Changes of macronuclei structure during encystment in the ciliated protozoan Climacostomum virens

Summary Ciliate cysts are an important object for investigating changes in cell morphology during cryptobiosis. In this work, we studied the ultrastructure of Climacostomum virens resting cysts using electron and confocal microscopy. The main attention was paid to the morphological changes in the macronucleus, nucleoli and chromatin. Vegetative cells, cysts 1-2 h after cyst wall formation, 2-months old cysts, one-year old cysts, and cells after excystment were investigated. The process of encystment in C. virens , as in other ciliates, is accompanied by a decrease in cell volume and the formation of a cyst wall. The volume of the macronucleus reduces, too. This reduction is likely to be related to the compaction of the macronuclear chromatin since extrusion of chromatin and nucleolar material into the cytoplasm during C. virens encystment was not observed. The nucleoli in young and 2-months old cysts retained their interphase morphology with well-defined fibrillar and granular zones. Only at the age of one year, the nucleoli in the cyst reduced in size and consisted of a dense homogeneous material. The chromatin structure in the cyst micronuclei was morphologically identical to macronuclear chromatin at most other cell cycle stages. Macronuclear chromatin in vegetative C. virens was mainly represented by individual bodies, 100–200 nm in size. As the age of the cysts increased, the chromatin bodies formed extended fibrils. In one-year old cysts, the entire space of the macronucleus was filled with fibrils 100–200 nm thick. Such fibrils resemble chromonemes in the chromosomes of higher eukaryotes and probably are formed by similar cellular mechanisms