In the June 11th online edition of Stem Cells, R. K. Suda et al from the Univ. of Pennsylvania School of Medicine published their study results on the ability to isolate osteogenic progenitor cells (OPCs) from the blood of patients with fibrodysplasia ossificans progressiva (FOP) experiencing episodes of heterotropic ossification (HO). The study results revealed that blood samples from patients prior to clinical onset of HO yielded higher levels of circulating COP. In vitro studies demonstrated that the OPCs were clonogenic and that cells derived from normal individuals can contribute to HO in genetically susceptible patients. These circulating OCPs appear to be bone marrow-derived hematopoietic cells release into circulation with osteogenic potential. The authors surmised that the circulating cells can serve as seeds to ossify tissues that have been damaged and undergoing a local inflammatory response. Examples of heterotropic bone formation is aortic valve disease, head and spinal cord injuries and after hip and knee replacements.