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 t he ciliate Paramecium putrinum was studied by LM cytochemistry, confoc al microscopy and EM. The nucleoli vary in size and number. Three main types of nucleoli can be distinguished: small compact ones, vacuolate d and large ''composite'' nucleoli (CN). The CN are often flattened at the Ma surface, but no blebbing dr nucleolar extrusion was observed w ith EM. The CN are composed of several ''nucleolar units'' represented by a ring of dense fibro-granular material with an electron dense bod y in the centre. The nucleolar units are embedded in loose fibro-granu lar material. Ag-NOR staining and EM data provide evidence for the loc ation of nucleolus organizer regions (NORs) at the CN periphery in the perinucleolar chromatin. Ag-NOR staining reveals a network, connectin g extrachromosomal NORs with each other and possibly presenting a skel eton for continuous strands of nucleolar material observed by confocal microscopy during certain stages of cell cycle. ''Free'' NORs are sca ttered through the Ma.