Postnatal new bone formation.

The morphologic events and macromolecular interactions in matrix-induced bone formation are comparable with those occurring in the development of fracture callus. Thus, bone induction by decalcified bone matrix is an experimental model for fracture healing and a new tool for research concerning the biochemistry of bone cell differentiation. Three conditions are necessary for bone cell differentiation in postnatal life: (1) a three-dimensional pattern of proliferation of mesenchymal cells; (2) anchorage-dependent microvilli extending the proliferating cells; and (3) a locally released bone morphogenetic protein (BMP). To date, BMP with a molecular weight of 17,500-18,500 daltons has been isolated from bone, and a BMP-like protein with a molecular weight of 22,000 daltons has been extracted from mouse osteosarcoma. It is difficult to separate BMP, a collagenase-resistant, trypsin-labile acidic polypeptide, from several other low molecular weight proteins.