Cell biomechanics can be both a direct consequence, and a regulating factor of the biological function and cellular architecture. Cells can generate and detect mechanical forces and respond to biochemical and physical cues by cytoskeletal re-organization leading to changes of a series of mechanical properties as: shape, volume, visco-elasticity, adhesion. Therefore, a lot of effort has been put in the last decade to develop new theoretical and experimental approaches which can provide new perspectives on the role of cell biomechanics in cell disease.
Here we report on a quantitative phase microscopy (QPM) approach to measure and monitor the cell volume and membrane elasticity. An interferogram (off axis hologram) obtained by the interference of a laser beam illuminating the sample/object and a reference beam, is first recorded on a digital camera. By using numerical algorithms, both the intensity and phase of the object beam are then reconstructed. The phase is a function of the optical path variations (cell height by the refractive index) and hence it provides 3D information of the cell. Note that in normal optical microscopy only the intensity is detected, while the phase information is lost, providing only 2D information of the cell.
We have built a custom inverted microscope to implement the technique described above. The laser beam in near IR to avoid damaging the cells. Microscope lenses of 40X, NA0.4 and 100X, NA1.2 can be used to provide good lateral resolution. We perform a time lapse imaging of the cell (5 min at 10 fps), reconstruct the phase and calculate the volume of the cell at each moment monitoring the changes of the volume of the cell, the top surface area of the cell, in two different situations: control and affecting the cytoskeleton by inhibitors of actin polymerization.
We present and discuss results obtained for neuroblastoma cell NG108. Moreover, using a fast camera, we could acquire interferograms at 500 – 1000 fps for 1 s, which is a good sampling to characterize the membrane fluctuations and derive the membrane elasticity. These results suggest the use of QPM as a strong tool to measure and characterize cell mechanics.