This could result in a major shift in orthopaedic methodologies to prevent or treat this disease. begin to express the tight junction molecule, claudin 5. The cells expressing both the osteoblast-specific transcription factor, osterix, as well as claudin 5, then disappear from blood circulation at approximately 3 to 4 4?days by extravasation into the site of new bone formation. These endoneurial osteoprogenitors express neural markers PDGFR, musashi-1, and the low-affinity nerve growth factor receptor p75(NTR) as well as the endothelial marker Tie-2. In a key experiment, cells that were obtained from mice that were injected with cells transduced with an empty vector, at 2?days after injection, contained 0.83% (SD, 0.07; 95% confidence interval [CI], 0.59C1.05) cells expressing claudin 5. However, cells that were obtained from mice 2?days after injection of BMP-2-producing cells contained 4.5% cells expressing claudin 5 (SD, 0.72%; 95% CI, 2.01C6.94; p?0.0015). Further analysis revealed that all of the cells expressing claudin 5 were found to be positive for osteoblast-specific markers, whereas cells not expressing claudin 5 were unfavorable for these same markers. Conclusions The findings suggest that the endoneurial progenitors are the major osteogenic precursors that are used for HO. They exit the nerve through the endoneurial vessels, circulation through vessels to the site of new bone formation, and then extravasate out of the vessels into this site. Clinical Relevance The biogenesis of osteoblasts in HO is very different than expected and shows that HO is usually, at least in part, a neurological disorder. This could result in a major shift in orthopaedic methodologies to Ritonavir prevent or treat this disease. The fact that nerves are intimately involved in the process may also provide clues that will lead to an explanation of the clinical fact that HO often occurs as a result of Ritonavir traumatic Ritonavir brain injury. Introduction Heterotopic ossification (HO) is the formation of bone at nonskeletal sites as a result of a variety of causes including traumatic injury, orthopaedic surgical procedures (eg, hip replacement), joint disease, and burns. We previously exhibited a link between peripheral nerves and HO in a murine model, which relies on sustained delivery of DNAJC15 bone morphogenetic protein-2 (BMP-2) through injection of AdBMP-2-transduced cells into muscle mass . Salisbury et al.  recognized the immediate expression of the pain mediators, material P and CGRP (calcitonin gene-related peptide), on delivery of the BMP-2, which leads to neural inflammation with resultant degranulation of local mast Ritonavir cells and remodeling of the epineurium of sensory nerves in the muscle mass near the injection site. Removal of the epineurium was correlated with migration of progenitors that reside in the perineurium that undergo brown adipogenesis , presumably for the purpose of patterning the new bone . Blocking this process either through delivery of inhibitors of mast cell degranulation  or inhibitors of the binding of pain mediators to their receptor  resulted in a significant decrease in Ritonavir HO. Blocking nerve remodeling led to the accumulation within the endoneurium of nanog+ Klf-4+ osterix+ progenitors . Osterix+ cells express the osteoblast-specific marker osterix  and therefore allow characterization of osteoprogenitors. The endoneurium contains the axons and their supporting glial cells, called Schwann cells, embedded in loose collagen fibrils within.