Silver staining of Pneumocystis carinii in the rat's lung.

The backscattered electron imaging mode of the scanning electron microscope was used to study the ontogenetic acquisition of argyrophilia in Pneumocystis carinii in rats. Silver staining continually increased from the late trophozoite to the mature cyst stage. The silver uptake began with a fine outline at the surface of the bodies of the late trophozoites; their cellular extensions, however, did not stain. The oblate precyst forms acquired the silver in heterogeneous patches. On spherical cysts the silver staining became more uniform and intense with at least one dense spot. The spherical and collapsed cysts also had short silver staining projections that may represent microvilli. Collapsed forms were paler than spherical ones and appear to be cysts that have undergone partial or complete release of sporozoites. These cell surface observations confirm and amplify previous transmission electron microscopical and histochemical studies indicating that silver staining correlates with the acquisition of the cell pellicle.