Nucleoli undergo structural and molecular modifications during hibernation

The nucleolus is a very dynamic structure able rapidly to adapt its activit y to the cellular metabolic state. An interesting physiological model chara cterized by drastic modifications of cellular metabolism is represented by hibernating animals. In the present study we investigated the hepatocyte nu clei of euthermic and hibernating edible dormice (Glis glis) with the aim o f revealing, by means of ultrastructural and immunocytochemical analyses, p ossible modifications of nucleolar components during hibernation. Our obser vations demonstrate that, in deep hibernation, nucleoli undergo structural and molecular modifications: (a) they show numerous nucleoplasmic invaginat ions and clumps of dense fibrillar component extend from the nucleolar surf ace; (b) they are frequently in contact with coiled bodies and fibro-granul ar material, two nuclear bodies usually occurring in the nucleoplasm; (c) t he dense fibrillar component contains significant amounts of small nuclear ribonucleoproteins, splicing factors usually distributed in the nucleoplasm . Taken together, these results suggest that during hibernation complex rel ationships are established between the nucleolus and nucleoplasm, probably related to functional activities peculiar to this physiological phase. Howe ver, since no evident nucleolar modification was found in early hibernating dormice, it seems Likely that the particular structural and molecular arra ngement of nucleoli establishes progressively during hibernation, becoming evident only in the deepest phase, and then disappears upon arousal.