A Structural and Biochemical Study of the Corneal Stroma

It is thought that the structural organisation of the cornea, in particular the uniform size and spacing of its constituent collagen fibrils, is a major factor responsible for its transparency. The roles of the various components of the corneal stroma in the maintenance of its structure are not currently well understood, although it is thought that proteoglycans an d the glyco-sylation of collagen may be involved in the control of fibril diameter. Protein components were removed from the corneal stroma, under mild and harsh conditions of extraction, and analysed using polyacrylamide gel electrophoresis and other biochemical techniques. A range of 0.15M NaCl- soluble proteins and proteoglycans could be removed under mild extraction conditions, including a proteoglycan of very high molecular weight which has not been reported elsewhere. Under harsh conditions of extraction, when 1% SDS was added to the extracting solution, an additional glyco¬protein, designated GP135, was removed. The effect of the extraction procedures on the structure of the cornea was studied using electron microscopy and X-ray diffraction. Electron mi¬croscopy showed increasing disruption to the ultrastructure of the cornea with more harsh extraction methods, and indicated that material had been removed from the gap region of the collagen fibrils. Loss of the interfibrillar X-ray diffraction pattern following extraction also indicated disruption of the ordered structure, and electron density profiles obtained from the meridional diffraction pattern suggested removal of material from the gap region. The coincidental removal of GP135 from the cornea with the removal of material from the gap region of the collagen fibrils indicated that it might be located here. Transparency measurements from extracted corneas showed that the loss of structural order produced by these methods might not be the most important factor in the loss of transparency for corneas at physiological hydration. Also, the transparency of highly h y d rated corneas is dependent on the type of solution they are immersed in