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6203961 
Journal Article 
The fine structure of the nucleoli of normal and actinomycin D-treated Amoeba proteus 
Flickinger, CJ 
1968 
0 
Journal of Ultrastructure Research

ISSN: 0022-5320 
23 
3 
260-271 
The fine structure of the nucleoli of normal and actinomycin D-treated Amoeba proteus was studied utilizing fixation in Karnovsky's glutaraldehyde—formaldehyde mixture. The multiple nucleoli of normal cells had a predominately granular substructure. Although some regions appeared to possess a fibrillar component as well, discrete areas of differentiated substructure were not identified. Nucleoli of actinomycin-treated cells first became more dense and regularly spherical and then exhibited segregation of nucleolar components. The center of the nucleoli was composed of ∼ 50 fibrils and was surrounded by a rim of 100–150 dense granules. Peripheral regions contained low density material mainly of an amorphous character. In cells treated with actinomycin for 3–4 days, nucleolar vacuoles became common. The similarity in the response of the presumed nucleoli of amoebae to that reported for many higher cells is discussed in terms of possible structural and functional analogies between the nucleoli of amoebae and other cells. It is suggested that normal amoeba nucleoli possess fine fibrils and granules characteristic of other nucleoli, and that the unusually uniform fine structure commonly observed is due to the close packing and intermixing of these components.