and, like the epithelium, show the normal complement of organelles. Glucose, which enters the lens by facilitated transport, I is its main energy supply, although energy may also derive from amino acids? The metabolism of the cortex is chiefly anaerobic, with 70% of the energy supply of the lens deriving from anaerobic glycolysis. If a lens is incubated in nutrient medium in anaerobic conditions with an adequate supply of glucose it remains transparent for a number of hours.3.4 The metabolism of the epithelium is aerobic. New lens fibres arise by cell division in the germi native zone in the pre-equatorial region of the lens. The epithelium and superficial cortical cells are major ...