Ectopic bone formation is enhanced in senescent animals implanted with embryonic cells

Ectopic bone formation induced by the subcutaneous implantation of demineralized bone matrix (DBM) is very significantly reduced in older Fischer 344 rats. Cells originating from calvaria of 20-day-old embryo donors were introduced into cylinders of DBM sealed at the ends with a Millipore filter or collagen sponges prior to subcutaneous implantation. Cells within the chambers had access to vascular channels that could penetrate through the interstices of the DBM. After four weeks of implantation in 26-month-old rats, the cylinders were full of bone. This bone was assessed by histologic techniques, by calcium and bone gamma-carboxyglutamic acid (gla) protein (BGP) concentrations, and by alkaline phosphatase activity. Cylinders to which no cells were added produced no bone. Bone marrow cells enclosed in similar cylinders or injected weekly at the implantation site also enhanced new bone formation but to a much lesser extent. Embryonic muscle cells formed large amounts of cartilage and less bone. Fibroblasts were inactive in this system. Prior treatment of the DBM with trypsin inhibited the myoblast response but not that of calvaria cells.