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PubMed1989

U3, U8 and U13 comprise a new class of mammalian snRNPs localized in the cell nucleolus.

Using anti-(U3)RNP autoantibodies, we have isolated and characterized two additional small nucleolar RNAs from HeLa cells, which are less abundant than U3 RNA. Both RNAs possess a trimethylguanosine cap as judged by precipitation with anti-TMG antibody, but are not precipitated by either anti-Sm or anti-La antibodies. In addition, both RNAs are not precipitable by anti-Th serum, which recognizes another nucleolar RNP autoantigen. Sequence analysis revealed that one of these RNAs, 136 nucleotides long, is the human U8 homolog; while the other, 105 nucleotides long, represents a novel species which we designate U13. Both RNAs share with U3 two conserved sequences (boxes C and D). The role of one or both of these boxes in binding the common 34 kd antigenic protein, otherwise known as fibrillarin, is discussed. Fractionation of HeLa cells revealed that U8 and U13, like U3, reside in the nucleolus. In glycerol gradients both RNAs cosediment with larger structures possibly representing ribosomal precursors. We propose that U3, U8 and U13 comprise a new subset of mammalian snRNPs whose roles in ribosome biogenesis are discussed.

Animals↗
PubMed2002

Combination of electron microscopic in situ hybridization and anti-DNA antibody labelling reveals a peculiar arrangement of ribosomal DNA in the fibrillar centres of the plant cell nucleolus.

The fibrillar centres (FCs) in the nucleoli of Allium cepa usually contained compact dense chromatin, which was always surrounded with light fibrous material (LFM). Distribution of 18S ribosomal DNA (rDNA) in the FCs was examined by in situ hybridization at the light and electron microscopic levels and the results were compared with those obtained by immunogold labelling with anti-DNA antibodies. Anti-DNA antibodies heavily labelled the dense chromatin of the FCs but scarcely labelled the LFM. However, electron microscopic in situ hybridization using the 18S rDNA probe showed that the label in the dense chromatin was extremely weak compared with that obtained by the anti-DNA antibody labelling: the specific label with anti-DNA antibodies of the dense chromatin was about 15 times as much as that of the LFM, whereas the specific label with in situ hybridization in the dense chromatin was only about 1.7 times higher than in the LFM. These results suggest that the rDNA encoding rRNA is preferentially released from the dense chromatin and that non-transcribed intergenic spacers remain in the dense chromatin as the anchoring sites of rDNA.

Antibodies↗
PubMed2003

In situ visualization of rDNA arrangement and its relationship with subnucleolar structural regions in Allium sativum cell nucleolus.

We used a DNA-specific staining technique to show the two states of DNA component distributed in the nucleolar region of Allium sativum cells. One state is the extended DNA fiber, and the other is the condensed DNA clump. In situ hybridization demonstrated that the extended DNA fiber was an rRNA gene. Anti-fibrillarin antibody immunolabeling revealed that these rRNA genes were located in the dense fibrillar component near the fibrillar center, including at the periphery of the fibrillar center. None was in the dense fibrillar component far away from the fibrillar center. The condensed DNA clump was located in the fibrillar center. Further observations showed that the rRNA genes in the nucleolus were all arranged around the fibrillar center and associated with the DNA clumps in the fibrillar center. Results of statistical analysis showed that the distribution region of rRNA genes occupied about one-third of the total dense fibrillar component region. Ag-NOR protein showed a similar distribution pattern to that of rDNA. Immunolabeling of an anti-RNA/DNA hybrid antibody demonstrated that the transcription sites of rRNA were located at the periphery of the fibrillar center and in the dense fibrillar component near the fibrillar center, and these sites were consistent with the location and arrangement of rDNA shown in situ. These results demonstrated that transcription of rRNA takes place around the fibrillar center and at the periphery, whereas the dense fibrillar component that was far away from fibrillar center was the non-transcription region. The DNA clumps within the fibrillar center were probably the anchoring sites for rDNA arrangement.

Antibodies↗
PubMed1988

Spontaneous in vitro neoplastic evolution of cultured Chinese hamster cells. Nucleolus organizing region activity.

Silver staining to demonstrate active nucleolus organizing regions (NORs) was performed at four different stages of the spontaneous tumorigenic progression in vitro of Chinese hamster WCHE/5 cells. The number of active NORs increased for fully transformed, highly tumorigenic, late passage cells. The increase of NOR material was due to additional NOR-bearing chromosomes or chromosome arms, i.e., trisomy 5, trisomy 8, and the marker chromosome i(3q). Intermediate stages of the neoplastic evolution showed changing patterns of NOR activity, but not an overall increase. We postulate that the increase of active rDNA enhances cell growth and provides undefined selective advantage, and that this supports our previous conclusion that selectable karyotype changes provide competitive advantages rather than being essential for neoplastic evolution in vitro.

Animals↗
PubMed1980

The critical role of the nucleolus in cell differentiation and stem cell development - an extension of the concept with a look at the mast cell and its possible role in zinc metabolism.

From the assumptions governing the behaviour of primitive cells, a set of corollaries is established and the corollaries are used in coming to an understanding of the growth and differentiation of the pancreas. A case is made out to show that there is a constant replacement of old acini by new throughout the life of the pancreas, and that new acini are derived from primitive cells (fixed reticulum cells) present in the stroma which envelopes the fine ducts. The part played by the mast cells in the process is discussed, and view put forward that circulating thymocytes are mast cell precursors, and that the function of the mast cell is to provide the primitive cells of the pancreas (and the primitive cells of certain other tissues) with Zn in an assimilatable form. The hypothesis is made that Zn can only be assimilated by primitive cells, and, that for organs other than the thymus, it is assimilated in the form of the histamine-Zn complex liberated by the mast cells. The fate of the mast cell is analysed, and the suggestion made that, following degranulation, the mast cell nucleus is transformed into a cell of the eosinophil series. The role of the basophil is also analysed, and the conclusion reached that basophils are mast cells whose development has been modified by plasma.

Basophils↗
PubMed1980

The critical role of the nucleolus in cell differentiation and stem cell development - the concept as it applies to erythropoiesis.

There is evidence to suggest that nucleated red blood cells (normoblasts) are derived from a fixed reticulum cell lining that marrow sinusoids. The reticulum cell is transformed into a nucleolated stem cell which gives rise to a cluster of normoblasts by a special process of clonal division. The problem, which is paramount to myeloid development, is what happens to the large numbers of nuclei which are extruded from the normoblasts during the freeing of the haemoglobinised plastid. They seem to disappear without a trace, and in order to explain this phenomenon the hypothesis is made that the extruded nuclei are transformed into cells of the neutrophil series, and evidence is presented to support this viewpoint.

Animals↗
PubMed1980

"The critical role of the nucleolus in cell differentiation and stem cell development - the concept as it applies to the malignant cell.".

Whereas growth prior to organ formation in the embryo is by replication of primitive cells, growth subsequent to organ formation is assumed to be due to replication of differentiated cells. The replication of both primitive cells and differentiated cells is said to conform to the classical mitotic cycle. But the cycle does not show how differentiation takes place, and to remedy this an alternative cycle incorporating clonal division is postulated. In order to explain malignant growth on the basis of the mitotic cycle, traditionalists have had to introduce the concept of dedifferentiation but the concept does not quite stand up to a critical analysis. Malignant change appears to be due to a nucleolar fault rather than a chromosomal one, and the behaviour of the malignant cell in adenocarcinoma of the lung confirms this. Smears made from the tumour mass indicate that the malignant stem cells are transformed into "specialised cells" by the same process of clonal division as already postulated for normal tissue growth. The presence of differentiating cells in adenocarcinoma greatly lessens the likelihood of dedifferentiation playing a part in malignancy. On the other hand, the presence of abnormal stem cells with bizarre nucleoli strengthens the case for malignancy being due primarily to a nucleolar fault.

Adenocarcinoma↗
PubMed1976

Cytochemical and ultrastructural studies of changes in the nucleus and nucleolus during cell differentiation in the root cortex of Haemanthus katharinae.

Dimensions,dry mass content, binding of [3H]actinomycin D ([3H]AMD), and [3H]uridine incorporation in the nuclei and nucleoli as well as their ultrastructure during growth and differentiation of the root cortex cells of Haemanthus katharinae were compared. The dry mass of the nuclei did not change, the condensed chromatin content was slightly augmented, and the ability of the nuclei to bind [3H]AMD became somewhat decreased, whereas a considerable reduction was observed in the [3H]uridine incorporation. The highest capability for the binding of [3H]AMD and the highest intensity of RNA synthesis were demonstrated by nuclei in the meristematic zone and during the most intensive cell growth. The size of the nucleoli, their dry mass, the amount of granular component, and the intensity of [3H]uridine incorporation underwent a steady decrease as the cells grew and differentiated. The progressing differentiation of the cells was accompanied by an increasing participation of arginine-rich histones in restraining the capacity for [3H]AMD binding.

Cell Differentiation↗
PubMed1988

Changes in nucleus, nucleolus and cell size accompanying somatic embryogenesis of Theobroma cacao L. I. Relationship between DNA and total protein content and size of nucleus, nucleolus and cell.

There was a linear relation between an increase in DNA content and size of nuclei, nucleoli and cells in callus and proembryos (Theobroma cacao L.). In callus the increase of DNA content was accompanied by proportional increase in nuclear size whereas in proembryos the increase in nuclear size did not match the increasing amount of DNA. The stimulation of embryogenesis by 10(-2) mg/l 2,4-D was associated with increase in nuclear and nucleolar size and with decrease in cell sizes. Inhibition of embryogenesis by 1.0 mg/l 2,4-D+10% coconut water did not change nuclear size, but increased cell size in relation to the control. The process of embryo formation was accompanied by changes in relationship between nuclear, nucleolar and cell size and the total (DNFB-stained) proteins content. In callus as well as in proembryo the increase in total protein content in nucleus was not equivalent to the increasing sizes of nuclei which leads to the decrease in nuclear protein concentration. Similar situation was observed for nucleoli. Differences were found in the concentration of cytoplasmic proteins between the callus and proembryo cells. The stimulation of embryogenesis by low concentration of 2,4-D resulted in decrease in concentration of total proteins in nuclei and nucleoli and the increase in cytoplasm.

Cacao↗
PubMed2005

The moving parts of the nucleolus.

The cell nucleolus is the subnuclear body in which ribosomal subunits are assembled, and it is also the location of several processes not related to ribosome biogenesis. Recent studies have revealed that nucleolar components move about in a variety of ways. One class of movement is associated with ribosome assembly, which is a vectorial process originating at the sites of transcription in the border region between the fibrillar center and the dense fibrillar component. The nascent preribosomal particles move outwardly to become the granular components where further maturation takes place. These particles continue their travel through the nucleoplasm for eventual export to the cytoplasm to become functional ribosomes. In a second kind of motion, many nucleolar components rapidly exchange with the nucleoplasm. Thirdly, nucleolar components engage in very complex movements when the nucleolus disassembles at the beginning of mitosis and then reassembles at the end of mitosis. Finally, many other cellular and viral macromolecules, which are not related to ribosome assembly, also pass through or are retained by the nucleolus. These are involved in nontraditional roles of the nucleolus, including regulation of tumor suppressor and oncogene activities, signal recognition particle assembly, modification of small RNAs, control of aging, and modulating telomerase function.

Animals↗
PubMed1988

Changes in nucleus, nucleolus and cell size accompanying somatic embryogenesis of Theobroma cacao L. II. Relation between basic protein content and size of nucleus, nucleolus and cell.

Embryo formation from callus of Theobroma cacao L. was associated with the changes in relationship between nuclear, nucleolar and cell sizes and the content of basic proteins (FG-FCF-stained). Together with the increase in nuclear size of callus and proembryo cells the increase in the amount of nuclear basic proteins was found. In the callus cells the increase in nucleolar protein content exceeded that in nucleolus size, which led to the rise in basic protein concentration in the nucleolus. However, in the early stage of embryogenesis the increase in protein content was not so marked as that in callus, which indicated that embryogenesis involved a decrease in concentration of nucleolar basic proteins. Differences between callus and proembryo cells were also observed in the concentration of cytoplasmic proteins. The increase in size of callus cells was the same as the increasing amount of cytoplasmic proteins. In proembryos a significant increase in cell size was accompanied by only slight changes in cytoplasmic proteins. The stimulation of embryogenesis by 2,4-D resulted in an increase of nuclear concentration of basic proteins in proembryos. The intensification of embryogenesis involved the decrease of the concentration of nucleolar proteins together with the increase in concentration of basic cytoplasmic proteins.

Cacao↗