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Previous works demonstrated that microfibrils stimulate blood platelets to aggregate. The present study compares the activation of platelets by human placental and bovine aortic microfibrils and by type III collagen. We studied the morphological changes occurring in in platelets during their activation and aggregation, as well as the kinetics of the release reaction and thromboxane B2 formation. As for collagen, the microfibrils-induced platelet aggregation followed a lag phase, during which progressive emission of pseudopodes and centralization of organelles occurred. Aggregation was associated with secretion of beta-thromboglobulin and adenylic adenylic nucleotides, and with formation of thromboxane B2; it was established that the kinetics of secretion and the aggregation curve were parallel. Microfibrils-induced aggregation was also inhibited by ethylenediamine tetraacetic acid, creatine phosphate-creatine phosphokinase, and aspirin, showing that it was calcium-dependent and required a secretion of ADP and formation of endoperoxide and thromboxane. The response to microfibrils was much more rapid than to collagen; placental microfibrils reacted faster than aortic microfibrils. The requirement of plasma in the microfibrils platelets interaction was confirmed: 10 microliters is the minimal amount of plasma necessary for an aggregation of platelets in 400 microliters of buffer. This fact supports the idea of the existence of two different pathways in the interaction between platelet and the subendothelium, depending on the vascular structure (microfibrils or collagen) involved, even though the sequence of events leading to the formation of an aggregate is similar.

14C serotonine labelled platetel rich plasma (PRP) incubated with fibrillar collagen is either gel filtered through a Sepharose 2B column (Sepharose test) or centrifugated on a 23% Ficoll layer (Ficoll test). In the Sepharose test, the adhesion is quantified by the determination of the per cent yield of free platelets eluted from the column and the release by a quantitative evaluation of the 14C serotonin. In the Ficoll test, the released seroton is measured from the distribution of radioactivity in the different layers (supernatant plasma, platelets adherent to collagen, Ficoll, platelet pellet) of the tube after centrifugation. Both methods are rapid and quite reproducible.