Developmental biology of Pneumocystis carinii, and alternative view on the life cycle of the parasite.

In this paper we present, based on elaborate ultrastructural studies, data on the existence of both intracellular and extracellular stages of Pneumocystis carinii, which result in a proposal of a new life cycle of the parasite. Up to now the formation of daughter cells in thick-walled pneumocysts is supposed to be the only way of multiplication. The present study shows that in rats treated with cortisone acetate the formation of daughter cells also takes place within thin-walled pneumocysts. In our opinion this way of multiplication is important for the understanding of the rapid increase in number of the parasites in an infected lung. The presence of pneumocysts inside the alveolar epithelial cells suggests that intracellular development of the parasites can occur, but the method of cell penetration, intracellular multiplication and parasite liberation is still unknown. Moreover our observations for the first time indicate a direct pathogenicity of the parasites in host cells.