Nuclear fine structure at interphase and during encystment in two forms of the testacean Arcella vulgaris

Arcella vulgaris and A. vulgaris var. multinucleata have two and seven vesicular nuclei, respectively. In early interphase, the nuclei are spherical, with a main central nucleolus and several small peripheral nucleoli. The main nucleolus has mixed fibrillar and granular components and no apparent chromatin bodies in the nucleolus-organizing regions (NORs). The nuclear chromatin is dispersed except for small heterochromatin lumps. Nuclear bodies of fibrous structure occur outside the main nucleolus. The nuclear envelope shows an internal lamina and an external layer of tangential fibres. In middle interphase, the nuclei become irregular in shape and adjacent to the plasma membrane at the dorsolateral cell surface. The condensation of the chromatin increases. The nucleolus is often eccentric and its NORs show conspicuous bodies of condensed chromatin surrounded by a halo and a fibrous transcription zone. By the end of interphase, the main nucleolus becomes polymorphic and segregated into a fibrillar basal part which contains a vacuolar area with dense inclusions, and a fibro-granular cap which contains many fibrous electron dense bodies. These are likely to be the nuclear chromatin elements. In early resting cysts, the nuclei detach from the cell membrane and approach one another. The main nucleolus segregates into large, mainly peripheral fibrillar blocks inside a granular mass. The NORs consist of condensed chromatin bodies without a transcription zone around them. The nuclear chromatin is condensed. Conspicuous intranuclear annulate lamellae bearing pore complexes occur only in cysts.