Fine-structural study of mitosis in the testacean Arcella vulgaris Ehrbg

The two nuclei of Arcella vulgaris divide synchronously by closed intranuclear orthomitosis during the formation and deployment of the thecagenous cytoplasmic bud. In prophase, the two nuclei are stacked one behind the other in front of the pseudostome, and markedly flattened, which brings the chromosomes near the equatorial plane. The nucleolus disperses, the nuclear lamina gradually disappears, and non-oriented microtubule bundles grow from intranuclear MTOCs. The condensing chromosomes already show attachment sites for one or more microtubules. In prometaphase, the nuclei are still stacked; the chromosomes further condense, the microtubules are oriented but short, leaving large polar regions of the nucleus filled with diffuse nucleolar material. At metaphase, the nucleus elongates to become spherical to barrelshaped; the spindle microtubules are longer but do not reach the nuclear envelope at the poles. In the polar regions, the nucleolar material aggregates into globular masses. The nuclear envelope remains continuous and devoid of lamina, but becomes sinuous. The kinetochores are improminent, associated with up to 6 microtubules. There are no nuclear pole bodies or extranuclear microtubules. At telophase, the nuclear envelope becomes very convoluted, suggesting that portions of the daughter nuclei bud off into the cytoplasm. Annulate lamellae occur inside the nuclei. The chromosomes decondense and the nuclear lamina is reformed, simultaneously with the appearance of new nucleoli which then fuse into one. Post-telophase nuclei undergo shuttle migrations with the cytoplasm between daughter cells.