A re-evaluation of the relationships between the fibrillar centres and the nucleolus-organizing regions in reticulated nucleoli: ultrastructural organization, number and distribution of the fibrillar centres in the nucleolus of the mouse sertoli cell

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Abstract
In mouse testis, the diploid Sertoli cell displays one large nucleolus flanked symmetrically by two heterochromatic masses. The hybridization in situ with [3H]rRNA confirmed that the ribosomal cistrons are localized within the central nucleolar mass. At the ultrastructural level this nucleolar mass appears to be reticulated and contains numerous fibrillar centres. These fibrillar centres are surrounded and interconnected by an electron-opaque fibrillar network, which constitutes the reticulated nucleolonema of the nucleolus. Ag-NOR staining reveals the presence of the argyrophilic proteins associated with active nucleolus-organizing regions (NORs) within both the fibrillar centres and the electron-opaque fibrillar component. Autoradiographic studies after [‘H]uridine incorporation show that ribosomal DNA transcription only takes place in this dense fibrillar component. Three-dimensional reconstruction of four Sertoli cell nucleoli after serial sectioning reveals that the size and number of the fibrillar centres are very variable from one cell to another (26, 35, 38 and 41 fibrillar centres). The analysis of the volume occupied by the fibrillar centres as compared to the whole nucleolar volume demonstrates that the larger the nucleolus, the more fibrillar centres it contains, but also the more numerous the fibrillar centres, the larger their total volume. While in each case the number of the NORs is virtually the same, i.e. ten. In the light of these results we concluded that, at least in reticulated nucleoli, there is no numerical relationship between the number of fibrillar centres and the number of NORs, and that the fibrillar centres cannot be considered only as the nucleolar counterparts of the NORs. Moreover, the increasing number of fibrillar centres from the smallest nucleolus to the largest one is difficult to explain by the previously postulated hypothesis of a reserve of inactive rDNA packaged in the fibrillar centres. These data led us to reconsider the role of the fibrillar centres in the transcriptional activity of reticulated nucleoli.