Collagen Orientation During Development of the Embryonic Avian Cornea

Purpose: During the latter stages of development, the secondary chick corneal stroma undergoes significant structural and compositional changes. The current study was designed to provide information about the orientation of collagen molecules (and thus fibrils) as the cornea matures before hatching. Collagen orientation is an important biomechanical parameter given that collagen lamallae consisting of parallel fibrils are thought to be strongest axially. Methods: Forty-eight corneas were obtained from fertilised chick embryos at developmental days 13 through 18 (n8 at each timepoint), stored frozen and later examined at the Synchrotron Radiation Source, Cheshire UK, where high-angle X-ray diffraction patterns were obtained. From the X-ray patterns the amount of X-ray scatter from aligned versus non-aligned (i.e. isotropic) collagen molecules was measured. Given that collagen molecules run approximately axially within fibrils, molecular orientation was taken to represent fibrillar orientation. Results: X-ray scatter from fibrillar stromal collagen (aligned and isotropic) as a proportion of X-ray scatter from all matrix elements measured 0.018 (day 13), 0.018 (day 14), 0.018 (day 15), 0.020 (day 16), 0.027 (day 17), and 0.033 (day 18). The increase after day 16 is indicative of increased deposition of fibrillar collagen. The amount of X-ray scatter from aligned fibrillar collagen as a proportion of scatter from all matrix elements (including aligned and isotropic collagen) measured 0.51 (day 13), 0.51 (day 14), 0.50 (day 15), 0.48 (day 16), 0.39 (day 17) and 0.30 (day 18). The decrease after day 16 points to lower levels proportionally of aligned collagen. Conclusions: The stromal matrix of the secondary chick cornea at day 13-16 of development contains a sizeable proportion of collagen fibrils that are preferentially aligned, often in a four-fold, orthogonal manner. Thereafter, this preferential alignment recedes, presumably because new collagen fibrils are being deposited in an non-orthogonal array, thereby masking the initial orthogonal template. A Wide Angle Fibre Diffraction Camera for station 14.1 of The SRS