Ageing of the human corneal stroma: structural and biochemical changes.

High and low angle X-ray diffraction patterns from the corneal stroma give information about the mean intermolecular spacing of the collagen molecules and the mean interfibrillar spacing of the collagen fibrils, respectively. X-ray data were collected, using a high intensity synchrotron source, from human corneas and sclera at approximately physiological hydration. The spacings were measured as a function of tissue age. Between birth and 90 years there is an increase in the cross-sectional area associated with each molecule in corneal collagen from approx. 3.04 nm2 to 3.46 nm2, and an increase in scleral collagen from approx. 2.65 nm2 to 3.19 nm2. These changes may be due to an increase in the extent of non-enzymic cross-linking between collagen molecules over the age range. We have investigated this possibility by measuring collagen glycation using the thiobarbituric acid assay and the subsequent advanced glycation end-products (AGEs) using fluorescence emission. The results obtained have shown an age-related increase in glycation and AGEs in both tissues. We have also demonstrated a decrease in the interfibrillar spacing of corneal collagen with increasing age which may be related to changes in the proteoglycan composition of the interfibrillar matrix.