The ultrastructure of mitotic nuclei of Blastocrithidia triatomae

The fine structure of mitotic nuclei of the flagellate Blastocrithidia triatomae has been studied by serial thin sections and three-dimensional reconstructions. The sequence of changes during the four stages of mitosis are described. A set of three dense plaques is constantly found in the equatorial stage of mitosis. The microtubular spindle is organized around these plaques. The plaques split into halves at the end of the equatorial stage, and the half-plaques migrate to the spindle ends. Elongation of the mitotic nucleus occurs after the division of the plaques. This elongation is associated with the formation of an interpolar bundle of microtubules. The equatorial spindle is formed by 26-28 microtubules and is 1.5 micrometers long. The nucleolus attaches itself to the nuclear envelope and persists up to the elongational stage; then it disintegrates and is reorganized in daughter nuclei. Mitotic events in B. triatomae are essentially similar to those in Trypanosoma cruzi epimastigotes. As in this hemoflagellate, the dense plaques behave as kinetochores. It is concluded that B. triatomae is probably a haploid organism that contains three chromosomes.