Activated platelets. Coloured scanning electron micrograph (SEM) of activated platelets, or thrombocytes, starting to adhere to exposed sub-endothelium on the inner surface of a vein. Platelets are fragments of white blood cells that under normal circumstances are small and biconcave in form. However, if there is a break in the surface of a blood vessel the platelets come into contact with molecules they are not used to and become activated. They become amorphous in form, with long projections (pseudopodia) that help them adhere to the vein and each other, forming a clot over the site of the break. Magnification: x50 when printed at 10 centimetres wide.