Platelets, also known as thrombocytes, are blood cells responsible for blood clotting. 


If a blood vessel wall becomes damaged, platelets will rush to the site of injury and form a plug or clot to stop the bleeding. If platelet count is low (a condition called thrombocytopenia), the risk of uncontrolled or prolonged bleeding increases. 


When there are too many platelets in the blood (a condition called thrombocytosis), it may lead to abnormal blood clot formation, which can be serious and life-threatening.


The platelets which appear "straggly" or "open" are "activated" platelets.


Platelet activation is a series of cascading responses which allow blood platelets to react to an injury. 


Platelets could be thought of as one of the body's emergency response teams; when a cut or burn occurs, the body sends out a distress signal, and platelets are among the many specialized cells which flock to the site. 


In people with low platelet counts, it can be difficult to control bleeding because not enough platelets can be activated. By contrast, a high platelet count can lead to increased clotting, which can become a problem when platelet activation occurs.


When the skin is broken, platelets are exposed to collagen, which causes them to activate. Platelet activation triggers the release of chemicals which cause additional platelets to activate. Activated platelets change shape, putting out fingers which cause them to take on a star shape. The platelets are also triggered to cluster and bind together, sealing the site of an injury to stop bleeding.


Below the surface of the skin, platelets coagulate during platelet activation, so that blood cannot flow out of a damaged vessel. Above the skin, they scab, forming a protective layer. In both cases, the platelets stop the bleeding and protect the body while it heals and rebuilds the injured area.