Real time observations of polylysine, dextran and polyethylene glycol induced mutual adhesion of erythrocytes held in suspension in an ultrasonic standing wave field.

A technique which enables cells to be observed in suspension for times of the order of minutes (employing acoustic radiation forces in a 1 MHz ultrasonic standing wave field) is described. Video recordings of the mutual adhesion of human erythrocytes in suspension have been analysed. Concave-ended cell doublets and linear rouleaux developed in 0.5-1.5% w/v Dextran T500 by a gradual (2.5-17 s) increase in the area of cell contact over the cell cross-section. The concave-ended rouleaux form was not seen in polylysine or in polyethylene glycol. In 5-7% dextran and in 20 micrograms/ml polylysine mutual adhesion was a two stage process. Cells first form a strong local contact which persists (without apparently growing in area) for a number of seconds following which the cell surfaces move suddenly to form a spherical doublet. The average initial contact time and engulfment time for cells in 7% Dextran T500 are 18 and 2.7s, respectively. The corresponding values for cells in 20 micrograms/ml, 14 kDa, polylysine are 2.7 and 0.3s. There was no initial contact delay during spherical doublet formation in 1 mg/ml polylysine. Electron microscopy showed that the intercellular seam for spherical doublets formed with all three agglutinating molecules was bent in a wavy lambda approximately equal to 4 micron) profile. The thickness of the intercellular space varied in a spatially periodic way (lambda approximately equal to 0.8 microns) for cells in polylysine. Examples of periodic intercellular spaces were seen by light microscopy in polyethylene glycol induced clumps. The role of interfacial instability in the adhesion processes is discussed.