Platelet. Coloured transmission electron micrograph (TEM) of a platelet in the lumen of a tumour capillary. A platelet (thrombocyte) is a small cell found in the blood that does not have a nucleus. It plays a major role in blood coagulation, having adhesive properties when activated. It aggregates in a cluster with many other platelets to form a clot and stem bleeding. The discoid shape of resting blood platelets is maintained by a marginal band of microtubules (not visible). Magnification: x12, 000 when printed at 10 centimetres wide.