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

Abstract

Evidence for meiosis was demonstrated by EM for the 1st time in P. 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 tannicacid as a fixative, spindle microtubles were also observed for the 1st 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 8 intracystic bodies are produced through meiotic reduction. The most characteristic feature of the precyst is a clump of mitochondria in the cytoplasm. The precyst phase was divided into 3 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 2 haploid nuclei and each of these divides at meiosis II producing 4 nuclei. After that, another postmeiotic mitosis takes place, resulting in 8 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. When the 8 intracystic bodies are complete, the precyst changes to a cyst. Intracystic bodies resulting from meiotic nuclear division are haploid and, after excystation, they are haploid trophozoites. This process may be called sporogony. Although the haploid and the diploid trophozoite could not be distingused, it is quite plausible that copulation occurs, probably in host alveoli.