Scanning electron micrograph (SEM) of a menstrual blood clot showing little granularity. Human red blood cells trapped in a fibrin blood clot. Platelets are cell fragments in the blood that play an essential role in blood clotting and wound repair. Platelets can also activate certain immune responses. Platelets are formed in the red bone marrow, lungs, and spleen by fragmentation of very large cells known as megakaryocytes. Platelets in the blood are small oval disks and are termed nonactivated platelets or thrombocytes. Platelets serve as the body's first line of defense to prevent excessive blood loss. When an injury such as a cut is sustained, platelets change their shape (now known as activated platelets), become sticky and build up on a vessel wall to form a plug. They are also involved in the secretion of a chemical platelet factor which helps produce threads of fibrin. Bloods cells collect in the entangled fibrin and platelet mass forming a blood clot at the site. Magnification: x1,600 when shortest axis printed at 25 millimetres.