Fine structure of nucleoli in the ciliate Didinium nasutum

Summary The macronuclear nucleoli of vegetative non-dividing cells of Didinium nasutum display inverted position of the main parts: the granular component is inside and the dense fibrillar one, in form of discrete bands, is mainly at the periphery. Before binary fission, the nucleoli are degranulated and their fibrillar bands scatter throughout the macronucleus, to be segregated during division between the daughter cells, where they begin to re-form the granular parts. In young resting cysts small nucleoli consist of granular material only, larger nucleoli show a clear segregation of its fibrillar and granular elements. During conjugation, nucleoli of the old macronucleus or of its fragments become segregated into granular and fibrillar parts and the latter are largely eliminated. The nucleoli lose contact with the chromatin bodies. In the developing anlagen of the new macronuclei the first nucleoli appear as fibrous bodies and only later develop granular parts; simultaneously the number of nucleoli increases.