The interpretation of interference-reflection images of spread cells: significant contributions from thin peripheral cytoplasm.

In interference-reflection microscopy, used for investigating cell-substratum separation, it is commonly believed that cytoplasmic thickness can be ignored, provided a high illuminating numerical aperture (INA) is used. It is shown here that even when a maximal INA is used, cytoplasmic lamellae of I micrometer or less can be major determinants of the image. The leading lamella of spreading tissue cells and large peripheral areas of Dictyostelium discoideum amoebae on adhesive substrata are less than I micrometer thick and it is argued that hitherto unexplained features of the interference images of these cells may be interpreted in terms of the theory used here.