The Ultrastructure of Rabbit Blood Platelet Aggregates^[*]

Summary

Different types of rabbit blood platelet aggregates formed in vitro were studied by means of electron microscopy.

1. Aggregates were produced by addition of small amounts of thrombin to citrate-PRP. The platelets, rich in pseudopods, were lying close together with no detectable fibrin between them. Fibrin was observed only in the periphery of the aggregate. Varying structural changes, with loss of granules and mitochondria, occured in the platelets. The appearance of these aggregates was similar to that of the haemostatic platelet plug formed in vivo.

In clots of PRP induced by greater concentrations of thrombin, fibrin was readily detectable as bundles of fibres. Platelets with complete loss of internal structure were scattered in the fibrin framework.

2. The platelets were usually loosely packed in thrombin-induced aggregates of washed platelets, with no visible fibrin between them. Considerable structural changes of the platelets were observed. Rods or filaments of electron-dense substance were observed inside the platelets.

3. The platelets were lying close together in ADP-induced aggregates, but several free spaces could be observed between them. Small and inconsistent structural changes of the platelets appeared.

4. Aggregates induced by saline extract of tendons demonstrated marked structural changes of the platelets, with loss of granules and mitochondria. Platelet membranes were mostly preserved. Several free spaces were observed between the platelets. No electron-dense substance indicative of fibrin was detected.

The appearance of the different types of aggregates is compared and discussed in relation to the ultrastructure of the haemostatic platelet plug.

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