Platelets. Scanning electron micrograph (SEM) of blood platelets or thrombocytes. These cell fragments are usually disc-shaped, but when activated extend numerous thread-like pseudopodia. Platelets, a component of the blood, are produced by the bone marrow. They are essential for the formation of blood clots, accumulating in large numbers at the site of an injured blood vessel. Magnification: x25,000 at 7x9.5 inch size.