The present invention provides muscle-derived progenitor cells that show long-term survival following transplantation into body tissues and which can augment non-soft tissue following introduction (e.g. via injection, transplantation, or implantation) into a site of non-soft tissue (e.g. bone). Also provided are methods of isolating muscle-derived progenitor cells, and methods of genetically modifying the cells for gene transfer therapy. The invention further provides methods of using compositions comprising muscle-derived progenitor cells for the augmentation and bulking of mammalian, including human, bone tissues in the treatment of various functional conditions, including osteoporosis, Paget's Disease, osteogenesis imperfecta, bone fracture, osteomalacia, decrease in bone trabecular strength, decrease in bone cortical strength and decrease in bone density with old age.
