Mechanism of basement membrane dissolution preceding corneal ulceration

In animal models for corneal ulceration, the degradation of extracellular matrix components of the stroma does not occur until after the basement membrane underlying the corneal epithelium has disappeared. Using a thermal-burn model, it was demonstrated that epithelial basement membrane is degraded actively by products of corneal cells in a process that does not require the participation of polymorphonuclear leukocytes. A new gelatinolytic metalloproteinase, preliminarily identified as the matrix metalloproteinase, MMP-9, is synthesized and secreted by corneal cells with a timing appropriate for a role in basement membrane degradation. During healing of ulcers, when new matrix is being deposited actively in the burned tissue, a second new gelatinase appears in the cornea, preliminarily identified as a stable activated form of the matrix metalloproteinase, MMP-2. The timing of expression suggests a role for this enzyme in appropriate deposition and remodeling of new matrix in the regenerating corneal tissue.