Ultrastructural studies on mitosis in Trypanosoma danilewskyi (Mastigophora: Zoomastigophorea)

Mitosis of Trypanosoma danilewskyi was studied using transmission electron microscopy. The basal body replicates prior to the kinetoplast. Nuclear division is coincident with the first part of cytokinesis. The kinetoplast DNA appears to disaggregate and then recondense as two strands. In the early phase of mitosis the chromatin and nucleolar material also disaggregate, but the nuclear membranes persist throughout mitosis. Electron-dense plaques, possibly kinetochores, were observed on the spindle microtubules in the polar regions of dividing nuclei. Additional electron-dense material was observed on the inner nuclear membranes adjacent to the plaques and microtubules but without connection to these latter structures. Daughter nuclei migrate but remain temporarily connected by a bridge composed of spindle microtubules enclosed by both nuclear membranes. During the final stages of karyokinesis the inner nuclear membrane reorganizes to exclude the microtubules of the bridge from the nucleoplasm. The nucleolus and chromatin reform, with the nucleolus being temporarily connected to the inner nuclear membrane by a chromatin strand. Cytokinesis is accomplished by transverse symmetrogenic fission, and the daughter cells are temporarily connected by a cytoplasmic isthmus.