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In mouse oocyte during meiotic prophase I, rRNA synthesis increases from midpachytene (onset of nucleologenesis) to the diplotene stage (both 4C DNA containing). During these stages the nucleoli undergone morphological changes from the ring-shaped structure to the nucleolonemal type. The results we obtained by 3-D reconstruction after serial sectionning of different nuclei during these stages, showed that there is an increase in the number of fibrillar centers (from 5-7 to 97-113) in correlation with the development of the nucleolonema. This result demonstrated that the fibrillar centers cannot be considered only as the interphasic counterparts of the nucleolar organizers in reticulated nucleoli. By a quantitative ultrastructural analysis of the respective volumes of fibrillar centers and other nucleolar components after serial sectionning we demonstrated that in oocyte nuclei and in mouse sertoli cell nuclei 1. there is a correlation between the number of fibrillar centers and their total volume, and 2. there is a relationship between the volume of fibrillar centers and the volume of the electronopaque fibrillar zone of the nucleolus, in which rDNA transcription occurs. This result, thus, demonstrated that the number and volume of the fibrillar centers are correlated, at least in reticulated nucleoli, with the activity of the nucleolar organizers. The more active the nucleolus, the more numerous the fibrillar centers and the larger their total volume. [ABSTRACT FROM AUTHOR]