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 bone marrow cells 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 become very platelet deficient, a condition that puts them at risk of bleeding and death. Platelets are transfused to these patients to raise their platelet count and reduce their risk of bleeding. About 75% of platelet products are used as an adjunct to cancer treatment.
The other primary use of platelets is to arrest bleeding in trauma and surgery cases.
Platelet products are produced by collecting blood from donors. There are two methods for producing platelets:
- Pooled platelets are created by collecting whole blood from donors. The blood is separated into red cells, plasma and platelets by centrifugation. The platelet fraction from four or five donors is combined to make a single platelet product. Donors must wait 56 days between donations to allow their bodies to regenerate the red cells that were removed.
- Apheresis uses a centrifugation process during the collection from a single donor. The method allows platelets to be separated from the other blood components during collection without the need for additional processing. As a result, more platelets can be collected from a single donor, and the donation does not need to be combined with donations from other individuals. Furthermore, platelets regenerate faster than red cells, so apheresis donors may donate every 14 days.