New aspects of the life cycle of Pneumocystis carinii

: Pneumocystis is recognized as a cause of pneumonitis in vertebrates, including man. So far an extra-cellular life cycle has been proposed; thick-walled cysts are the site of the formation of so-called intracystic bodies. These intracystic bodies are set free and are supposed to develop into free living thin-walled Pneumocysts ("trophic" stage) (Seifert and Pliess, 1960; Vavra and Kucera, 1970; Campbell, 1972). In this paper we present, based on elaborative ultrastructural studies, data on the existence of both intra-cellular and extra-cellular stages of the parasite which result in a new proposal for the life cycle of the parasite. This short communication will be followed by a more detailed description of our observations.