Cytological approach to the nucleolar functions detected by silver staining

Functional ultrastructure of the plant nucleolus

Nucleoli are nuclear domains present in almost all eukaryotic cells. They not only specialize in the production of ribosomal subunits but also play roles in many fundamental cellular activities. Concerning ribosome biosynthesis, particular stages of this process, i.e., ribosomal DNA transcription, primary RNA transcript processing, and ribosome assembly proceed in precisely defined nucleolar subdomains. Although eukaryotic nucleoli are conservative in respect of their main function, clear morphological differences between these structures can be noticed between individual kingdoms. In most cases, a plant nucleolus shows well-ordered structure in which four main ultrastructural components can be distinguished: fibrillar centers, dense fibrillar component, granular component, and nucleolar vacuoles. Nucleolar chromatin is an additional crucial structural component of this organelle. Nucleolonema, although it is not always an unequivocally distinguished nucleolar domain, has often been described as a well-grounded morphological element, especially of plant nucleoli. The ratios and morphology of particular subcompartments of a nucleolus can change depending on its metabolic activity which in turn is correlated with the physiological state of a cell, cell type, cell cycle phase, as well as with environmental influence. Precise attribution of functions to particular nucleolar subregions in the process of ribosome biosynthesis is now possible using various approaches. The presented description of plant nucleolar morphology summarizes previous knowledge regarding the function of nucleoli as well as of their particular subdomains not only in the course of ribosome biosynthesis.

The 5-methyl-deoxy-cytidine (5mdC) localization to reveal in situ the dynamics of DNA methylation chromatin pattern in a variety of plant organ and tissue cells during development

DNA methylation of cytosine residues constitutes a prominent epigenetic modification of the chromatin fiber which is locked in a transcriptionally inactive conformation leading to gene silencing. Plant developmental processes, as differentiation and proliferation, are accompanied by chromatin remodeling and epigenetic reprogramming. Despite the increasing knowledge gained on the epigenetic mechanisms controlling plant developmental processes, the knowledge of the DNA methylation regulation during relevant developmental programs in flowering plants, such as gametogenesis or embryogenesis, is very limited. The analysis of global DNA methylation levels has been frequently conducted by high performance capillary electrophoresis, and more recently also by ELISA-based assays, which provided quantitative data of whole organs and tissues. Nevertheless, to investigate the DNA methylation dynamics during plant development in different cell types of the same organ, the analysis of spatial and temporal pattern of nuclear distribution of 5-methyl-deoxy-cytidine (5mdC) constitutes a potent approach. In this work, immunolocalization of 5mdC on sections and subsequent confocal laser microscopy analysis have been applied for in situ cellular analysis of a variety of plant cells, tissues and organs with different characteristics, e.g. hardness, heterogeneity, cell accessibility, tissue compactness, etc.; the results demonstrated the versatility and feasibility of the approach for different plant samples, and revealed defined DNA methylation nuclear patterns associated with differentiation and proliferation events of various plant cell types and developmental programs. Quantification of 5mdC immunofluorescence intensity by image analysis software also permitted to estimate differences in global DNA methylation levels among different cells types of the same organ during development.

snRNP: Rich Nuclear Bodies in Hyacinthus orientalis L. Microspores and Developing Pollen Cells

The aim of the present work was the characterization of nuclear bodies in the microspore and developing pollen cells of Hyacinthus orientalis L.. The combination of Ag-NOR, immunofluorescence and immunogold techniques was used in this study. The obtained results showed the presence of highly agyrophylic extranucleolar bodies in microspore and developing pollen cells, which were finally identified as Cajal bodies. In all cases, a strong accumulation of snRNP-indicating molecules including TMG cap, Sm proteins and U2 snRNA, was observed in the examined nuclear bodies. In contrast to their number the size of the identified structures did not change significantly during pollen development. In the microspore and the vegetative cell of pollen grains CBs were more numerous than in the generative cell. At later stages of pollen development, a drastic decrease in CB number was observed and, just before anthesis, a complete lack of these structures was indicated in both pollen nuclei. On the basis of these results, as well as our previous studies, we postulate a strong relationship between Cajal body numbers and the levels of RNA synthesis and splicing machinery elements in microspore and developing pollen cells.

Nuclear localization of NORs and centromeres in mouse oocytes during folliculogenesis

Mouse oocytes at the germinal vesicle stage are characterized by one of two nuclear morphologies: surrounded nucleolus (SN), in which the nucleolus is surrounded by a rim of Hoechst positive chromatin and not surrounded nucleolus (NSN), in which this rim is essentially absent. This morphological difference has a biological relevance as NSN oocytes are transcriptionally active, yet incapable of development beyond the two-cell stage. Whereas SN oocytes, which are transcriptionally inactive, are capable of development to the blastocyst stage. To further our understanding of the nuclear organization of the mouse oocyte during folliculogenesis, we have conducted a series of investigations employing silver methods that stain nucleolus organizer region (NOR), centromeres, and heterochromatin, as well as, specific antibodies for centromeres. Results obtained by a variety of microscopic methods (light, electron, immunochemical, and confocal) demonstrate: (1) a changing pattern of NOR staining during folliculogenesis that is specific to follicular type, and (2) significant differences in the organization of NORs and centromeres of isolated, antral NSN, and SN oocytes. These observations suggest possible means by which, chromosomes of mature, germinal vesicle oocytes are organized with respect to the nucleolus.

Simultaneous production of triploid and haploid eggs by triploidSqualius alburnoides(Teleostei: Cyprinidae)

The hybrid minnow Squalius alburnoides comprises diploid and polyploid forms with altered modes of reproduction. In the present paper, we report a cross where a triploid female generated both large, triploid and small, haploid eggs simultaneously, which were fertilized with S. pyrenaicus sperm. Although the large eggs were rarer (15%), they originated offspring with higher survivorship, so that tetraploids were dominant among the surviving siblings. The cross yielded apparently all female progeny. Inheritance patterns were inferred using four microsatellite markers and NORs (Nucleolus Organizer Regions) phenotypes, and suggested that haploid eggs were probably produced by an atypical hybridogenesis, in which the elimination of the unmatched genome permitted random segregation and recombination between the homospecific genomes, while the triploid eggs were clonal. The present results suggest that the occurrence of triploid unreduced eggs may be a new route for the natural tetraploidization in the complex.

Activation of the ribosomal RNA genes late in the third cell cycle of porcine embryos

In porcine embryos, nucleoli are first observed during the third postfertilization cell cycle, i.e., at the 4-cell stage. However, direct studies of the initiation of rRNA transcription have not been reported. This transcription was investigated in the present study by simultaneous visualization of the rRNA genes and the rRNA by fluorescent in situ hybridization using a porcine 28S rDNA probe and subsequent visualization of argyrophilic nucleolar proteins by silver staining of extracted and fixed nuclei from in vivo-derived porcine embryos (n = 229). Nucleologenesis was observed by transmission electron microscopy. In general, the 2-cell and 4-cell embryos fixed at 10 and 20 h postcleavage (hpc) showed no signs of rRNA transcription. Four small clusters of fluorescein isothiocyanate (FITC) labeling were visible in interphase nuclei, consistent with hybridization to the rRNA gene clusters only; there was no silver staining at the sites of the rRNA genes and nucleolus precursor bodies. From 30 hpc onwards, most 4-cell embryos had medium size to large clusters of FITC-labeled areas colocalized with silver staining of rRNA gene clusters and fibrillogranular nucleoli. These observations indicate that rRNA transcription had been initiated. These signs of rRNA synthesis could be blocked by actinomycin D, which is a strong inhibitor of RNA polymerase I. The rRNA transcription of porcine embryos is initiated between 20 and 30 hpc, corresponding to the end of the S-phase or the beginning of the G2 phase during the third cell cycle.

Simultaneous localization of transcription and early processing markers allows dissection of functional domains in the plant cell nucleolus

Nucleolar transcription in isolated onion cell nuclei was visualized, after Br-UTP incorporation, under the conventional fluorescence microscope, the confocal microscope, and the transmission electron microscope. The confocal microscopy study of transcription was combined with immunodetection of fibrillarin, a component of the RNP complex involved in the early processing of pre-rRNA. Superposition of transcription and fibrillarin images from the same optical section showed some small "black holes" in the nucleolus, around which a lateral and radial differentiation of labeling was observed: laterally, zones corresponding to transcription labeling alternated with zones of fibrillarin labeling; radially, areas of transcription gradually became areas of colocalization of transcription and fibrillarin, and, further outward, of fibrillarin alone, which occupied the major part of the labeled nucleolar area. Three-dimensional reconstruction of the nucleolar transcription labeling, from confocal optical sections, showed clusters of foci arranged around an area of low or no labeling. Thin labeled extensions, connecting single foci, were observed. Visualization of transcription at the ultrastructural level identified the black holes as fibrillar centers, in view of their size and the absence of labeling in them. In fact, most of the labeling was observed in discrete areas of the dense fibrillar component, near fibrillar centers, including the transition area between these two components. This observation was supported by a quantitative study. Otherwise, the outline of fibrillar centers did not appear entirely surrounded by particles, and a minor proportion of particles was detected dispersed throughout the dense fibrillar component. As a complementary study, the transcription factor upstream binding factor (UBF) and the protein NopA64, a plant nucleolin homologue, were immunolocalized. Small foci of UBF localization alone and other foci in which the two protein markers overlapped were observed. The outer areas of the nucleolus showed the exclusive presence of NopA64. Under the electron microscope, UBF labeling, quantitatively assessed, appeared as clusters of particles, most of them surrounding fibrillar centers. A graphic model is presented to give a molecular interpretation of these data.

Microwave irradiation improvements in the silver staining of the nucleolar organizer (Ag-NOR) technique

The well-known technique of silver staining of the nucleolar organizer (Ag-NOR) is improved in contrast, selectivity and speed when performed with microwave irradiation. The Ag-NOR technique is a very useful tool for studies on the functional morphology and molecular architecture of the nucleolus, and is reputed to be one of the best techniques for diagnosis and prognosis of cancer lesions. To test the generality of the enhancing effects, our study has involved the use of both mammalian and plant cells. Two steps in the process are improved quantitatively by microwave irradiation: fixation and staining itself. Fixation with the ethanol-based reagent, Kryofix, for 3 min in the microwave oven, resulted in good structural preservation at the optical level, and enhanced the contrast and selectivity of silver staining. On the contrary, we found that neither glutaraldehyde fixation, nor a treatment of sections with Carnoy's solution, improved Ag-NOR staining. After an analysis of the effects of the different substances involved in sample preparation, we conclude that ethanol is an essential factor for fixation for nucleolar staining, particularly if aldehydes are eliminated from fixative solutions. The process of staining was performed with a drop of staining solution on a semithin section of plastic-embedded tissue in the microwave oven for 1 min. Staining under these conditions always improved the visualization of nucleoli, regardless of the fixation procedure. Therefore, microwave irradiation at both steps is recommended for giving the best results. Microwave irradiation probably enhances fixation by controlled heat, whereas the increase in reactivity of the staining solution is a direct effect by the microwaves on the silver ions themselves. We used this method to study nucleolar materials during mitosis in proliferating plant cells. Current applications of Ag-NOR staining can be improved with this technical modification.

Cytochemistry and immunocytochemistry of nucleolar chromatin in plants

This review attempts to document the most relevant data currently available on the in situ localization of nucleolar chromatin on plant cells. The data provided by the most powerful and recent in situ techniques, such as DNA specific ultrastructural staining, immunogold labelling, in situ molecular cytochemistry, in situ hybridization or confocal microscopy, are summarized and discussed in the light of the potential and limitations of each individual methodology. The presence of DNA in both fibrillar centres and regions of the dense fibrillar component is extensively documented. Data on the nucleolar distribution of other important macromolecules involved in ribosomal transcription are also shown and referred to with regard to the location of DNA. The comparison with the available data on the animal cell nucleolus points towards models of similar functional organization in both plant and animal nucleoli.

Identification and localization of a nucleolin homologue in onion nucleoli

A protein homologous to nucleolin, a major nucleolar protein with multifunctional features involved in pre-rRNA synthesis and early processing, has been identified and localized in situ in onion root meristematic cells by different techniques, which have included the use of an antibody raised against hamster nucleolin. The protein was identified on Western blots of nucleolar proteins as a 64-kDa band, by means of the anti-nucleolin antibody, bismuth staining, and the silver staining-nucleolar organizer (Ag-NOR) method. The experiments also suggested that nucleolin could be a target of these two cytochemical stainings. Although the 64-kDa band corresponds to a major nucleolar protein, it is a minor one among total nuclear proteins. The same techniques were used in situ at the ultrastructural level, and the immunogold detection of the nucleolin homologue was quantitatively evaluated. The protein accumulates in the transition area from nucleolar fibrillar centers to the dense fibrillar component, which is considered to be the structural result of ribosomal gene transcription. Out of this transition area, the dense fibrillar component may be divided into two regions, proximal and distal with respect to fibrillar centers, which show, respectively, the significant and unsignificant presence of nucleolin; we interpret this fact as the expression of the topological arrangement of pre-rRNA processing. Fibrillar centers themselves showed a weak but significant labeling with the anti-nucleolin antibody. However, bismuth staining was absent from the interior of fibrillar centers, indicating that the nucleolin in them is not phosphorylated. Ag-NOR staining uniformly covered fibrillar centers and the dense fibrillar component (at least in its proximal region), but it did not stain condensed chromatin inclusions in heterogeneous fibrillar centers, showing that the binding of nucleolin to chromatin is associated with its decondensation. This work provides additional evidence of the high phylogenetic conservation of molecular motifs which take part in ribosome biogenesis.

Nucleolar organizer function in developing potato calli

The number of transcriptionally active nucleolar organizer regions (NORs) of one monohaploid, one dihaploid and two tetraploids of Solanum tuberosum and one diploid S. phureja was established by the silver staining of metaphases in root meristems and in in vitro-cultured leaf explants. The maximum number of active NORs per cell was one per haploid set of chromosomes. One or more NORs could be inactive in cells of the tetraploid meristems and in non-polyploidized and polyploidized cells of the dihaploid and tetraploid explants. Inactivation was determined by genotype and tissue and could remain constant during in vitro culture.

Ultrastructural localization of silver-staining nuclear proteins at the onset of transcription in early bovine embryos

Argyrophilic nuclear proteins, known to be functionally associated with ribosomal genes, were localized, in four-, eight-, and 16-cell bovine embryo blastomere nuclei using two different silver-staining procedures. Within the eight-cell cleavage stage by the process of embryonal nucleologenesis in the cow embryo the full-capacity ribosome-producing machinery is established. In the four-cell embryo, many patches and islands of argyrophilic (Ag+) material were detected in the nucleoplasm. The nucleolus-precursor bodies (NPBs), composed uniformly of a homogeneous compact mass, were completely devoid of any silver staining. On the other hand, clear-cut localization of argyrophilic proteins was detected during the eight-cell stage either inside the transforming NPBs or in the close vicinity, or in the already differentiated nucleolus. In compact, nonvacuolated NPB, an intensive Ag+ area was detected, in the form of a lenticle, at the periphery of the NPB. During and following vacuolation of the NPB, no Ag+ was detected inside these vacuoles. It was seen, however, in the dense fibrillar nucleolar component surrounding the smaller vacuoles formed at the time of the establishment of nucleolar structure. Ag+ areas were seen repeatedly in the vicinity of NPBs, probably a part of the nucleolus-associated chromatin or, alternatively, representing the extranucleolar bodies. In blastomere nuclei of 16-cell embryos, already possessing reticulated nucleoli known from intensively synthesizing somatic cells, the silver-staining pattern corresponded to the usual situation in differentiated cells: slight staining of fibrillar centers, heavy labelling in the dense fibrillar component, and absence of silver deposits in the granular component.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunolocalization of DNA at nucleolar structural components in onion cells

Intranucleolar DNA, including ribosomal DNA (rDNA), was localized in situ in proliferating onion cells under the electron microscope using an anti-DNA monoclonal antibody and a postembedding indirect immunogold procedure. In the interphase nucleolus of this species, characterized by a very high amount of rRNA genes, we found DNA concentrated mostly in fibrillar centres (FCs) and in the region of the dense fibrillar component (DFC) immediately surrounding them. Clusters of gold particles were frequently seen covering both of these structural components of the nucleolus at the same time. Moreover, the same technique, applied to transcriptionally arrested quiescent onion cells, showed the nucleolar DFC devoid of DNA. Also, in mitotic cells at telophase, the prenucleolar material, which has the same morphological and cytochemical features as the DFC, does not contain DNA. These data suggest the existence of at least two subcomponents of the DFC in the onion cell nucleolus, one associated with pre-rRNA synthesis, and the other, with further processing of transcripts, already released from the rDNA template. We conclude that the first subcomponent forms part of the "transition between FC and DFC", which is the in situ structural counterpart of pre-rRNA synthesis. This transition is morphologicaly sizeable in onion cells, because of their high number of rRNA genes and the large size of the DFC mass; however, it would be largely detectable in situ in other cell systems, where the whole DFC comprises only a thin layer and the amount of rDNA is considerably reduced.

Extra-ribosomal spacer sequences in Triturus

We show that, in Triturus vulgaris meridionalis, sequences homologous to the rDNA "non-transcribed" spacer (NTS) are clustered at chromosomal loci where they are not associated with 18 S or 28 S rDNA genes: these sequences are referred to as the extra-ribosomal spacer sequences. Genomic clones containing such extra-ribosomal spacer sequences have been isolated. As shown by restriction mapping, these clones appear to consist mostly of repetitive BamHI fragments that are, in turn, internally repetitious and highly homologous to each other. The structure of the clones was confirmed by nucleotide sequence analysis, which also demonstrates the high degree of conservation between the BamHI elements and the homologous NTS sequences. An intriguing 12 base-pair homology between the extra-ribosomal spacer sequences and a Xenopus NTS enhancer sequence is reported. The possibility that a repetitive octanucleotide motif found within the BamHI elements could act as a recombination hotspot by virtue of its similarity with the Escherichia coli chi sequence is discussed.

Structural and functional relationships between nuclear bodies and the nucleolus-DNA body complex in the oocyte of Amphiporus lactifloreus

The localization, structure and function of two types of nuclear bodies have been investigated by cytological and cytochemical electron microscopy methods in oocytes from the Hoplonemertean, Amphiporus lactifloreus. Type I nuclear bodies differentiate in contact with the nucleolus-DNA body complex, whereas type II nuclear bodies develop close to the diplotenic chromosomal axes. The structure of type I and type II spherical nuclear bodies, 4-5 micron in width, results from the association of a fibrillar reticulum with some dense included regions. The cytochemical findings following the use of osmium-ammine reaction for DNA and silver reaction for NOR proteins support the hypothesis that type II nuclear bodies, derived from the extranucleolar area, as well as type I nuclear bodies, derived from the nucleolar complex, may be involved in ribosomal biogenesis.

Development of the nucleolus in early goat embryos

In vivo nucleologenesis was studied in goat embryos from the pronuclear stage to the blastocyst stage by light and electron microscopy. Ultrastructural changes of the nucleoli were characterized by the following progression: homogeneous electron-dense fibrillar primary nucleoli in the pronucleus; small, dense fibrillar masses dispersed in clusters of chromatin at the two-cell stage; ring-shaped nucleoli made up of a fibrillar center surrounded by a layer of dense fibrillar components at the four-cell stage; reticulated nucleoli composed of a three-dimensional network of fibrillar components surrounded by small amounts of granular components at the eight-cell stage; fully developed compact-type nucleoli consisting of several fibrillar centers each surrounded by a layer of fibrillar components and abundant granular components in morulae and blastocysts. Moreover, it was concluded that activation of rRNA transcription, as evidenced by specific silver nitrate staining of the nucleolus organizer regions of metaphase chromosomes, occurs at the two- to four-cell stage and that the morphological changes accompanied a substantial increase in nucleolar transcriptional activity up to the blastocyst stage. This study provides evidence that a structure-function relationship exists during nucleologenesis in goat embryos.