Measuring optical and mechanical properties of a living cell with defocusing microscopy

Defocusing microscopy (DM) is a recently developed technique that allows quantitative analysis of membrane surface dynamics of living cells using a simple bright-field optical microscope. According to DM, the contrast of defocused images is proportional to cell surface curvature. Although, until now, this technique was used mainly to determine size and amount of membrane shape fluctuations, such as ruffles and small random membrane fluctuations, in macrophages, its applications on cell biology extend beyond that. We show how DM can be used to measure optical and mechanical properties of a living macrophage, such as cell refractive index n, membrane bending modulus K-c, and effective cell viscosity eta for membrane-actin meshwork relaxation. Experimental data collected from defocused images of bone marrow-derived macrophages were used to evaluate these parameters. The obtained values, averaged over several different macrophages, are n =(1.384 +/- 0.015), K-c approximate to 3.2 +/- 10(-19) J, and eta approximate to 459 Pa . s. We also estimate the amplitude of the small fluctuations to be of the order of 3 nm, which is around the step size of a polymerizing actin filament.