Bone Marrow Adiposity and Heterogeneity in Outbred Mice

We utilized skeletally immature mice CD1 mice at 4 weeks of age to characterize the distribution of adipocytes within femurs and tibiae. Lipid laden cells were stained with osmium and found in association with the endocortical surfaces and in abundance within the distal tibia. Osmium data were supported with histological evaluation and adipocyte gene expression analysis. Collectively, this data suggest true skeletal and regional differences in adipocyte distribution. We hypothesized that adipocytes contribute to progenitor niches. To begin to address this question, we isolated and cultured bone marrow stromal cells from the proximal and distal regions of femurs and tibiae. The proximal femur and distal tibia demonstrated the most robust response to adipogenic and osteogenic stimuli suggesting that bone marrow is heterogeneous. The identities of these progenitor cells in the proximal and distal tibia were explored through progenitor depletion cultures. The distal tibia had progenitors with restricted adipocyte and osteoblast potential. The adipocyte population may be derived from a pre-adipocyte lineage where the osteoblasts may originate from a multi-potent mesenchymal progenitor population lacking adipogenic capacity. The proximal femur had significant osteogenic and adipogenic capacity but did not have abundant adipocytes like the distal tibia. This suggests that adipocytes are not necessary to maintain rich mesenchymal progenitor populations. Alternatively, adipocytes might discriminate between the functional activities of the two regions by secreting adipokines that have been shown to stimulate angiogenesis. We proposed that the distal tibia might show signs of increased vascularity relative to the proximal tibia. This hypothesis was evaluated by both FACS analysis and immunohistochemistry. Significant findings were an increased percentage of CD31+ cells, marking the endothelial lineage, in the distal relative to the proximal tibia as well increased numbers of CD31+ cells with associated luminal structures in the distal tibia.