Sites of transcription of ribosomal RNA in HeLa cells were visualized by electron microscopy. Cells were either incubated with Br-uridine, or permeabilized and then incubated with BrUTP, before sites containing Br-RNA were immunolabeled with gold particles. Short incubations ensured that most incorporated analogue remained at synthetic sites. Fibrillar centres were unlabelled except at their periphery; label was concentrated over certain regions of the surrounding dense fibrillar component. These results suggest that the dense fibrillar component is the site of rRNA transcription. After dispersing the granular component and the dense fibrillar component by a hypotonic treatment, removal of most chromatin and preparation of resinless sections, fibrillar centres remained fixed to a nucleoskeleton. These structural and functional features are incorporated into a model for rRNA transcription.

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Site of transcription of ribosomal RNA and intranucleolar structure in HeLa cells

Does the Synthesis of Ribosomal RNA Take Place within Nucleolar Fibrillar Centers or Dense Fibrillar Components? A Critical Appraisal

Cytochemistry and immunocytochemistry of nucleolar chromatin in plants

In vivo nucleologenesis was studied in bovine embryos by electron microscopic immunogold labelling of DNA, RNA, protein C23 and protein B23. We have used the classification of Kopecny et al. (1989b) and Kopecny (1990) dividing nucleologenesis in four steps : compact nucleolar precursor body (NPB), monovacuolated NPB, NPB containing secondary vacuoles and fully reticulated nucleolus. These different features of early bovine embryo nucleologenesis were mainly observed during the eight-cell stage. In the first step of nucleolar development, the association of compact NPB with DNA structures was observed. DNA was also labelled in some small secondary vacuoles appearing during the third developmental step. From the second step onward, the labelling of protein C23 was observed in the compact fibrillar network of theNPB. Protein B23 started to be labelled in the compact fibrillar mass at the third step. RNA labelling was also observed for the first time in NPB containing secondary vacuoles. Labelled RNA was located in the peripheral region of compact fibrillar mass as well as along the border of the vacuoles. In the reticulated nucleolus, the dense fibrillar component was found to contain both proteins and RNA.

Nucleologenesis in the cleaving bovine embryo: immunocytochemical aspects.

Exceptionally high DNA contents were found in supramedullary neuron (SN) nuclei of the pufferfish Diodon holacanthus by quantitative microfluorimetric assay. This phenomenon has been explained by endoreplication, the functional significance of which is still unclear. In this view, the peptidergic nature and large dimensions make the teleostean clustered SN an interesting model for investigating the relationships between endoreplication, nuclear morphology and biosynthetic cellular activity. In this paper, we present a cytochemical and ultrastructural study on the SN of D. holacanthus (Tetraodontiformes). The nucleolar and nucleus structures suggest an intense production of ribosomal components in order to satisfy high cellular demands for protein synthesis. Accordingly, the cytoplasmic compartment presents an extensive rough endoplasmic reticulum, well-developed Golgi apparatus and a remarkable vesicular traffic. These features suggest that SN are engaged in an intense process of protein biosynthesis. The SN are completely surrounded by processes of different types of glial cells. The glial cells may be considered part of the SN cluster.

Ultrastructural and cytochemical features of the supramedullary neurons of the pufferfish Diodon holacanthus (L.) (Osteichthyes)

Nucleoli were noted by biologists very early (Fontana, 1781) and a lot of data has been accumulated on the structure of nucleoli in the last decades. Due to the work of Perry (1962; Perry et al., 1961) it is known that the nucleolus is the site of ribosome biogenesis. Its visibility both in the light-and the electron microscope is due to the fact that the genes for ribosomal RNA are not only transcribed but also processed and complexed with ribosomal proteins in nucleoli, rendering them morphologically different from the rest of the karyoplasm.

P. Hozak

Papers

Ribosomal genes and nucleolar morphology