Ultrastructural changes of the carp ( Cyprinus carpio ) hepatocyte nucleolus during seasonal acclimatization

Background information . The eurythermal fish carp ( Cyprinus carpio ) adjusts to the seasonal changes in the temperature and photoperiod of its habitat through diverse cellular and molecular mechanisms. We have observed that ribosomal biogenesis is modulated during the acclimatization process and correlates with profound phenotypic changes, reflecting a seasonal‐dependent ultrastructural appearance of the nucleolar components. Previous studies using classical techniques showed that in winter‐adapted carp the nucleolus appears to be segregated. In the present work, we have reassessed the nucleolar ultrastructural organization of the carp in summer‐ and winter‐adapted fish by using more specific cytochemical and immunocytological techniques. Results . The acetylation method provided evidence that the nucleolar organization is different between winter‐ and summer‐adapted carp. In winter‐adapted fish the fibrillar component appears as a unique mass surrounded by several granular caps, whereas in summer‐adapted carp the fibrillar component forms few cordons surrounded by granular masses. The nucleolar structure and distribution of the condensed chromatin observed varies upon seasonal acclimatization. In winter the nucleolar chromatin is densely packed in masses that surround the nucleolus, whereas during summer it displays a rather looser organization formed by filaments that not only surround the nucleolus, but also go through the nucleolar body. Using the TdT (terminal deoxynucleotidyl transferase)‐immunogold labelling technique, we detected condensed and decondensed nucleolar chromatin, and found some labelling of fibrillar components in both seasons. When liver tissue from summer‐adapted carp was treated with AMD (actinomycin D), we observed that the rearrangement of the nucleolar components and condensed chromatin were similar to that found in winter‐adapted fish, with differences in the distribution of the perinucleolar chromatin. Conclusions . The acetylation and TdT—immunogold labelling experiments indicated that the rearrangement of the nucleolar components of winter‐adapted carp is very similar to the AMD‐treated summer‐adapted carp nucleolus, with the latter representing the repression of the ribosomal biogenesis that occurs during the cold season. Nevertheless, the distribution of the condensed perinucleolar chromatin in winter‐adapted carp compared with AMD‐treated cells suggests that the transcription of rRNA genes in winter‐adapted fish is less strongly inhibited and does not lead to the classical segregation of the nucleolus of that described after AMD treatment. In addition, we have confirmed that carp hepatocyte nucleoli comprise only two main structural compartments: a fibrillar component and a granular component. Fibrillar centres were not observed.