Hypothetical model of skeletal metamorphosis in fibrodysplasia ossificans progressiva. (Adapted from Kaplan FS, et al. (2007) Morphogen receptor genes and metamorphogens: skeleton keys to the metamorphosis. Ann N Y Acad Sci 1116:113–133 [38].) PGs, prostaglandins; pO2, tissue oxygen tension; HIF, hypoxia-inducing factor; CTPs, connective tissue progenitor cells; HSC, hematopoietic stem cells; T, T cells; B, B cells; Mo, monocytes; MA, mast cells; IL-1β, interleukin-1β; IL-6, interleukin 6; TGF-β, transforming growth factor-beta; straight arrows, progression; wavy arrows, influence