DENNIS KUNKEL MICROSCOPY / SCIENCE PHOTO LIBRARY DENNIS KUNKEL MICROSCOPY / SCIENCE PHOTO LIBRARY
Activated (filopodial extensions) and non-activated (smooth, discoid) platelets around a red blood cell, scanning electron micrograph (SEM). Channels of the open canalicular system are still present. Platelets are blood cell fragments that play an essential role in blood clotting and wound repair, and can also activate certain immune responses. Platelets in the blood are small oval disks and are termed non-activated platelets or thrombocytes. They are the body's first line of defence against excessive blood loss. When an injury such as a cut is sustained, platelets (now known as activated platelets) change their shape, become sticky and build up on a vessel wall to form a plug. They are also involved in the secretion of a chemical platelet factors which helps produce threads of fibrin. Blood cells collect in the entangled fibrin and platelet mass forming a blood clot at the site. Magnification: x1,600 when shortest axis printed at 25 millimetres.
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