Ultrastructural finding of vascular degeneration in fibrodysplasia ossificans progressiva (FOP).

The masseter and temporalis muscles were investigated ultrastructurally in 2 patients having fibrodysplasia ossificans progressiva (FOP). Results showed atrophy and necrosis of muscle fibres. Both cases contained cartilage which was calcified in the second case and was associated with degenerating chondrocytes. The blood vessels showed various changes, with haematoma formation and large fibrin deposits. Some of the fibrin deposits seen in the second case were surrounded with cartilage. This suggests that the cartilage probably forms as a reaction to an old haemorrhage. It is possible that the effect of the mutant gene found in FOP is on blood vessels rather than the normal regulatory mechanisms of the inducible osteogenic precursor cells. These degenerating blood vessels often result in haemorrhage and fibrin deposits; the fibrosis, cartilage or bone formation are probably secondary changes. Amianthoid fibres found in the tendon of the temporalis muscles of the second case, together with the other changes of fibrosis and cartilage formation, account for the severe restricted mouth opening suffered by these patients.