Distribution of glycoprotein IIb-IIIa complexes in the surface membranes of human platelets and megakaryocytes.

The distribution of glycoprotein (GP) IIb-IIIa complexes in the surface membranes of human platelets and megakaryocytes was investigated by transmission electron microscopy of cells that had been incubated with Fab fragments of a human alloantibody (IgG L) specific for the complex. Binding was visualized by a second antibody conjugated to peroxidase or adsorbed onto gold particles. Initial studies showed that the peroxidase reaction product and the gold particles were to be found at the outer surface of unactivated platelets. The occasional small cluster of particles was present. A positive reaction, more apparent with peroxidase labelling, was also seen in the channels of the platelet open canalicular system. Gold particles were abundant on the outer surface of mature megakaryocytes, and their distribution resembled that on unactivated platelets. As with platelets, peroxidase-labelled antibodies penetrated better, and revealed GP IIb-IIIa in the demarcation membrane system. A double immunofluorescence study, involving Fab fragments of IgG L and rhodamine-conjugated antibodies to factor VIII R:Ag, demonstrated the presence of GP IIb-IIIa in megakaryocyte precursor cells. Our results show that the GP IIb-IIIa complex is present in megakaryocyte membranes and that it appears at the same time as the other platelet antigens.