Blood cell formation. Transmission electron micrograph of blood cell precursors in mouse bone marrow. The cells with multilobed nuclei and granular cytoplasm are maturing granulocytes, a type of white blood cell. The grey cell (centre right) with no nucleus is a red blood cell. Continual renewal of blood cells is necessitated by their relatively short lifespan. They are produced by a process called haemopoiesis. The five main cell types (erythrocytes, lymphocytes, granulocytes, monocytes and platelets) all derive from a single type, the multipotential stem cell. Magnification x7600 at 10x8 inch size.