Fibronexus junctions associated with in vivo human endothelium

The fibronexus is recognized as a characteristic marker of the myofibroblast. However, it is not completely specific for this cell, having been seen in aortic smooth muscle (in attenuated form) and endothelium in experimental animals. This paper documents fibronexus junctions in human in vivo endothelium. Ultrastructural observations were made on the vasculature of a desmoplastic and focally neurotropic malignant melanoma. Cross-sectioned fibronectin fibrils were seen outside the stromal surface of the endothelial plasmalemma. Often, they were positioned directly opposite the actin-filament bundles in the peripheral cytoplasm. Neoplastic and in vitro cultured cells apart, endothelium is the only nonmyofibroblastic cell type to show well-developed fibronexus junctions. Mostly, they have been documented in aortic endothelium in experimental animals, where they possibly constitute an adaptation to hemodynamic stress, and where they might more securely anchor endothelium on to subjacent connective tissue. They might also function as mechanotransducers of extracellular stress in the extracellular milieu. The present observations constitute a further, rare example of endothelium-associated fibronexuses in reactive human vessels.