CELLULAR INVASION AND COLLAGEN TYPE-IX IN THE PRIMARY CORNEAL STROMA IN-VITRO

During different stages in the development of the avian cornea, variou s collagen types have been shown to participate in matrix formation an d have been implicated in morphogenesis. One of these is the fibril-as sociated collagen type IX. This molecule is present when the primary c orneal stroma is in a compact state, but rapidly disappears just prior to stromal swelling and its invasion by mesenchymal cells. The tempor ospatial pattern of the disappearance of type IX collagen in the devel oping cornea suggests that this molecule may be involved in stabilizin g the primary corneal stromal matrix by interacting either with other type IX collagen molecules or with other matrix components. To explore further whether the removal of type IX collagen is involved in stroma l swelling, we have employed an in vitro culture system in which swell ing of the primary stroma and mesenchymal cell invasion can be experim entally manipulated by culturing chick corneal explants on a Nuclepore filter support in the presence or absence of an associated lens. We h ave also examined the effect of exogenously added human recombinant ti ssue inhibitor of metalloproteinases (TIMP-1) on the presence of type IX collagen and cellular invasion. When stage 25-26(+) corneal explant s were cultured with an associated lens, the primary stroma did not sw ell; immunohistochemically detectable type IX collagen was still prese nt, and mesenchymal cell invasion failed to occur. Conversely, when th e same stages of corneal explants were cultured without an associated lens, the primary stroma swelled; type IX collagen disappeared, and me senchymal cell migration occurred. Under both conditions, however, the type II collagen of the stroma, which is known to be a component of t he striated fibrils, remained clearly detectable and with time even se emed to increase in amount. This result is consistent with the proposi tion that type IX collagen is one factor involved in maintaining the p rimary stroma as a compact matrix, possibly by functioning as a bridgi ng/stabilizing factor. When TIMP was added to cultures of corneal expl ants, type IX collagen remained detectable in focal regions, suggestin g that one or more metalloproteinases are involved in the removal of t he type IX collagen. In addition, some of these type IX-containing reg ions contained mesenchymal cells, suggesting that in addition to type IX collagen other factors are likely to be involved in regulating mese nchymal cell migration. (C) 1994 Wiley-Liss, Inc.