- A tiny, disc-shaped piece of cell that is found in the blood and spleen. Platelets are pieces of very large cells in the bone marrow called megakaryocytes. They help form blood clots to slow or stop bleeding and to help wounds heal. Having too many or too few platelets or having platelets that don’t work as they should can cause problems. Checking the number of platelets in the blood may help diagnose certain diseases or conditions. Also called thrombocyte.