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Fine structure and cytochemistry of the mitotic plaques of Trypanosoma cruzi and Crithidia fasciculata.

The mitotic plaques are double, electron-dense structures which are located at the equator of the nucleus during the equatorial (metaphase) stage of mitosis in Trypanosoma cruzi, Crithidia fasciculata and other trypanosomatids. Each part of the equatorial plaques separates from the other and becomes an hemiplaque at the beginning of nuclear elongation. Variations of size of the plaques in different species of Trypanosomatidae are restricted to a limited range (less than 30% of the average thickness). At least two different components are found in the plaques with cytochemical methods: a) a basic protein with a high affinity for ethanolic-phosphotungstic acid, which is located in a narrow band towards the cleavage plane in each hemiplaque; and b) an osmiophilic component (possibly a protein) with a low affinity for uranyl acetate and which is located throughout the body of the plaque. The affinity for uranyl acetate can be abolished by methylation and acetylation, but remains after extraction with cold perchloric acid. No cytochemical evidence for the presence of DNA in the plaques is found. However, electron microscopy and cytochemical observations show that the PTA-affine band of the plaques is associated at its sides with chromatin fibers. Thus plaques, as the outer layer of kinetochores in higher eukaryots, have a component with high affinity for phosphotungstic acid, strengthening the hypothesis that these structures are phylogenetically related.