Figure 1.

The bone marrow niche is a multicellular compartment of mesenchymal and hematopoietic progenitors that is highly vascularized. Blood supplies basic nutrients that are needed for each of the cell types shown. The marrow adipocytes are located on the endosteal surface of bone as well as throughout the marrow, and are increased in conditions such as calorie restriction and diabetes. It is conceivable that marrow adipocytes could arise from vascular endothelial cells, pericytes, bone lining cells or mesenchymal precursors. Notwithstanding their unclear origin, both osteoblast and adipocyte precursors require nutrients to fuel the needs of differentiation. This need links the marrow compartment with the peripheral sources of energy, particularly fat depots and the liver. Hence, both transcriptional and metabolic programming determine the fate of mesenchymal progenitors, which in turn, define the marrow adiposity phenotype.