Fibrin mesh in a blood clot. Coloured scanning electron micrograph of fibrin fibres in a blood clot. When blood vessels become damaged, blood cells called platelets turn into spiky spheres theat clump together and release chemical factors into the blood. These factors turn the soluble protein fibrinogen into insoluble protein fibres, fibrin, which mesh together to form a dense network which catches blood cells and tightens into a solid clot.