Abstract
In rabbits, after long-bone growth is complete and the cambium layer regresses, mesenchymal-type cells with embryonic potential (competence) for bone development persist in the adventitial layer of periosteum. These cells are not determined osteoprogenitor cells (stem cells) because bone tissue differentiation does not occur when adult periosteum is transplanted into a heterotopic site. In this respect, adventitial cells differ from bone marrow stroma cells. In a parosteal orthotopic site in the space between the adult periosteum and diaphysis, implants of bone morphogenetic protein (BMP) and associated noncollagenous proteins (BMP/NCP) induce adventitia and adjacent muscle connective-tissue-derived cells to switch from a fibrogenetic to a chondroosteoprogenetic pattern of bone development. The quantity of induced bone is proportional to the dose of BMP/NCP in the range from 10 to 50 mg; immature rabbits produced larger deposits than mature rabbits in response to BMP/NCP. Preoperative local intramuscular injections of citric, edetic, or hyaluronic acids in specified concentrations markedly enhanced subperiosteal BMP/NCP-induced bone formation. The quantity of bovine or human BMP/NCP-induced bone formation in rabbits is also increased by very low-dose immunosuppression but not by bone mineral, tricalcium phosphate ceramic, inorganic calcium salts, or various space-occupying, unspecific chemical irritants. Although composities of BMP/NCP and allogeneic rabbit tendon collagen increased the quantity of bone in a parosteal site, in a heterotopic site the composite failed to induce bone formation. In a parosteal site, the conditions permitting BMP/NCP-induced bone formation develop, and the end product of the morphogenetic response is a duplicate diaphysis. How BMP reactivates the morphogenetic process in postfetal mesenchymal-type adventitial cells persisting in adult periosteum (including adjacent muscle attachments) is not known.
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