Application of atomic force microscopy to ultrastructural and histochemical studies of fixed and embedded cells

Various methods of sample preparation were used to study the cellular ultrastructure with an atomic force microscope (AFM). A rather satisfactory fine structure of cellular cross section, that was comparable to low to medium magnification electron micrographs, was obtained when ultrathin sections of epoxy embedded materials cut with a diamond knife were examined by use of a non-contact mode. No surface treatment was necessary to image the surface contour of ultrathin sections. According to this method, cellular membranes appeared as an elevation in contrast to depression of the intercellular space and cytoplasm. The chromatin and nucleolus showed an elevated image against the depression of nucleoplasm, resulting in the formation of images comparable to those obtained by conventional transmission electron microscopy. In the section treated for post-embedding immunohistochemical reaction, colloidal gold particles larger than 10-15 nm in diameter could be detected as elevations. By means of Kelvin force probe microscopy (KFM), enhancement of the surface potential was detected at exactly the same location as topographically detected gold particles. Application of KFM is expected to become a valuable tool for novel histochemistry with the use of AFM.