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. Author manuscript; available in PMC: 2013 Jan 22.
Published in final edited form as: Nat Rev Rheumatol. 2010 Aug 10;6(9):518–527. doi: 10.1038/nrrheum.2010.122

Figure 3.

Figure 3

Schematic histologic representation of the stages of endochondral heterotopic ossification in FOP. Lesion formation in FOP involves inflammation and the destruction of connective tissues (phase 1) followed by a replacement phase of new tissue development (phase 2). The initial histologic evidence of lesion induction is the presence of abundant perivascular lymphocytes (stage 1A) in connective tissue such as skeletal muscle. Lymphocytes expand into the tissue (stage 1B) and loss of the connective tissue structure follows (stage 1C). As the tissue is degraded, it is rapidly replaced by fibroproliferative cells (stage 2A). Angiogenesis and vascularization occur (stage 2B), followed by chondrogenesis and osteogenesis (stage 2C) and the formation of heterotopic bone. Abbreviation: FOP, fibrodysplasia ossificans progressiva.