Intranodal myofibroblastoma: study of a case suggesting smooth-muscle differentiation.

A case of intranodal myofibroblastoma with amianthoid fibres was studied by histology, immunohistochemistry and electron microscopy, in a Canadian Caucasian male presenting with an inguinal mass which appeared following a sports injury. Tumour cells were spindled and formed haphazard interlacing fascicles with intervening areas of haemorrhage. They were positive for vimentin, alpha-smooth-muscle actin and HHF35. By electron microscopy, they displayed moderate numbers of rough endoplasmic reticulum cisternae, fine actin-sized filaments in cell processes, and discrete stretches of unambiguous lamina ('external' lamina), sometimes in association with short attachment plaques. The fibronectin fibrils and fibronexus junctions characteristic of myofibroblasts were not seen. The absence of fibronexus junctions and the presence of surface features typical of smooth-muscle cells (attachment plaques with overlying lamina) suggest that this tumour is not myofibroblastic but is exhibiting a degree of smooth-muscle differentiation. The findings confirm earlier observations suggesting that some intranodal myofibroblastomas are not myofibroblastic, but show a form of smooth-muscle differentiation.