Regulation of haematopoietic progenitor cell proliferation and survival: The involvement of the osteoblast.

Haematopoietic progenitor cells (HPC) traffic between the circulation and the bone marrow. Through contact with osteoblasts in the bone marrow niche, their survival, maintenance and proliferation is regulated. This review summarizes recent observations regarding the interaction between osteoblasts and HPCs, and the resulting downstream effects on signaling and niche maintenance. Using live imaging, amongst other techniques, HPCs were found to make prolonged contact with the osteoblast, via a specialized region of their membrane with high expression of prominin 1, CD63 and rhodamine PE. Following contact, portions of the HPC membrane expressing these molecules were phagocytosed by the osteoblast into SARA-positive signaling-endosomes. In response, Smad signaling was downregulated in the osteoblasts, leading to increased production of SDF-1; a chemokine involved in progenitor cell homing to the bone marrow, and thus regulating progenitor cell trafficking. The study summarised here presents important findings regarding progenitor cell trafficking, maintenance, proliferation and survival in the bone marrow and potentially other niche microenvironments, following signaling events initiated and propagated through single cell interactions.