High osmotic pressure enables fine ultrastructural and cytochemical studies on Pneumocystis carinii. I. Epon embedding.

High osmotic pressure was used to preserve the ultrastructure of rabbit-, SCID mouse-, and rat-derived Pneumocystis carinii organisms from osmotic stress during fixation. Organelles and cytosol were well preserved within the tonicity range of 850-1,300 mosmol. Under these experimental conditions, we determined that the endoplasmic reticulum was well developed in all parasite stages and could observe the Golgi complex, autophagic vacuoles, dense bodies, type II endoplasmic saccules, and the recently described outer surface membrane, which was found in all parasite stages. The biological implications of these findings are discussed.