Demonstration of the outer surface of freeze-etched normal human platelets: correlation with buoyant density.

The external surface of the normal human platelet shows clusters of spherical particles in a reticular pattern with smooth and regular surface membrane in adjacent areas. Labelling with cationic ferritin demonstrates a diffuse distribution of ferritin particles which stop at the fracture face of the intramembranous surface. The intramembranous surface is not labelled. Previously discrepant observations on the plasma membrane surface are probably due to different techniques of platelet preparation for freeze-etch electron microscopy. Density-dependent platelet sub-populations do not differ with respect to the ultrastructural features of their external surface and intramembranous portions of the plasma membrane. Based upon the relationship between platelet density and age, it appears that the ultrastructure of the normal platelet plasma membrane does not change with in vivo ageing.