Heterotopic ossification - replacement of one tissue with another.

Our body is composed of multiple organ systems that use specific types of cells for specialized functions. Bone and cartilage cells are normally restricted to the skeleton, however in heterotopic ossification, osteogenesis and chondrogenesis occurs within tissues such as skeletal muscle, tendons, and ligaments, and leads to the formation of bone tissue outside of the skeleton. Heterotopic ossification is most commonly associated with severe trauma such as following hip arthroplasty, traumatic brain and spinal cord injury, and high impact blast injury. It is difficult to predict who will respond to these traumas by forming heterotopic bone, and there are currently no effective preventions or treatments. However, genetic forms of heterotopic ossification are providing new insights into the cellular and molecular mechanisms that control postnatal de novo formation of bone. Our investigations of a rare genetic disorder of heterotopic ossification, fibrodysplasia ossificans progressive (FOP), have not only identified key cell signaling pathways that regulate bone and cartilage cell differentiation, but also revealed the importance of the tissue microenvironment in supporting and directing aberrant cell differentiation. Our data support that heterotopic ossification involves more than mis-directed cell differentiation, but is a multi-step, multi-cell type process leading to new tissue formation.