Tachykinins in the emerging immune system: relevance to bone marrow homeostasis and maintenance of hematopoietic stem cells.

The mammalian tachykinins mediate crosstalk within the neural-immune-hematopoietic axis. Hematopoiesis occurs in the adult bone marrow (BM) and it is described as the method by which the immune system is replenished by a finite number of hematopoietic stem cells (HSC). These cells are found in the BM close to the endosteum where the oxygen level is the lowest. The BM is also resident to mesenchymal stem cell (MSC). The functions of HSC depend on the MSC to generate the supporting stromal cells. This review discusses possible mechanisms by which the MSC act as the 'gatekeeper' in the BM and regulate immune cells in and out of the BM. The roles of the tachykinins are discussed in the context of homeostasis in the BM and as mediators of BM disruption. The involvement of the tachykinins within the BM microenvironment and the development of immune cells in the BM are explained.