Pneumocystis carinii pneumonitis

The cyst forms of Pneumocystis carinii in specimens stained with methenamine silver contain single or paired discrete foci of enhanced staining that measure 1-2 micron in maximum size. The nature of these foci and their location within the cysts have been disputed. We demonstrate by electron microscopy of silver-stained sections that the darkly stained foci correspond to a focal thickening of the cyst wall and are unrelated to sporozoites and other intracystic organelles. The morphology of these structures by light microscopy is characteristic, and their recognition is helpful in identifying P. carinii cysts and in differentiating them from yeast-form fungi and argyrophilic tissue elements in histologic sections and cytology specimens.