The fibrodysplasia ossificans progressiva lesion

Clinically, the lesions in fibrodysplasia ossificans progressiva (FOP) follow spontaneous or injury-induced exacerbations and are characterized by painful swellings in soft connective tissue that progress to form mature heterotopic bone. Heterotopic ossification (HO) progresses in characteristic anatomic and temporal patterns, and the location of HO dictates the severity of functional consequences. The histological stages of lesion formation are well described and include, in order of progression: perivascular lymphocytic infiltration (stage 1A), lymphocytic migration into affected muscle and myonecrosis (stage 1B), early reactive fibroproliferation (stage 1C), intense fibroproliferation, neovascularity, and angiogenesis (stage 2A), cartilage formation (stage 2B), and endochondral bone formation (stage 2C). Possible mechanisms for FOP lesion formation, including the origin of bone-forming cells, are discussed.