Fresh plasma freed of intact platelets can be shown to contain minute particulate material (platelet-dust) which can be separated by ultracentrifugation. Much of this material is rich in phospholipid and shows coagulant properties resembling those of Platelet Factor 3. The supernatant plasma, freed of these particles, shows a corresponding deficiency in Platelet Factor 3 activity.
Electron microscopic studies suggest that the lipid-rich particles may originate from the osmophilic granules of platelets and that they are extruded on storage of platelets, even in the absence of coagulation. It is suggested that the liberation of this material is the basis of the process of ‘activation’ of platelets known to occur with storage.
When clotting occurs, coagulant particulate material is released from platelets, considerably in excess of what is required for thrombin generation and it can be detected in serum. Its presence accounts for the platelet-like activity of serum.
The material can be distinguished from intact platelets, red cell stroma and chylomicra. It was observed that chylomicra, freed of platelet-dust, were inactive in a thrombin generation test, which thus appeared responsive only to platelet type lipid. Both chylomicra and platelet-dust were effective in a Stypven time test. Attention is drawn to possible errors of interpretation which may arise from the use of the latter test as an index of Platelet Factor 3 activity.
Preliminary immunological investigation of platelets revealed the presence of fibrinogen and at least three other constitutents which appear to be primarily platelet components.