ULTRASTRUCTURE OF MACRONUCLEUS AND DISTRIBUTION OF RNA-CONTAINING STRUCTURES IN RESTING CYSTS OF A CILIATE BURSARIA-TRUNCATELLA

Ultrastructure of macro- and micronucleus, macronuclear chromatin, and nucleolar apparatus was studied in resting cysts and vegetative cells of B. truncatella. Macronuclear chromatin forms electron-dense clumps (CC). The CC size is twice larger in cysts than in vegetative B. trun catella cells. Regions of aggregated larger CC and dispersed smaller C C are seen within the cyst macronucleus. Aggregated CC often form stru ctures resembling chromonemae of higher eukaryotic chromosomes. The st ructural organization of CC at the cyst stage is different from that k nown for other stages of the cell cycle. The changes in nucleolar morp hology at the cyst stage are analyzed. The distribution of RNA-contain ing structures in cysts is described and the processes of segregation and extrusion of some nucleoli into the cytoplasm are evaluated cytoch emically using RNase A colloidal gold complexes.