Dense collagen matrices obtained by using the property of type I collagen to form liquid crystals at high concentrations, were shown to be colonized by human dermal fibroblasts (Biomaterials 23 (2002) 27). In order to evaluate them as possible tissue substitutes, we investigated in this study the mechanism of cell colonization. Fibroblasts were seeded at the surface of collagen matrices at concentrations of 5 and 40 b mg/ml. Cell density and migration were estimated from histological sections over 28 days within 500 microm thick matrices. At day 14, migration started in the 40 mg/ml matrices, attaining 320 microm in distance and 5500 cell/mm(3) in density at day 28. As zymography and western blot techniques demonstrated production of collagenase 1 (MMP1) and gelatinase A (MMP2) in culture medium, collagen hydrolysis was required for cells to penetrate the collagen network. Furthermore, the presence of MMP1 and MMP2 and their tissue inhibitors TIMP1 and TIMP2 was revealed by immunohistochemistry. We presently show that 40 mg/ml collagen matrices are colonized by human dermal fibroblasts and reach, at day 28, a density close to that measured in human dermis.