Imaging of cell/substrate contacts of living cells with surface plasmon resonance microscopy

We have developed a new method for observing cell/substrate contacts of liv ing cells in culture based on the optical excitation of surface plasmons. S urface plasmons are quanta of an electromagnetic wave that travel along the interface between a metal and a dielectric layer. The evanescent field ass ociated with this excitation decays exponentially perpendicular to the inte rface, on the order of some hundreds of nanometers. Cells were cultured on an aluminum-coated glass prism and illuminated from below with a laser beam . Because the cells interfere with the evanescent field, the intensity of t he reflected light, which is projected onto a camera chip, correlates with the cell/substrate distance. Contacts between the cell membrane and the sub strate can thus be visualized at high contrast with a vertical resolution i n the nanometer range. The lateral resolution along the propagation directi on of surface plasmons is given by their lateral momentum, whereas perpendi cular to it, the resolution is determined by the optical diffraction limit. For quantitative analysis of cell/substrate distances, cells were imaged a t various angles of incidence to obtain locally resolved resonance curves. By comparing our experimental data with theoretical surface plasmon curves we obtained a cell/substrate distance of 160 +/- 10 nm for most parts of th e cells. Peripheral lamellipodia, in contrast, formed contacts with a cell substrate/distance of 25 +/- 10 nm.