Abstract
The eye lens grows throughout life by cell accrual on its surface and can change shape to adjust the focussing power of the eye. Varying concentrations of proteins in successive cell layers create a refractive index gradient. The continued growth of the lens and age-related changes in proteins render it less able to alter shape with loss of capacity by the end of the sixth decade of life. Growth and protein ageing alter the refractive index but as accurate measurement of this parameter is difficult, the nature of such alterations remains uncertain. The most accurate method to date for measuring refractive index in intact lenses has been developed at the SPring-8 synchrotron. The technique, based on Talbot interferometry, has an X-ray source and was used to measure refractive index in sixty-six human lenses, aged from 16 to 91 years. Height and width were measured for forty-five lenses. Refractive index contours show decentration in some older lenses but individual variations mask age-related trends. Refractive index profiles along the optic axis have relatively flat central sections with distinct micro-fluctuations and a steep gradient in the cortex but do not exhibit an age-related trend. The refractive index profiles in the equatorial aspect show statistical significance with age, particularly for lenses below the age of sixty that had capacity to alter shape in vivo. The maximum refractive index in the lens centre decreases slightly with age with considerable scatter in the data and there are age-related variations in sagittal thickness and equatorial height.
Highlights
The lens of the eye is a key optical element that, together with the cornea, refracts light to the retina
Ageing is a natural physiological process that leads to some well-defined changes in the body, such as loss of skin elasticity, bone resorption, retardation in mechanisms of repair and is a major risk factor of many chronic and acute diseases
It is a process that continues to occupy a wide range of research fields as the more specific and the subtle manifestations of ageing on the structure and function of various body systems require far greater understanding that current knowledge permits
Summary
The lens of the eye is a key optical element that, together with the cornea, refracts light to the retina. The lens has the capacity to alter ocular focus so as to enable clarity of vision for objects over a range of distances This functional capacity, termed accommodation, which decreases progressively with age leading to a loss of ability to focus on near objects, is achieved by the lens changing its shape under forces mediated by the contraction and relaxation of the ciliary muscle. The growth of the lens continues throughout life by accrual of layers of fibre cells over existing tissue without concomitant cellular loss creating a lamellar structure with the oldest cells in the central layers and the most recently synthesised cells on the lens surface This renders the lens an ideal tissue for the study of ageing and pathologies of ageing given that chronic systemic conditions of protein folding such as diabetes have manifestations in the eye lens [3]. A suggested link between Alzheimer’s disease and cataract [4] was not supported by findings from more recent studies www.impactjournals.com/oncotarget [5, 6], perhaps indicating that a systemic condition needs to be present for a longer term than is Alzheimer’s disease before any associated signs are seen in the lens
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