Foci of amorphous/granulofilamentous matrix in the extracellular domain of tumours. 2. Immunohistochemical and immunogold characterization of a fibronectin-rich matrix component.

The term FEAM (foci of extracellular amorphous matrix) has been used for discretely outlined areas of moderately dense material having a filamentous/granular substructure located in the extracellular matrix of tumours. In spite of being widespread in mesenchymal tumours especially, and often abundant, they have received little attention in terms of structure, composition and origin. Mostly, they have been regarded as a variant or a product of lamina ('basement membrane material'). However, they also appear in tumours whose cells should and do lack a lamina, such as giant-cell fibroblastoma and solitary fibrous tumour. This paper describes their fine structure in a variety of predominantly mesenchymal tumours, and documents their composition using light microscope immunostaining and immunogold labelling. Small amounts of type IV collagen and laminin were found focally and inconsistently among the five tumours by light microscope immunostaining, but fibronectin was strongly and consistently identified. Strong fibronectin staining was also identified by immuno-electronmicroscopy. These data suggest that FEAM represent a fibronectin-rich matrix constituent, which might be a common final product of either lamina or the external component of the subplasmalemmal linear density (focal adhesion). There is little support light microscopically for a relationship to immune-complexes or cryoglobulins.