Platelets are the smallest of the three major types of blood cells. These are only about 20% of the diameter of red blood cells. The normal platelet count is 150,000-350,000 per microliter of blood, but since platelets are so small, they make up just a tiny fraction of the blood volume. The principal function of platelets is to prevent bleeding.
Platelets are produced in the bone marrow, the same as the red cells and most of the white blood cells. Platelets are produced from very large bone marrow cells called megakaryocytes. As megakaryocytes develop into giant cells, they undergo a process of fragmentation that results in the release of over 1,000 platelets per megakaryocyte. The dominant hormone controlling megakaryocyte development is thrombopoietin (often abbreviated as TPO).

In addition to being the smallest blood cell, platelets are also the lightest. Therefore they are pushed out from the center of flowing blood to the wall of the blood vessel. There they roll along the surface of the vessel wall, which is lined by cells called endothelium. The endothelium is a very special surface, like Teflon, that prevents anything from sticking to it. However when there is an injury or cut, and the endothelial layer is broken, the tough fibers that surround a blood vessel are exposed to the liquid flowing blood. It is the platelets that react first to injury. The tough fibers surrounding the vessel wall, like an envelop, attract platelets like a magnet, stimulate the shape change that is shown in the pictures above, and platelets then clump onto these fibers, providing the initial seal to prevent bleeding, the leak of red blood cells and plasma through the vessel injury.

Single Donor Platelets
Platelets can be collected directly from a donor by means of a specific machine. This machine draws whole blood in a special reservoir and separates the platelets from the rest of the blood by centrifugation. The platelets are collected and stored in a special bag which allows for gaseous exchange; while the rest of the blood (that is red cells and plasma) is returned to the donor. This constitutes one cycle. To make one unit of platelets the machine performs 6-8 cycles, each lasting about 10 minutes. This unit of platelets thus collected is called ‘single donor platelets’ since, as the name implies, platelets are derived from just one donor.
Another type of platelet component is referred to as ‘pooled platelets’. When donors visit a donation session to donate whole blood (WB), this is taken to our laboratories for processing. The bag is split up into red cells, a buffy coat (platelet rich plasma) and plasma, which will each become components in their own right for transfusion purposes. The buffy coat is where the platelets are found. Five such buffy coats (of the same ABO and RhD blood group) are ‘pooled’ together to form one unit of ‘pooled platelets’ (since the platelets are derived from multiple donors). So, in reality, we are all platelet donors!

All platelets are stored at room temperature (20-24ºC) with gentle agitation until issue. The shelf life is 5 days from the date of collection. Pooled Platelets expire 4 hours from the time of preparation (pooling if required) pooling.