Collagen fibrils in the human corneal stroma: structure and aging

Transparency and biomechanical properties of the cornea depend on the structure and organization of collagen fibrils. The authors determined diameter, axial period, and lateral molecular spacing of collagen fibrils in human corneal stroma as a function of age.

Seventeen normal human corneas were investigated in their native state by means of small-angle and wide-angle x-ray scattering.

The mean radius of collagen fibrils, the axial period of collagen fibrils, and the lateral intermolecular Bragg spacing were found to be age dependent. The authors determined fibril radii of 16.1 +/- 0.5 nm in persons older than 65 years of age (n = 10) and 15.4 +/- 0.5 nm (mean +/- SD) in persons younger than 65 years (n = 7) (P < 0.022). The related age-dependent values were 66.4 +/- 0.7 nm (> 65 years) and 65.2 +/- 0.8 nm (< 65 years) for the axial period (P < 0.006) and 1.515 +/- 0.010 nm (> 65 years) and 1.499 +/- 0.013 nm (< 65 years) for the intermolecular Bragg spacing (P < 0.022).

Aging is related to a three-dimensional growth of collagen fibrils in the human corneal stroma. The age-related growth of the fibril diameter was mostly a result of an increased number of collagen molecules and, in addition, to some expansion of the intermolecular Bragg spacing probably resulting from glycation-induced cross-linking. The observed expansion of the fibrils in an axial direction may result from reduction of the molecular tilting angle within collagen fibrils. The observed alterations of the collagen framework may have implications for refractive surgery and ocular tonometry achieved through related changes in the biomechanical properties of the cornea.