ULTRASTRUCTURAL ALTERATIONS ON THE PLATELET SURFACE INDUCED BY COMPLEMENT MEMBRANE ATTACK COMPLEX, DEMONSTRATED WITH SERIAL SECTIONS AFTER CRYOFIXATION AND CRYOSUBSTITUTION

The membrane attack complex C5b-9 (MAC) induces cell permeabilization accompanied by shedding of “microparticles” from the plasmalemma. We used cryofixation and examination of serial sections to demonstrate the ultrastructural details of the complement mediated alterations. The complement system was activated by incubation of citrated platelet rich plasma with the antibody IgM FN 52 to CD9. The experiment was monitored with an aggregometer, and arrested by rapid freezing during (1) shape change and (2) increasing light transmission. Phase 1 was characterized by filopodia formation, degranulation, and irregularities of the plasmalemma. Sequestration of cytoplasmic fragments was detected infrequently. In phase 2, the cytosol became electron lucent. Sequestration of cytoplasmic fragments from the platelet body appeared frequently. Membrane-attached electron dense deposits and distinct particles with a dimension similar to that of the MAC were recognized on the membranes. In the neck region of sequestered fragments, stretched membrane-like lines inserted angularly into the membrane were found. From their structural and dimensional characteristics, it was concluded that they represent an end to (membrane) site position of the MAC during sequestration. The findings suggested that membrane alterations were induced in early phases by permeabilizing precursors, and later by the incorporation of the complex into the membrane. This led to a decrease in cytosol density, which agreed with increasing light transmission in the aggregometer.