VISCOSITY OF PASSIVE HUMAN NEUTROPHILS UNDERGOING SMALL DEFORMATIONS

At issue is the type of constitutive equation that can be used to desc ribe all possible types of deformation of the neutrophil. Here a neutr ophil undergoing small deformations is studied by aspirating it into a glass pipet with a diameter that is only slightly smaller than the di ameter of the spherically shaped cell. After being held in the pipet f or at least seven seconds, the cell is rapidly expelled and allowed to recover its undeformed, spherical shape. The recovery takes approxima tely 15 s. An analysis of the recovery process that treats the cell as a simple Newtonian liquid drop with a constant cortical (surface) ten sion gives a value of 3.3 X 10(-5) cm/s for the ratio of the cortical tension to cytoplasmic viscosity. This value is about twice as large a s a previously published value obtained with the same model from studi es of large deformations of neutrophils. This discrepancy indicates th at the cytoplasmic viscosity decreases as the amount of deformation de creases. An extrapolated value for the cytoplasmic viscosity at zero d eformation is approximately 600 poise when a value for the cortical te nsion of 0.024 dyn/cm is assumed. Clearly the neutrophil does not beha ve like a simple Newtonian liquid drop in that small deformations are inherently different from large deformations. More complex models cons isting either of two or more fluids or multiple shells must be develop ed. The complex structure inside the neutrophil is shown in scanning e lectron micrographs of osmotically burst cells and cells whose membran e has been dissolved away.