Abnormalities of the Extracellular Matrix in Keratoconus Corneas

Purpose: To study alterations of the extracellular matrix (ECM) and basement membrane (BM) components in human keratoconus corneas.

Methods: Fifteen normal and 13 keratoconus corneas were characterized by immunofluorescence with antibodies to 23 ECM and BM components.

Results: Keratoconus staining patterns for posterior nonscarred regions and Descemet's membrane were normal. We focused on three areas of keratoconus corneas: (a) nonscarred anterior corneal regions, (b) scarred anterior and posterior corneal regions, and (c) gaps in Bowman's layer. In each of these areas, consistent ECM and BM changes could be found. Nonscarred regions showed decreased staining of the epithelial BM for entactin/nidogen, fibronectin, alpha 3-alpha 5 chains of type IV collagen, and chains of laminin-1. In contrast, scarred regions had greater than normal staining of the epithelial BM for these same components and also for laminin-5, perlecan, and type VII collagen. In the Bowman's layer gaps/breaks, focal fibrotic deposits containing type VIII collagen, fibrillin-1, tenascin-C, alpha 1-alpha 2 type IV collagen, and normal stromal ECM and epithelial BM components were seen. Fibrotic regions were largely restricted to areas where, because of the lack of Bowman's layer, the epithelium was in contact with the stroma.

Conclusions: In a single keratoconus cornea, abnormalities in the ECM/BM patterns were not uniform. This may reflect locally increased protease activity (where few if any BM components are found) and ongoing wound healing (where more BM or ECM components or both are found).