Changes in HL-60 cell deformability during differentiation induced by DMSO.

We have investigated changes in cellular deformability during promyelocytic leukemic HL-60 cell maturation. HL-60 cells were induced to mature with 1.25% dimethyl sulfoxide. Cellular deformability was evaluated by single-cell micropipette aspiration at one day, four days and seven days after induction. HL-60 cells were found to decrease in size and increase in deformability with maturation. When tested under the same aspiration pressures (0.5-1.3 kPa), cytoplasmic viscosity was found to vary from 210 to 85 Pa.s for cells prior to induction; it varied from 85 to 40 Pa.s for cells seven days after induction. Further, cytoplasmic viscosity exhibits power-law dependence on shear rate, mu = mu c (gamma m/gamma c)-b, where mu is cytoplasmic viscosity, gamma m is mean shear rate during cell entry, mu c is the characteristic viscosity at the characteristic shear rate, gamma c, and b is a material coefficient. Cells of all maturities showed similar dependence on shear rate (b approximately 0.5), but the characteristic viscosity decreased with maturation except for Day 1. When gamma c was set to 1 s-1, mu c = 236 +/- 5 Pa.s for cells prior to induction, mu c = 239 +/- 7, 209 +/- 7 and 175 +/- 14 Pa.s for cells on Days 1, 4 and 7 of induction, respectively.