FIG. 1.

Model for the role of the osteoblast lineage in the bone marrow hematopoietic niches. Cells of the osteoblast lineage are in blue, whereas hematopoietic cells are red. Within each lineage, darker color intensity reflects a more advanced stage of differentiation. In this model, mature osteocalcin-expressing osteoblasts provide a niche for HSCs, whereas more-differentiated pro-B cells are supported by cells expressing both osterix and IL-7. However, many questions await further study. Are the osterix⁺IL-7⁺ cells the same as osterix⁺ osteoprogenitors? Cells of the osteoblast lineage can produce CXCL12; are CXCL12⁺ cells that contact prepro-B cells a subset of the osteoblast lineage? Finally, do HSCs require direct contact with mature osteoblasts and do earlier cells in the osteoblast lineage have any role in supporting HSCs?