An X-ray diffraction investigation of corneal structure in lumican-deficient mice

PURPOSE. The corneas of mice homozygous for a null mutation in lumican, a k eratan sulfate-containing proteoglycan, are not as clear as normal. In the present study, mutant corneas were examined by synchrotron x-ray diffractio n to see what structural changes might lie behind the loss of transparency. METHODS. X-ray diffraction patterns were obtained from the corneas of 6-mon th-old and 2-month-old lumican-null and wildtype mice. Measured in each cor nea were the average collagen fibril diameter, average collagen fibril spac ing, and the level of order in the collagen array. RESULTS. The x-ray reflection arising from regularly packed collagen was we ll-defined on all x-ray patterns from 6-month-old wildtype corneas. Pattern s from 6-month-old lumican-deficient corneas, however, contained interfibri llar reflections that were measurably more diffuse, a fact that points to a widespread alteration in the way the collagen fibrils are configured. The same distinction between mutant and wild-type corneas was also noted at a-m onths of age. Average collagen fibril spacing was marginally higher in corn eas of 6-month-old lumican-null mice than in corneas of normal animals. Unl ike x-ray patterns from wild-type corneas, patterns from lumican-deficient corneas of both ages registered no measurable subsidiary x-ray reflection, evidence of a wider than normal range of fibril diameters. CONCLUSIONS. The spatial arrangement of stromal collagen in the corneas of lumican-deficient mice is in disarray. There is also a considerable variati on in the diameter of the hydrated collagen fibrils. These abnormalities, s een at 2 months as well as G months of age, probably contribute to the redu ced transparency.