The bone marrow supports haemopoiesis of all blood cell types and delivers mature cells to the blood. Haemopoiesis is characterized not only by the differentiation and proliferation of haemopoietic stem cells but by a number of physically associated cell types. These include macrophages, lymphocytes and, when haemopoiesis is intense, a multinucleate branched stromal cell. The venous vasculature of the bone marrow is associated with both haemopoiesis and the delivery of blood cells to the circulation. The wall of the vascular sinus consists of an endothelium lying upon a basement membrane. On the outside surface of the basement membrane lie adventitial cells or pericytes which branch out into the haemopoietic space forming a scaffolding upon which haemopoietic clusters are arranged. These cells move away from the wall of the vascular sinus to permit maturing blood cells to penetrate the endothelium and enter the circulation. Under other circumstances, adventitial cells accumulate fat, becoming the adipocytes of marrow.