Freeze-fracture studies on the plasma membranes of normal human, thrombasthenic, and Bernard-Soulier platelets

Abstract

The description of severe molecular deficiencies of different membrane glycoproteins in thrombasthenic and Bernard-Soulier platelets has led us to investigate the intramembrane organization of their plasma membranes using freeze-fracture electron microscopy. An initial examination of the cleaved plasma membranes of freeze-fractured normal human platelets revealed randomly distributed MAPs on the fracture faces of both the outer and inner phospholipid leaflets of the bilayer. Particle densities of 925 +/- 52/micrometer22 on the EF and 427 +/- 29/micrometer2 on the PF were calculated with a computer-linked picture analyzer. The particle size was heterogeneous on both fracture faces, and the number of particles decreased exponentially in the 5 to 13 nm size range. Examination of the platelets of three thrombasthenic patients revealed a low particle density (36% to 69% of the normal range) on the PF of the cleaved plasma membrane and a reduced particle coefficient between the two fracture faces (PF/EF). This abnormality was accompanied by a preferential decrease in the larger sized particles. In marked contrast 8 to 13 nm particles predominated on both fracture faces of the plasma membranes of the platelets of a Bernard-Soulier patient, and a greater concentration of particles on the PF rather than the EF was uniquely observed. The results further define the modified structure of thrombasthenic and Bernard-Soulier platelet plasma membranes and suggest a structural heterogeneity within the total MAP population of the membranes of normal human platelets.