Extrachromosomal Nucleolar Apparatus in the Macronucleus of the Ciliate Paramecium putrinum: LM, EM and Confocal Microscopy Studies

The extrachromosomal nucleolar apparatus in the macronucleus (Ma) of the ciliate Paramecium putrinum was studied by LM cytochemistry, confocal microscopy and EM. The nucleoli vary in size and number. Three main types of nucleoli can be distinguished: small compact ones, vacuolated and large “composite” nucleoli (CN). The CN are often flattened at the Ma surface, but no blebbing or nucleolar extrusion was observed with EM. The CN are composed of several “nucleolar units” represented by a ring of dense fibro-granular material with an electron dense body in the centre. The nucleolar units are embedded in loose fibro-granular material. Ag-NOR staining and EM data provide evidence for the location of nucleolus organizer regions (NORs) at the CN periphery in the perinucleolar chromatin. Ag-NOR staining reveals a network, connecting extrachromosomal NORs with each other and possibly presenting a skeleton for continuous strands of nucleolar material observed by confocal microscopy during certain stages of cell cycle. “Free” NORs are scattered through the Ma.