Ultrastructural basis of the mechanism of rouleaux formation.

Abstract The degree of aggregation of red blood cells (RBC) in dextrans with various molecular weights and at different concentrations was quantified by direct microscopic observation, determination of erythrocyte sedimentation rate, and measurement of low-shear viscosity. With increasing molecular weights, dextran fractions became increasingly more effective in causing rouleaux formation. Electron microscopy showed that the adjacent cell surfaces in the rouleaux were parallel to each other with a rather uniform intercellular distance, which varied in the same direction as the molecular size of the dextran used. The results have led to the postulation of a model of red cell aggregation by monolayer bridging of macromolecules. The stability of the aggregation depends upon the balance of the surface-bridging force (between the macromolecule and the cell) vs. the electrical repulsive force, the mechanical shear force, and the membrane-bending force. The larger molecular dextrans are more effective in inducing red cell aggregation because of (a) a weaker electrical repulsion due to the longer intercellular distance, and (b) a stronger bridging force due to the larger adsorption area on the cell surface.