Blood clot. Coloured scanning electron micrograph (SEM) of red blood cells (erythrocytes) trapped in a fibrin mesh. Fibrin is produced during blood clotting. Some of the red blood cells have become crenated (spiked). This may be symptomatic of uraemia (accumulation of nitrogenous waste in the blood), which causes the blood cells to become dehydrated. However, it is often the result of prolonged storage of a blood specimen. Magnification: x3000 when printed 10cm wide.