Blood flow to the haemopoietic bone marrow.

In order to maintain an adequate haemopoiesis, the bone marrow is dependent on a sufficient blood supply. Our understanding of the regulation of marrow perfusion remained patchy for several decades, mainly because of technological difficulties. Recent advances in methodology have permitted more detailed analyses of bone marrow perfusion. Enhanced stimulation of erythropoietic or leucopoietic activity leads to increased blood flow to the bone marrow. There is evidence that this vasodilatory effect is mediated by the cytokines erythropoietin and granulocyte colony-stimulating factor; however, a direct link has yet to be firmly established. The presence of receptors for several other cytokines on vascular endothelial and smooth muscle cells suggests that these may also have a regulatory role. The vasodilating factor nitric oxide (NO) regulates bone marrow blood flow both under normal conditions and during accelerated haemopoiesis, and NO is possibly induced via activation of cytokine receptors. There are conflicting reports as to how catecholamines affect marrow vascular tone. Although there is firm evidence that the bone marrow vasculature is innervated, no definite role of this innervation in vasoregulation has been documented. Advances in bone marrow/blood stem cell transplantation technology and the development of other therapeutic strategies for bone marrow failure may in part be dependent on optimization of the blood supply to the haemopoietic bone marrow.