It is unclear whether cell division is driven by cortical relaxation outside the equatorial region or cortical contractility within the developing furrow alone. To approach this question, a technique is required that can monitor spatially-resolved changes in cortical stiffness with good time resolution. We employed atomic force microscopy (AFM), in force-mapping mode, to track dynamic changes in the stiffness of the cortex of adherent cultured cells along a single scan-line during M phase, from metaphase to cytokinesis. Video microscopy, which we used to correlate the AFM data with mitotic events identified by light microscopy, indicated that the AFM force-mapping technique does not perturb dividing cells. Here we show that cortical stiffening occurs over the equatorial region about 160 seconds before any furrow appears, and that this stiffening markedly increases as the furrow starts. By contrast, polar relaxation of cells does not seem to be an obligatory event for cell division to occur.

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By applying an external electric field, we have studied the dc Stark shifts at the two states of a low-temperature single-molecule optical switch. We observe a reversible change from a mainly quadratic field dependence to a linear field dependence. The data analysis provides the S1−S0 dipole moment and polarizability differences of both the original and photoproduct state. On the basis of these data, a detailed microscopic scenario for the underlying nonphotochemical hole burning mechanism is discussed.