Freeze-fracture study of mast cell secretion.

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Abstract
Within seconds after exposure of rat peritoneal mast cells to polymyxin B, bulges appeared on the surface of the cells. Freeze-fracture EM revealed that each bulge overlies a mast cell granule. In contrast to the even distribution of intramembranous particles in the plasma membrane of unstimulated cells, the intramembranous particles in the stimulated cells were unevenly distributed in the membrane of the bulges with large patches of membrane lacking intramembranous particles. The membranes over the most prominent bulges were entirely free of intramembranous particles, and in some instances there was an increased concentration of intramembranous particles at the margins of the bulges. Perigranule membranes exhibited the same changes in distribution of intramembranous particles. EM of thin sections of rapidly fixed, stimulated mast cells showed a peculiar structure of the membrane overlying some bulges; instead of the pentalaminar membranes previously demonstrated, the membrane at these sites of presumptive fusion of perigranule and plasma membrane assumed the form of a single dense lamina with a fine fuzzy coating on either side. It seems possible that membrane fusion and subsequent pore formation proceed in the stimulated mast cell through a stage of flight of intramembranous particles and molecular rearrangement of the other membrane components.