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

Abstract

Objective: Bone marrow stromal cells (BMSCs) include multi-potent 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 micro-environment. 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 particles, utilizing a spectrum of dosages, into immunotolerant mice; the transplants were followed for up to 29 months. Bone formation and the presence of hematopoiesis was graded on a previously validated semi-quantitative scale (range 0-4). 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 were necessary for extensive bone formation or any hematopoiesis (Figure 1). Hematopoiesis only occurred in conjunction with extensive bone formation, and no hematopoiesis occurred where bone formation was poor (Figure 2, b=bone, p=particle, h=hematopoeisis). Additionally, transplants underwent a change in bone morphology but not bone content after 8 weeks; this constitutes a novel description of long-term human BMSCs transplantation. Conclusions: Our results have provided evidence that the formation of both hematopoiesis and a mature bone organ are 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.