CORRELATIVE IMMUNO-LIGHT AND ELECTRON-MICROSCOPIC STUDY ON THE TYPE-ICOLLAGEN IN THE BONE MORPHOGENETIC PROTEIN-INDUCED CARTILAGE

Bone morphogenetic protein (BMP) partially purified from bovine bone w as implanted into the thigh muscles of AKR-strain mice to induce an ec topic endochondral ossification tissue. The extracellular matrix compo nents around the hypertrophic chondrocytes in the induced tissue were examined by electron microscopy and immunohistochemistry for type I co llagen. At 14 days after implantation, chondrocytes became hypertrophi c with calcifying matrix around them. Some hypertrophic chondrocytes s howed no degenerative appearances. They were ringed by a pericellular matrix with no metachromasia which was distinct from the metachromatic cartilage matrix. The localization of type I collagen was demonstrate d by immune-staining in this matrix. Ultrastructurally, ring-shaped ma trix consisted of interwoven collagen fibrils on which a D-periodic ba nding pattern could be discerned. Immunoelectron microscopy demonstrat ed immunoreactivity of type I collagen on these fibrils. These results suggest that some of the BMP-induced hypertrophic chondrocytes mainta in their activity and produce type I collagen-rich matrix before their lacunae are eroded.