Scanning electron microscopic studies of single blood platelets and platelet aggregates showed:
Immediately fixed platelets were disc-shaped with no or very few, small pseudopods. The surface of the platelets was rough with holes, probably corresponding to openings from the surface connecting system. Due to the centrifugation, platelets were lying close together, but there was no evidence of platelet aggregation. Platelets in citrated plasma stirred for 5 minutes in an aggregometer at 37° C were mostly discoid, but some irregular forms with pseudopods occurred. A few small platelet clumps were observed. Platelets appearing to adhere to red cells were also found.
Platelets in citrated plasma stored at 0–2° C for 30 minutes were irregular, spherical with numerous buds and pseudopods. Thus the appearance of these platelets was completely different from that of the above mentioned specimens.
Aggregates induced by ADP or collagen showed spherical platelets with a meshwork of interlacing pseudopods.
The conclusion is drawn that chilling has a pronounced effect on platelet shape, a fact which should not be overlooked. The meshwork of pseudopods observed in the aggregates may be of importance for the stability of the aggregates in vitro as well as in vivo.