Regulation of hematopoiesis in endosteal microenvironments.

After birth, the hematopoietic system develops along with bone formation in mammals. Osteolineage cells are derived from mesenchymal progenitor cells, and differentiate into several types of bone-forming cells. Of the various types of cell constituents in bone marrow, osteolineage cells have been shown to play important roles in hematopoiesis. Early studies have identified osteoblasts as a hematopoietic stem cell niche component. Since that time, the role of endosteal microenvironment as a critical regulator of hematopoietic stem/progenitor cell (HSC/HPC) behavior has been appreciated particularly under stress conditions, such as cytokine-induced HSC/HPC mobilization, homing/engraftment after bone marrow transplantation, and disease models of leukemia/myelodysplasia. Recent studies revealed that the most differentiated osteolineage cells, i.e., osteocytes, play important roles in the regulation of hematopoiesis. In this review, we provide an overview of recent advances in knowledge of regulatory hematopoietic mechanisms in the endosteal area.