Ultrastructural analysis of collagen fibrils and proteoglycans in keratoconus

Purpose: To investigate ultrastructural alterations in the distribution of collagen fibrils (CFs) and proteoglycans (PGs) in the keratoconus cornea. Methods: Four normal corneas (donor age 24-75 years) and four severe and one mild keratoconus corneas (donor age 24-47 years) were fixed in 2.5% glutaraldehyde containing 0.05% cuprolinic blue dye for electron microscopy. Analyses were carried out on approximately 39 000 CF and 66 000 PG filaments in the anterior, middle and posterior stroma, using analySIS (R) soft imaging software. Results: In severe keratoconus, stromal lamellae were seen to undulate in most regions, whereas in mild keratoconus only the middle and posterior lamellae were affected. In keratoconus corneas the mean diameter and interfibrillar spacing of CFs was reduced in all zones (p < 0.0001) and the CF and PG number density and area fractions were significantly increased (p < 0.0001) compared with in normal corneas and were higher (p < 0.0001) in the corneas with severe keratoconus than in that with mild keratoconus. The lamellae contained microfibrils (8-9 nm wide) and, in addition, PGs embedded within CFs. Degenerate keratocytes containing PGs were found in all keratoconus corneas. Conclusions: These studies suggest that as keratoconus progresses, the PG content of the stroma increases, whereas fibril diameter is reduced. The altered stromal content of PGs may influence CF diameters and their organization in keratoconus, weakening lateral cohesion and resulting in significant disorder of CF packing.