Bone and bone marrow are closely aligned physiologic compartments, suggesting that these tissues may represent a single functional unit with a common bone marrow progenitor that gives rise to both osteoblasts and hematopoietic cells. Although reports of multilineage engraftment by a single marrow-derived stem cell support this idea, more recent evidence has challenged claims of stem cell transdifferentiation and therefore the existence of a multipotent hematopoietic͞osteogenic progenitor cell. Using a repopulation assay in mice, we show here that gene-marked, transplantable marrow cells from the plastic-nonadherent population can generate both functional osteoblasts͞osteocytes and hematopoietic cells. Fluorescent in situ hybridization for the X and Y chromosomes and karyotype analysis of cultured osteoblasts confirmed the donor origin of these cells and excluded their generation by a fusion process. Molecular analysis demonstrated a common retroviral integration site in clonogenic hematopoietic cells and osteoprogenitors from each of seven animals studied, establishing a shared clonal origin for these ostensibly independent cell types. Our findings indicate that the bone marrow contains a primitive cell able to generate both the hematopoietic and osteocytic lineages. Its isolation and characterization may suggest novel treatments for genetic bone diseases and bone injuries.

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Despite their anatomic proximity and recognized physiologic relationship, the interactions between bone and bone marrow (BM) are incompletely understood. Consistent with our data from clinical BMT in children with osteogenesis imperfecta, murine models show significant engraftment of transplanted donor BM cells in the osteogenic compartment early after BMT, which declines to a nearly undetectable level by 9 months post-BMT. To gain insight into the mechanism of donor BM cell engraftment in bone, we studied the kinetics of hematopoietic and osteopoietic engraftment. Lethally irradiated (1100 cGy) FVB/N mice were transplanted with 2x106 BM cells transduced with a retroviral vector encoding the green fluorescent protein (GFP). Transduction efficiency, assessed by flow cytometry, was 53%. Engraftment in the hematopoietic and osteopoietic compartments was analyzed at 2, 4, or 6 weeks post-transplant (wpt). Flow cytometric analyses of blood leukocytes, as a marker of functional hematopoietic marrow, showed 66% ± 2% GFP+ (mean ± SEM) at 2 wpt (n = 8 for each time point), increased at 4 wpt to 73% ± 2% and plateaued at 77% ± 2% by 6 wpt (p < 0.01). Erythrocytes and platelets exhibited a similar profile. BM cellularity, estimated by microscopic evaluation, increased from approximately 50% to 80% while GFP expression was observed in about 50% of marrow cells and remained at that level from 2 to 6 wpt. Analysis of 5 immunostained bone sections per animal revealed 40% ± 4% GFP+ osteoblasts at 2 wpt with a decline to 20% ± 2% by 6 wpt (p < 0.01). Mature osteocytes were 20% ± 3% GFP+ at 2 wpt, then declined to 10% ± 3% by 6 wpt (p < 0.05). Thus, both hematopoietic and osteopoietic engraftment are early events; however, donor-derived hematopoietic BM proliferates over the first 6 weeks post-BMT while the donor-derived osteopoietic cell population declines. Our data suggest that either the transplantable marrow osteopoietic cell is a progenitor capable of a single burst of differentiation or that bone specific mechanisms related to the BMT procedure underlie the capacity of BM cells to engraft in the osteopoietic environment. Elucidating this cellular pathway will be a key element to the development of BM cell therapy for mesenchymal tissues.