Abstract:The morphological features at the light and electron microscopic level and distribution of fibronectin and laminin using immunohistochemical methods were evaluated in chondro-osteogneic tissue induced by bone morphogenetic protein (BMP) in mouse skeletal muscle. Satellite cells were seen at the muscle fibers adjacent to the site of BMP implantation 3 days after BMP-implantation. At the ultrastructuraI level, the skeletal muscle had a continuous basal lamina, satellite cells were partially enveloped by basal lamina-like structure, and fibroblast-like cells with abundant mitochondria and endoplasmic reticulum were adjacent to the myofibrils. An enhanced immunoreactivity for laminin and fibronectin was observed at the basement membrane of muscle fibers and extracellular ~natrix of neighboring connective tissue on day 3. On the 7th day, the enhanced immunoreactivity of laminin was persistent on the muscle fibers adjacent to the BMP and myoblast-like satellite cells, however, fibronectin showed decreased reaction. On the 14th day specimen, the newly formed osteo-chondroid matrix with numerous chondroid cells and osteoblasts showed positive immunoreaction for laminin and fibronectin. During the process of resorption of the induced osteoid tissue, no immunoreactivity for laminin and fibronectin was observed. An enhanced immunoreactivity of these glycoproteins during the early stages of BMP induced chondro-ossification suggests that they play an important role in the differentiation of myo-fibroblasts and organization and integration of ground substances of connective tissue and chondro-osseous matrix.