Immunogold labeling of ultrathin sections of cells remaining in the depleted bone marrow of the transgenic MIIb-tk mice after GCV treatment. Labeling was performed using rat MoAb to murine markers and bound IgG were shown by goat IgG coupled to gold particles. (a) A typical small Sca-1⁺ cell (4 to 7 μm diameter) with a high nucleus/cytoplasm ratio. Sca-1 labeling (arrowheads) is visible on the plasma membrane of the cell. (b) A CD34⁺cell labeled on the plasma membrane (arrowheads) also presents the morphological features of an immature cell. (c and d) Double-labeling for Sca-1 and CD45R. Sections were first incubated with MoAb directed against Sca-1 whose binding was shown by goat antibody to rat IgG coupled to 10-nm gold particles. After rapid fixation, sections were then incubated with MoAb directed against CD45R whose binding was shown by goat antibody to rat IgG coupled to 5-nm gold particles. An example of a Sca-1⁺ CD45R⁻ cell is illustrated in (c). Only 10-nm gold particles (arrowheads) showing the presence of the Sca-1 antigen are present on the plasma membrane of this cell. In (d) is shown an example of a Sca-1⁻ CD45⁺ cell present in the same preparation. The labeling is intense for CD45R and numerous 5-nm gold particles (arrowheads) are exclusively found associated with the plasma membrane of this cell. Bars = 0.5 μm.