Photoelectron imaging is a novel way of imaging the distribution
of specific cell surface components. The basic principle is
the photoelectric effect, in which electrons are ejected from
the specimen by UV light. There are two primary forms of image
contrast in photoelectron microscopy: material contrast and
topographical contrast. Material contrast, provided by differences
in photoelectron quantum yields, makes it possible to select
labels that appear bright against the darker background of the
cell surface. Topographical contrast ensures that label
distribution can be interpreted in the context of cellular
structures. By varying the wavelength of the exciting light,
the contrast between label and cell surface can be controlled
(tuned) to produce images ranging from a primarily topographical
view of the surface to those showing only the label distribution.
Being an emission-based technique, photoelectron imaging shares
with fluorescence microscopy the ability to image label
distributions at low magnification. However, unlike fluorescence
microscopy the photoelectron image is formed by electrons, which
generates a greater level of attainable resolution. Label contrast
and wavelength-dependent tunability of contrast is illustrated
with images of silver-enhanced immunogold-labeled cell surfaces,
including selective labeling of cells in mixed-cell co-cultures,
the cell surface CD44 adhesion protein on human glioma cells,
fibronectin patterns on human fibroblasts, and the human
transferrin receptor on MCF-7 human breast carcinoma cells.