Osteoprogenitor cells within skeletal muscle

The formation of ectopic bone within skeletal muscle is a widely observed p henomenon. However, the source of the osteoprogenitor cells responsible for ectopic bone formation remains unknown. This study was designed to test fo r osteogenic differentiation among cells isolated from skeletal muscle tiss ue. Different subpopulations of cells derived from an adult mouse skeletal muscle were tested for induction of alkaline phosphatase activity after exp osure to bone morphogenetic protein-2 in vitro. A responsive subpopulation was identified, transduced with a retrovirus encoding for beta -galactosida se (Rv-lacZ) and an adenoviral construct encoding for one bone morphogeneti c protein-2, and injected into the hindlimb of immune compromised (severe c ombined immunodeficient, or SCID) mice. The injected cells appeared to acti vely participate in the ectopic bone formation. The existence of lacZ-posit ive muscle-derived cells colocalized with osteocalcin-producing cells withi n lacunae of newly formed bone matrix suggests osteoblast and osteocyte dif ferentiation. Although a specific cell was not isolated, these data support the contentions that osteoprogenitor cells reside within skeletal muscle a nd that muscle may represent a source other than bone marrow for the harves t of these cells.