Activated platelets. Coloured Scanning Electron Micrograph (SEM) of activated platelets in human blood. Platelets or thrombocytes (yellow) are small non-nucleated cells found in large numbers in the blood. When inactive they are round or oval discs; activated they have dendritic processes, or pseudopodia, and are star-shaped (as seen here). When activated they clump together and are involved in blood clotting. Leucocytes or white blood cells (light blue) play a role in the immune response of the body. At top is a red blood cell. The endothelial wall of a blood vessel is dark blue. Magnification: x4,510 at 6x7cm size. Magnification: x5,700 at 4x5 inch size.