DNA localization in nucleoli of the sea urchin Paracentrotus lividus oocytes

IL is known that, in oocytes of P. lividus, nucleolus contains a single gia nt nucleolus of unusual structure, where intensive rRNA synthesis occurs. H owever, the questions of structural and functional relationships between th e nucleolar compartments anti the sites of active rDNA transcription still remain open. In the present work, we studied the chromatin organization in the nucleoli of P. lividus oocytes using Feulgen`s and amine osmium stainin g procedures. Our results indicate that nucleolar chromatin of small (immat ure) oocytes differs from that in large mature oocyte. At early stage of de velopment, the DNA filamentous network 0.2-0.5 mu m in diameter is formed i n the nucleoli. Profound changes in the structure of the nucleolar DNA are observed in the course of oogenesis. Thus, nt late stage of development, th e nucleolar chromatin forms characteristic ring-like structures, which indi cates a nonuniform distribution of active ribosomal genes. A model of struc tural organization of the P. lividus nucleolus is discussed.