The fibronexus in reactive and tumoral myofibroblasts: further characterisation by electron microscopy

Forty two surgical specimens containing myofibroblasts were studied to clarify the criteria for identifying the fibronexus, an ultrastructural feature regarded as a marker for myofibroblastic differentiation. Granulation tissue, tumour stroma, fibro-proliferative lesions (nodular fasciitis, myofibromatosis, inflammatory myofibroblastic tumour) and malignancies (myofibrosarcoma and fibrosarcoma) were studied. Comparable results were found throughout these specimens, although fibronexus junctions were better developed in reactive compared with tumoral myofibroblasts. By electron microscopy, myofibroblasts were identified by abundant rough endoplasmic reticulum, peripheral smooth-muscle myofilaments with focal densities, and fibronexus junctions. The latter were recognised as the points of convergence on the myofibroblast surfaces of intracellular myofilaments and extracellular fibronectin fibrils. The fibronectin fibrils were often co-linear with myofilaments. Also, fibronectin fibrils were dark-staining, straight and rigid-looking, and had a longitudinal filamentous substructure. A striking feature was the tendency of fibronectin fibrils to project into the surrounding extracellular space, away from the myofibroblast surface: in these respects, they differed significantly from lamina ("basement membrane"). The presence of fibronectin fibrils correlated positively with fibronectin immunostaining by light and electron microscopy. Laminin and collagen IV showed variable and weak staining in the intercellular spaces in a minority of cases and never strongly stained myofibroblast surfaces. The data emphasise that the fibronexus has a number of distinctive features permitting identification, and constitute a reference-point for pathologists wishing to use electron microscopy to refine light microscopy diagnoses of putative myofibroblastic lesions. The role of the fibronexus in the definition of the myofibroblast is discussed.