Formation of hematopoietic territories and bone by transplanted human bone marrow stromal cells requires a critical cell density

Objective Bone marrow stromal cells (BMSCs) include multipotent cells with the ability to form mature bone organs upon in vivo transplantation. Hematopoiesis in these bone organs has been ascribed to the action of skeletal stem cells, which are capable of differentiating towards bone and hematopoiesis-supporting stroma. Yet, the creation of hematopoietic territories may be in part a natural consequence of the formation of a sufficiently mature and large bone microenvironment. Here, we describe, for the first time, a relationship between BMSC numbers and the extent of bone/hematopoiesis formation in heterotopic transplants. Methods Human BMSCs were transplanted along with hydroxyapatite/tricalcium phosphate, utilizing a spectrum of dosages, into immunotolerant mice; the transplants were followed for up to 29 months. Results The extent of bone and hematopoiesis formation increased with increasing BMSC numbers; however, the relationship was sigmoid in character, and a threshold number of BMSCs was necessary for extensive bone formation or any hematopoiesis. Hematopoiesis only occurred in conjunction with extensive bone formation, and no hematopoiesis occurred where bone formation was poor. Consistent with our earlier studies of long-term BMSC transplantation, the transplants underwent a change in bone morphology but not bone content after 8 weeks. Conclusion Our results have provided evidence that the formation of both hematopoiesis and a mature bone organ is as much a consequence of a sufficiently high local density of bone marrow stromal cells as it is the product of skeletal stem cell action.