Nucleolar activity and RNA metabolism in previtellogenic and vitellogenic oocytes of Xenopus laevis: A biochemical and autoradiographical, light and EM study

Abstract

Biochemical findings on isolated germinal vesicles and autoradiograms for the light and electron microscopes, after labelling with [ 3H]uridine, have given corroborating results for both previtellogenic and vitellogenic oocytes. Ribosomal RNA (rRNA) is synthesized throughout the previtellogenic period (in oocytes ranging from 10 to 300 μm in diameter), but the density of nucleolar labelling decreases progressively during this period. In the final previtellogenic stage (200 to 300 μm oocytes), it is extremely low and the nucleoli display fibrillar structure; the cytoplasm is almost devoid of ribosomes. Autoradiographical and biochemical methods have permitted quantitative estimations of labelled RNA in previtellogenic oocytes about 100 μm in diameter. Ribosomal (nucleolar) RNA represents only 5% of the labelled RNA which accumulates in the germinal vesicles after several hours of [ 3H]uridine incorporation. The situation is reversed when vitellogenesis begins (oocytes 400 μm in diameter): 30S rRNA makes up more than 40% of the total nuclear label. The many large nucleoli are densely labelled; the cytoplasm is crowded with ribosomes. The morphology of the nucleoli during the previtellogenic stages is consistent with the possibility of inhibition of rDNA transcription during previtellogenesis. This hypothesis is discussed.