On the origin of cells in heterotopic bone formation.

Previous experiments have demonstrated that decalcified, lyophilized bone matrix is a powerful substratum for differentiation of mesenchymal cells into new bone. The present experiment employs parabiosis as a tool to distinguish between cells of hemotogenous origin and mesenchymal cell populations derived from the site of implantation of bone matrix. The results indicate that the osteoinductor releasing cells are blood-borne monocytoid cells which enter the tissues by diapedesis and become histiocytes, macrophages, matrixclasts and osteoclasts; their precursors are derived from bone marrow at sites remote from the area of bone induction. The cell populations responding to the osteoinductor released by this mechanism develop into osteoblasts and osteocytes and are the progeny of perivascular mesenchymal cells.