Platelets are the tiny fragments of special cells formed up to 7 days in the normal body and it was form in the bone marrow of the humans. Platelets do not have nuclei. Platelets help in the coagulation of blood (stop the blood when it flow, clotting of blood) in a cut or wound. For example, when our hand was cut from knief then the bleeding starts but after some time the bleeding automatically stop because platelets help clot the blood (make the blood semi-solid) due to which further bleeding stops. All the blood cells are made in the bone marrow from the cells called stem cells.