Ectomesenchyme of ameloblastic fibroma reveals a characteristic distribution of extracellular matrix proteins

The distribution of collagens type I, IV and VI, procollagen type III and of undulin was studied in four cases of ameloblastic fibroma (AF). The ectomesenchyme of AF revealed an as yet unobserved organization of these extracellular matrix proteins with collagen type VI clearly predominating over collagen type I, procollagen type III and undulin, that showed a weak and amorphous distribution throughout the tumor stroma. Undulin, a glycoprotein that is associated with mature collagen fibrils and with differentiated tissues, was not detectable in the tumor stroma of AF except for a slow expression around capillaries and in areas with a high cellularity. We could demonstrate that the characteristics of extracellular matrix composition allowed a clear distinction between the ectomesenchyme of AF and the adjacent normal mesenchymal stroma. Due to the specific staining patterns it was possible to detect epithelial tumor islands outside the typical ectomesenchymal stroma. Our findings furthermore indicate that epithelial cells of AF invade the adjacent normal mesenchyme possibly inducing de novo formation of ectomesenchymal tumor stroma.