The avian cornea provides a model which has the major features of the human cornea, including all of the distinct layers of extracellular matrix: Bowman's membrane, stroma, and Descemet's membrane. The corneal stroma develops in stages which involve the deposition, modification, and maturation of two collagenous matrices. The first of these is the primary stroma, an epithelially-derived matrix which serves as a template for the mature, secondary stroma, which is produced by the stromal fibroblasts. This application proposes to study further the development of the primary stroma and the changes in this matrix that result in formation of the mature stroma. The primary stroma is produced as a compact matrix which subsequently undergoes swelling. This swelling, which is a critical event required for subsequent development of the mature stroma, results in separation of the collagenous layers which, in turn, allows for migration of periocular mesenchymal cells into the matrix. These cells then differentiate into the corneal fibroblasts that produce the mature stroma. Our previous observations suggest a model in which the primary stroma, when newly synthesized, remains compact due to bridging/crosslinking of collagen fibrils in adjacent layers by a fibril-associated collagen (collagen type IX). Other of our observations suggest that enzymatic cleavage of this molecule allows for subsequent fibril separation, matrix swelling, and mesenchymal cell invasion, and that this cleavage most likely involves matrix metalloproteinase (MMP) activity. MMPs are a family of enzymes whose activity can be regulated at multiple levels, including positive regulation by their synthesis and proenzyme activation, and negative regulation by naturally occurring inhibitors (TIMPs). Preliminary observations also suggest that more than one of these proteinases may be involved. Studies will be done to identify which enzymes are involved, what controls their synthesis, and how their activities are regulated. These studies potentially have broad significance, since matrix proteinases, and especially the MMPs, have been implicated in many aspects of development in both ocular and non-ocular tissues, in tissue injury and repair, and in cancer metastasis.
