What are Platelets?

Platelets (also known as thrombocytes) are small discoid cells and one of the major constituents of blood. They are formed by fragmenting from cells found in bone marrow called megakaryocytes, and similar to red blood cells, they do not have a nucleus. Platelets have an average life of 8-12 days, and finish their life cycle when they are absorbed by the spleen.

Platelets are a natural source of growth factors and make an interesting contribution to the serotonin system, but they are best known for their critical role in hemostasis (blood clotting). When platelets are activated, they change from their smooth discoid shape to a spiky sphere, which helps to form clots and stop bleeding. Platelets become activated by encountering one of several activating factors that are typical of a bleeding wound: contacting collagen, being exposed to thrombin, undergoing shear stress, being cooled to a low temperature, or contacting a negatively charged surface. After a clot has formed, the bleeding stops and the body can begin to heal the site.

Platelets are important in the medical context because many patients, such as cancer patients, can end up with an extremely low platelet count, a condition that puts them at risk of bleeding and death. In order to help these patients, whole blood is collected from donors and separated into plasma, platelet concentrate and red blood cells. The platelet concentrate is then transfused into the patient to raise their platelet count.