Creation of New Bone by the Percutaneous Injection of Human Bone Marrow Stromal Cell and HA/TCP Suspensions

Background: The in vivo transplantation assay has become a valuable tool for assessing the osteogenic potential of diverse cell populations. It has required that cells are cotransplanted with a matrix into recipient animals using large incisions and extensive dissections. Here, we demonstrate that transplants of an osteogenic cell population, bone marrow stromal cells (BMSCs), are capable of assembling into mature bone organs when injected as suspensions of cells and a particulate matrix. Methods: Human BMSCs, along with hydroxyapatite/tricalcium phosphate (HA/TCP) particles, were placed either into the dorsal subcutaneous space or onto the calvarium of immunodeficient mice, either via injection or via a wide operative exposure. Transplants were harvested from 7 to 110 weeks later; their histologic and mechanical properties and their cellular origin were analyzed. Results: A total of 43 transplants were evaluated. The extent of new bone and hematopoiesis, the bone's adherence to the underlyingmouse calvarium, and the bone elasticmodulus and hardness were comparable between the two groups. In situ hybridization confirmed a human origin of the new bone. Conclusions: Our data indicate that BMSCs and HA/TCP particles, when injected as a suspension, can assemble into mature bone organs, and that this bone has histologic and mechanical properties similar to bone formed in standard transplants delivered through a large incision. These results open the possibility for assessing the osteogenic capacities of cell populations, for modeling bone formation and repair and for treating bone deficits, all in the context of minimal surgical intervention or soft tissue disruption.