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 of normal and partially hepatectomized rats, to allow their ultrastructure and transcriptional activity to be precisely correlated. Transcriptional activity was estimated from the intensity of incorporation of [14C]-UTP during periods of maximal RNA-polymerase I activity. RNA synthesis in hepatocyte nucleoli was maximal at 6 and 22 h after partial hepatectomy. The changes in transcriptional activity coincided with changes in nucleolar ultrastructure. Pseudonucleolonemal nucleoli, in which the prominent nucleolonemal network has a large dense fibrillar component and a small granular part, were first seen 6 h after the operation and showed the highest transcriptional activity. After 22 h, a second peak of RNA synthesis was recorded. Nucleolar size had almost doubled, and most hepatocytes showed hypertrophy of the granular component, indicative of intensified pre-rRNA processing.