Platelets (thrombocytes), coloured scanning electron micrograph (SEM). are made in the bone marrow. In their non-activated state they are disc shaped but when a blood vessel becomes damaged, it will send out a signal to thrombocytes who respond by travelling to the damaged area and transforming into their activated form. They change shape and form long tentacle-like protrusions. Fibrin is a protein formed from fibrinogen during the clotting of blood. The thrombocytes clump together and together with the polymerized fibrin, form a clot over the wound. Magnification: x10500 when printed at 10cm wide.