The concept of a hematopoietic niche was first proposed in 1978, and the overall concept of a stem cell niche was first demonstrated in Drosophila gonads (1–3). Within mammalian bone marrow, hematopoietic stem and progenitor cells (HSPCs) interact with a variety of cells and signals, which constitute their niche or microenvironment. Cells of the microenvironment, through either direct contact or through secreted factors, can influence HSPC behaviour in the marrow. These micro environmentally

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... osed signals can regulate stem cell fate decisions, self-renewal, and residence in the marrow and are critical to maintaining the stem cell pool. Disruption of these signals in the microenvironment can lead to stem cell depletion, altered haematopoiesis, and malignancy. Over the past 10 years, numerous cell types and molecules of the HSPC niche have been identified and are discussed in several comprehensive reviews. Our research will focus on the cellular components of the hematopoietic stem cell (HSC) niche that are targets for hormonal signals (specifically, mesenchymal stem cells [MSCs] and the osteoblastic lineage as well as adipocytes) and how hormonal signals and signalling pathways are integrated in the bone marrow microenvironment.