False-colour scanning electron micrograph (SEM) of human red blood cells (erythrocytes), showing the initial clotting process, the formation of fibrin filaments that comprise a clot or thrombus. Clotting is a complex, multi-step process: essentially, thromboplastin released from blood platelets & around the injured vessel, in presence of calcium ions, converts prothrombin to thrombin. The presence of thrombin results in transformation of blood protein fibrinogen into insoluble fibrin threads. Numerous erythrocytes surround a clot; some undergo deformation (crenation) in the concentrated plasma environment. Magnification: x675 at 35mm size.