TRANSCRIPTIONAL ACTIVITY AND ULTRASTRUCTURE OF MORPHOLOGICALLY DIFFERENT TYPES OF NUCLEOLI ISOLATED FROM HEPATOCYTES OF NORMAL AND HEPATECTOMIZED RATS

Morphologically different types of nucleoli were isolated from liver o f normal and partially hepatectomized rats, to allow their ultrastruct ure and transcriptional activity to be precisely correlated. Transcrip tional activity was estimated from the intensity of incorporation of [ C-14]-UTP during periods of maximal RNA-polymerase I activity. RNA syn thesis in hepatocyte nucleoli was maximal at 6 and 22 h after partial hepatectomy. The changes in transcriptional activity coincided with ch anges in nucleolar ultrastructure. Pseudonucleolonemal nucleoli, in wh ich the prominent nucleolonemal network has a large dense fibrillar co mponent and a small granular part, were first seen 6 h after the opera tion and showed the highest transcriptional activity. After 22 h, a se cond peak of RNA synthesis was recorded. Nucleolar size had almost dou bled, and most hepatocytes showed hypertrophy of the granular componen t, indicative of intensified pre-rRNA processing.