DNA localization in the nucleoli of sea urchin Paracentrotus lividus oocytes

It is known that in oocytes of P. lividus the nucleus contains a single giant nucleolus of unusual structure where intensive rRNA synthesis occurs. However, the questions of structural and functional relationships between the nucleolar compartments and the sites of active rRNA transcription still remain open. In the present work, we studied the chromatin organization in the nucleoli of P. lividus oocytes using Feulgen's and osmium amine staining procedures. Our results indicate that nucleolar chromatin of small (immature) oocytes differs from that in large mature oocytes. At the early stage of development, the DNA filamentous network 0.2-0.5 microm in diameter is formed in the nucleoli. Profound changes in the structure of the nucleolar DNA are observed in the course of oogenesis. Thus, at the late stage of development, the nucleolar chromatin forms characteristic ring-like structures, which indicates a non-uniform distribution of active ribosomal genes. A model of structural organization of the P. lividus nucleolus is discussed.