Ultrastructural Studies of Platelet ‘Activation’ and Aggregation in Normal Spleen from Different Species

Ultrastructural studies of immersion fixed specimens of spleen from dogs, rabbits and mice were carried out in order to explore morphologically platelet storage and destruction. In normal spleen from non‐treated animals in all species loose platelet aggregates were found in the sinusoids, whereas only single platelets were found in the Billroth cords. Both single and aggregated platelets showed morphological evidence of alterations with shape changes, pseudopod formation, and centralization and occasional loss of organelles. Administration of heparin, epinephrine and platelet antiserum appeared to increase the tendency of platelet aggregation. Macrophage phagocytosis of platelets could be demonstrated, but presence of identifiable platelets within vacuoles of the cells was not a frequent finding. Studies of aortic and splenic vein platelets revealed slight morphological differences, as the latter were more irregular in shape than the aortic platelets. Lower values of platelet serotonin were found in splenic vein than aortic blood, but platelet counts, aggregation ability, and electrophoretic mobility revealed no significant differences. It is concluded that the platelets in the spleen morphologically show evidence of ‘activation’, and that they are partly stored as loosely packed aggregates. This was, however, not reflected by the functional tests carried out.