Intracellular elasticity and viscosity in the body, leading, and trailing regions of locomoting neutrophils.

To investigate the mechanisms underlying pseudopod protrusion in locomoting neutrophils, we measured the intracellular stiffness and viscosity in the leading region, main body, and trailing region from displacements of oscillating intracellular granules driven with an optical trap. Experiments were done in control conditions and after treatment with cytochalasin D or nocodazole. We found 1) in the body and trailing region, the granules divided into a "fixed" population (too stiff to measure) and a "free" population (easily oscillated; fixed fraction 65%, free fraction 35%). By contrast, the fixed fraction in the leading region was <5%. 2) In the body and trailing region, there was no difference in stiffness or viscosity, but both were sharply lower in the leading region (respectively, 20-fold and 5-fold). 3) Neither cytochalasin D nor nocodazole caused a decrease in stiffness, but both treatments markedly reduced the fixed fraction in the body and trailing region to <20% and <40%, respectively. These observations suggest a discrete lattice structure in the body and trailing region and suggest that the developing pseudopod has a core that is more fluidlike, in the sense of a much lower viscosity and an almost total loss of stiffness. This is consistent with the contraction/solation hypothesis of pseudopodial formation.