Ultrastructure of Pneumocystis carinii: a review of internal and surface characteristics.

Electron microscopy (EM) shows unique characteristics of Pneumocystis carinii (PC) that are impossible to appreciate by light microscopy. EM's major contribution is in its elucidation of the PC life cycle, the host-parasite interaction of PC, and less so, of PC's taxonomy. Difficulty with EM stems from the propensity of PC to undergo degeneration and from the dearth of studies of well-preserved organisms. Ultrastructurally, the foamy alveolar cast (FAC) is composed of intimately associated cysts and trophozoites and contains little or no fibrin. By transmission EM (TEM), PC conglomerates are held together by slender membroanotubular extensions (MTE) growing from their concave and convex surfaces. Voids, formed by the juxtaposition of the uneven contours of the organisms and their intertwined MTE, contribute significantly to the FAC. The interior of the organisms lacks a complex organelle system typical of protozoa, but may contain intracystic bodies. Neither the MTE nor thicker processes of the cyst wall shows microtubules or any other cytoskeletal element. Trophozoites are even less rigid and adapt closely to the surface of type I alveolar epithelial cells. By scanning EM (SEM), gnome's hat-shaped organisms with beaded borders correspond to the crescent-shaped cysts noted by TEM. Slender processes are noted in rough surfaces of the organisms, whereas smooth surfaces display occasional budding. Elongated filopodia act as interparasitic fibrils but contribute little to contact with host cells. Together with argyrophilia and the Pap-stained fluorescence by UV light, the ultrastructure of PC suggests the organism is a fungus. This is in agreement with results of recent ribosomal RNA sequence studies that show PC to be related to sacharomyces.