Early clot formation. Coloured transmission electron micrograph (TEM) of a section through a capillary containing a numbers of aggregating platelets. Platelets plug defects in the walls of blood vessels, and are involved in clotting. They also release serotonin which constricts blood vessels. Platelet cells (thrombocytes) are formed in the bone marrow, and circulate in blood in large numbers. In an inactivated state they are round/oval, whereas activated platelets develop pseudopodia or extensions from the cell wall. Platelets contain cytoplasmic organelles such as mitochondria, endoplasmic reticulum and granules. Magnification: x12000 when printed 10cm wide