A light and electron microscopic study of ectopic tendon and ligament formation induced by bone morphogenetic protein-13 adenoviral gene therapy

Object. Bone morphogenetic proteins (BMPs) are involved in the growth and d evelopment of many tissues, but it is their role in skeletal development an d their unique ability to induce ectopic and orthotopic osteogenesis that h ave attracted the greatest interest. Expression of the BMP-13 gene is predo minantly localized to hypertrophic chondrocytes in regions of endochondral bone formation during development, as well as in mature articular cartilage in the adult. In addition, the application of BMP-13 on a collagen carrier induces neotendon/neoligament formation when delivered subcutaneously or i ntramuscularly in rodents. The aim of the present study was to determine th e histological and ultrastructural. changes that occur after the intramuscu lar injection of a first-generation BMP-13 adenoviral vector. Methods. Athymic nude rats were injected with 3.75 X 10(10) plaque-forming units of adenovirus (Ad)-BMP-13 or Ad-beta -galactosidase in the thigh musc ulature, and the region was examined using light and electron microscopy at various time points between 2 days and 100 days postinjection. As early as 2 days after injection of Ad-BMP-13, progenitor cells were observed infilt rating between the transduced muscle fibers. These cells subsequently proli ferated, differentiated, and secreted large amounts of collagenous extracel lular matrix. By 100 days postinjection, the treated tissue displayed the h istological and ultrastructural appearance of neotendon/neoligament, which was clearly demarcated from the surrounding muscle. Small foci of bone and fibrocartilage were also seen within the treated tissue. A short-term bromo deoxyuridine study also demonstrated rapid mesenchymal cell proliferation a t the Ad-BMP-13 injection site as early as 48 hours postinjection. At all t ime points, the control AD-beta -gal injection sites were found to contain only normal muscle, without evidence of inflammation or mesenchymal cell pr oliferation. Conclusions. The results of this study indicate that in the future the use of the BMP-13 gene may have therapeutic utility for the healing of tendon a nd ligament tears and avulsion injuries.