Healing corneas express embryonic fibronectin isoforms in the epithelium, subepithelial stroma, and endothelium

The cornea is a simple, nonvascularized structure, advantageous for studying the molecular components of epithelial and stromal wound repair. Fibronectin (Fn), of uncertain source and composition, accumulates in healing corneas. We postulated that local synthesis of Fn occurs, as exogenous plasma/tear-derived Fn, which lack the embryonic EIIIA and EIIIB segments, have no consistent beneficial effect on healing. Two contrasting corneal wounds were examined by in situ hybridization: a wound of the anterior stroma, basement membrane, and epithelium (anterior excimer laser keratectomy) and a superficial wound restricted to the epithelium that preserved the basement membrane (mechanical scrape). Both wounds heal without scarring. In normal corneas, only the endothelium had detectable Fn mRNA, containing the V and EIIIB domains, sporadically and at low levels. After anterior keratectomies, extensive expression of Fn mRNA occurred in a specific distribution that changed during the phases of healing. Before re-epithelialization (days 1 and 2) V+, EIIIA+, and EIIIB+ isoforms were diffusely found in stromal cells under and adjacent to the wound. After re-epithelialization (days 3 to 42) and reconstitution of laminin in the regenerating basement membrane zone, V+, EIIIA+, and EIIIB+ isoform synthesis was largely restricted to subepithelial stromal cells at the epithelial/stromal interface. In addition, the corneal epithelial cells focally expressed Fn mRNA. The endothelium showed increased levels of V+, EIIIA+, and EIIIB+ Fn mRNA in open and recently re-epithelialized wounds. At 12 weeks after keratectomy, Fn mRNA expression returned to control levels. In contrast, scrape wounds had only a modest increase of stromal and endothelial Fn mRNA (EIIIA+, EIIIB+, and V+) during the first 7 days and no evidence of epithelial Fn synthesis. Embryonic Fn isoforms are synthesized transiently by the cornea in response to even the most superficial wounds and are likely to be relevant to corneal healing and restoration of structure without scar formation.