Blood cell formation. Coloured transmission electron micrograph (TEM) of a section through blood cell precursors in mouse bone marrow. The three cells at left with multilobed nuclei and granular cytoplasm are maturing granulocytes, a type of white blood cell. The pink cell 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: x3000 when printed at 10 centimetres tall.