Sporogony in Pneumocystis carinii: Synaptonemal Complexes and Meiotic Nuclear Divisions Observed in Precysts1

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Abstract
Evidence for meiosis was demonstrated electron microscopically for the first time in Pneumocystis carinii in rat alveoli by the observation of synaptonemal complexes followed by nuclear divisions. Synaptonemal complexes indicating meiotic nuclear divisions were observed in uninuclear precysts. Additionally, owing to the use of tannic acid as a fixative, spindle microtubules were also observed for the first time in the precyst. Based on these facts, a new life cycle of the organism is proposed. The precyst has generally been considered an intermediate form between the trophozoite and the cyst. The present paper proposes that the precyst is additionally defined as the cell in which eight intracystic bodies are produced through meiotic reduction. The most characteristic feature of the precyst is a clump of mitochondria in the cytoplasm. We divide the precyst phase into three forms, which are named early, intermediate, and late. Synaptonemal complexes were only observed in the early precyst, which is a uninuclear cell with a thin pellicle. In the intermediate precyst, nuclear divisions are observed as follows: meiosis I produces two haploid nuclei and each of these divides at meiosis II producing four nuclei. After that, another postmeiotic mitosis takes place, resulting in eight haploid nuclei. In the late precyst, a delimiting membrane originates from the mother plasmalemma and surrounds the daughter nuclei and a small portion of the adjacent cytoplasm. Finally, when the eight intracystic bodies are complete, the precyst changes to a cyst. Thus, we deduce that intracystic bodies resulting from meiotic nuclear division are haploid and, after excystation, they are haploid trophozoites. We consider that this process can be called sporogony. Although we could not distinguish between the haploid and the diploid trophozoite, it is quite plausible that copulation occurs, probably in host alveoli.