Ultrastructure of a model of thrombogenesis induced by mechanical injury.

Injured cortical arteries were observed by electron microscopy. Haemostasis was brought about by both platelets and fibrin, but the intravascular thrombus contained only platelets. Some platelets adhered to the exposed subendothelium but did not form a continuous layer. Such platelet adhesion does not provoke a thrombus, which appears only at the opening in the artery. There was a gap of 7 micrometers between the thrombus and the intact arterial wall. The thrombus was built up progressively by concentric accumulation around the main injury. Central platelets were closely packed and the more distal ones loosely gathered but not touching and not activated. This structure was very different from that observed in in vitro aggregates which formed rapidly and whose platelets are all at the same stage of development and disposed radially. This implies a different sequence in the physiological evolution of platelets submitted to either mode of activation. The results obtained with the present model differ in several respects from those obtained with other models.