[Introduction] Immunocytochemical labeling of cryosections, especially immunofluorescence microscopy using semi-thin (0.5-μm) cryosections, has been a powerful technique for detection of cellular antigens in situ and has been widely employed in cell and molecular biology studies. In many cases, immunofluorescence provides sufficient resolution and sensitivity to answer the question being addressed. However, in certain cases the increased resolution of the electron microscope using ultrathin (90-nm) cryosections may be required to define more precisely the localization of specific molecules. Recently, a unique fluorescent ultrasmall immunogold probe, FluoroNanogold (FNG), has been developed for use as a secondary antibody in immunocytochemical applications. It consists of a Fab' fragment of an antibody to which a 1.4-nm gold particle and fluorochromes are conjugated. FNG permits correlative microscopic observation of a sample stained in a single labeling procedure by multiple optical imaging. Recently, we have shown FNG immunocytochemistry on ultrathin cryosections to be valuable for high-resolution correlation of immunofluorescence and immunoelectron microscopy. In the present study, we have examined the utility of FNG as a secondary antibody for immunolabeling of myeloperoxidase (a marker protein for the azurophillic granules) in ultrathin cryosectioned human neutrophils.

[Materials and Methods] Purified human neutrophils were fixed with paraformaldehyde, embedded in gelatin, infiltrated with sucrose, cut as ultrathin cryosections, and then collected on formvar film-coated nickel EM grids as described previously. Grids containing ultrathin cryosections were incubated with antimyeloperoxidase and then incubated with FNG.