Platelets, also called thrombocytes, are the smallest blood component at around 2-4 μm two to four micrometers in diameter. They are formed in the bone marrow when long extensions of Megakaryocyte cells break off into small pieces, each of which forms an individual, irregularly shaped platelet. As platelets are only cell fragments, they do not contain a nucleus (they are anucleate).

When blood vessels are damaged, platelets are activated and rush to the site of injury. They clump together (a process called agglutination) to form a platelet plug which blocks off the damaged blood vessel and reduces blood loss (see Figure 1).

Platelets also release clotting and growth factors, which help repair damage and stimulate tissue regeneration.

Illustration of the interior of a blood vessel. There are two types of cells present inside: red blood cells, which are flat, red, and disk-like shaped; and platelets, which have an irregular star-like shape, are smaller than red blood cells, and are white.

Figure 1: Red blood cells and platelets in the bloodstream.