Abstract: Objective To study the life cycle and morphology of Pneumocystis carinii by ultrastructural observation. Methods Wistar rat model of P.carinii infection was established by subcutaneous injection with dexamethasone. Lung tissue of the infected rats was used for the transmission electron microscopical study. Results The organisms were mainly present in the lung alveolar cavity, and also in the alveolar septem, pulmonary macrophages and neutrophils. More trophozoites of P. carinii attached to the type Ⅰ alveolar epithelial cells, and rarely to the type Ⅱ alveolar epithelial cells. Most of these trophozoites showed pseudopodial evaginations on their pellicles. The nucleus-associated organelle and spindle microtubules were observed in some trophozoites. The precyst phase was in three forms: early, intermediate and late form. Synaptonemal complexes indicating meiotic nuclear divisions and a clump of mitochondria were also observed in the precyst. The pellicle of the cyst has a thickened portion with a pore. There were nucleus with nucleolus, mitochondrion, vesicles, endoplasmic reticulum and numerous ribosomes in the organisms, and tubular expansions on its surface. Conclusion The life cycle of P. carinii consists of trophozoite, precyst and cyst stages. The presence of a single pore in the cyst wall reveals that pore formation may be a mode of excystation for intracystic bodies of P. carinii.

Key words: Pneumocystis carinii, Ultrastructure, Transmission electron microscopy