The comparative role of demineralized bone matrix placement on the periosteum versus in the muscle with and without bone morphogenetic protein 2

Demineralized bone matrix (DBM) is an allograft material used in orthopedics that promotes endochondral bone formation. While the placement of DBM on either the periosteal surface of a bone or within a skeletal muscle promotes the recruitment of stem cells that can form skeletal tissues through the temporal progression of endochondral bone development, it remains unclear to what degree these processes are different between the two sites. In this study, we utilize a comparative in vivo model of endochondral ossification by implanting the DBM on the periosteum and in the muscle. Within the muscle we further compared the effects of DBM with and without Bone morphogenetic protein-2 (BMP-2), a primary morphogenetic factor involved in the differentiation of skeletal stem cells. The mice were harvested at various time points after DBM implantation in order to analyze the development of the bone. Analysis included X-ray imaging, microCT imaging, and mRNA expression. Plain x-ray and micro-CT imaging analysis showed mineralized bone formation in the implant on the periosteum and in the muscle with BMP-2, but no growth in the muscle when BMP-2 was not added to the DBM. The mechanisms for bone development were further analyzed by qRT-PCR to determine temporal patterns and levels of expression of various stem cell and differentiated skeletal cell associated genes. The stem cell gene expression varied between implant placement locations suggesting different mechanisms for stem cell recruitment. Interesting, while DBM implants in the muscle without BMP did not induce mineralized tissue specific mRNA expression; specific stem cell and early skeletal cell lineage commitment genes were present. These results suggest that while DBM in muscle is capable of recruiting stem cells that higher BMP-2 levels are needed to promote the progression of cartilage to mineralized bone in muscle tissues