Quantitative ultrastructural analysis of fibrillar centers in the mouse: Correlation of their number and volume with nucleolar organizers-activity

Abstract

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.