ROLE OF THE MEMBRANE CORTEX IN NEUTROPHIL DEFORMATION IN SMALL PIPETTES

The simplest model for a neutrophil in its ''passive'' state views the cell as consisting of a liquid-like cytoplasmic region surrounded by a membrane. The cell surface is in a state of isotropic contraction, w hich causes the cell to assume a spherical shape. This contraction is characterized by the cortical tension. The cortical tension shows a we ak area dilation dependence, and it determines the elastic properties of the cell for small curvature deformations. At high curvature deform ations in small pipets (with internal radii less than 1 mu m), the mea sured critical suction pressure for cell flow into the pipet is larger than its estimate from the law of Laplace. A model is proposed where the region consisting of the cytoplasm membrane and the underlying cor tex (having a finite thickness) is introduced at the cell surface. The mechanical properties of this region are characterized by the apparen t cortical tension (defined as a free contraction energy per unit area ) and the apparent bending modulus (introduced as a bending free energ y per unit area) of its middle plane. The model predicts that for smal l curvature deformations (in pipets having radii larger than 1.2 mu m) the role of the cortical thickness and the resistance for bending of the membrane-cortex complex is negligible. For high curvature deformat ions, they lead to elevated suction pressures above the values predict ed from the law of Laplace. The existence of elevated suction pressure s for pipets with radii from 1 mu m down to 0.24 mu m is found experim entally. The measured excess suction pressures cannot be explained onl y by the modified law of Laplace (for a cortex with finite thickness a nd negligible bending resistance), because it predicts unacceptable hi gh cortical thicknesses (from 0.3 to 0.7 mu m). It is concluded that t he membrane-cortex complex has an apparent bending modulus from 1 x 10 (-18) to 2 x 10(-18) J for a cortex with a thickness from 0.1 mu m dow n to values much smaller than the radius of the smallest pipet (0.24 m u m) used in this study.