Whole-cell-mount cytochemistry by the colloidal gold labeling method. Combined transmission and scanning electron microscopic study of ConA binding sites in mouse macrophages.

Binding, redistribution, and endocytosis of colloidal gold (CG)-labeled concanavalin A (ConA) were studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Mouse peritoneal macrophages were cultured on Formvar-coated platinum grids. Either fixed or unfixed cells were labeled by the indirect ConA-CG labeling method. Specimens were critical-point-dried and observed by TEM and SEM in the same region. Surface-bound ConA-CG was easily seen by SEM. Stereomicroscopic observation by TEM clearly showed the three-dimensional distribution of ConA on the cell surface as well as in the cytoplasmic vesicles and vacuoles. In the prefixed cells, CG was distributed randomly on the cell surface. When unfixed cells were labeled at 0 degree C, a similar binding pattern was observed, although the density of bound CG was decreased. When cells labeled with ConA-CG at 0 degree C were further incubated at 37 degrees C, redistribution and endocytosis of the label were seen. Endocytosed CG in the cytoplasmic vesicles and vacuoles was clearly seen by TEM. In addition, three-dimensional location and relationship with other organelles were easily observed. Combined TEM and SEM observation of CG-labeled whole-cell-mount specimens is a useful method to study the dynamics of cell-bound ligands.