Thrombus formation following penetrating injury. In this depiction of thrombus formation induced by laser or sharpened glass probe, injury results in the exposure of phosphatidylserine on the endothelial cell surface and initiates assembly of the prothrombinase complex. A core of tightly packed platelets (blue) forms at the injury site and restricts the transport of agonists, including thrombin. A shell of loosely adherent, unactivated platelets (red) forms around the core. The core expands over time as agonists diffuse out of the core into the shell, creating a transition zone (dotted lines) in which activation of loosely adherent platelets occurs. Professional illustration by Luk Cox, Somersault1824.