Early stage of blood clot formation: Coloured scanning electron micrograph (SEM) showing an aggregation of red blood cells and large numbers of platelets (yellow) on the endothelium of a large blood vessel. Clotting is a repair process signalled by the release of thromboplastin from platelets (a type of blood cell) and injured endothelial cells. The formation of a clot, a mesh of blood cells bound by threads of fibrin, is a complex multi-step process. No fibrin is visible at this early stage. Magnification: x400 when printed 10 centimetres wide.