Ag-staining of nucleolus organizer regions on human prematurely condensed chromosomes from cells with different ribosomal RNA gene activity

The age-dependent epigenetic and physiological changes in an Arabidopsis T87 cell suspension culture during long-term cultivation

Plant cell suspension cultures represent good model systems applicable for both basic research and biotechnological purposes. Nevertheless, it is widely known that a prolonged in vitro cultivation of plant cells is associated with genetic and epigenetic instabilities, which may limit the usefulness of plant lines. In this study, the age-dependent epigenetic and physiological changes in an asynchronous Arabidopsis T87 cell culture were examined. A prolonged cultivation period was found to be correlated with a decrease in the proliferation rate and a simultaneous increase in the expression of senescence-associated genes, indicating that the aging process started at the late growth phase of the culture. In addition, increases in the heterochromatin-specific epigenetic markers, i.e., global DNA methylation, H3K9 dimethylation, and H3K27 trimethylation, were observed, suggesting the onset of chromatin condensation, a hallmark of the early stages of plant senescence. Although the number of live cells decreased with an increase in the age of the culture, the remaining viable cells retained a high potential to efficiently perform photosynthesis and did not exhibit any symptoms of photosystem II damage.

Changes in DNA methylation patterns and repetitive sequences in blood lymphocytes of aged horses

It is known that aged organisms have modified epigenomes. Epigenetic modifications, such as changes in global and locus-specific DNA methylation, and histone modifications are suspected to play an important role in cancer development and aging. In the present study, with the well-established horse aging model, we showed the global loss of DNA methylation in blood lymphocytes during juvenile-to-aged period. Additionally, we tested a pattern of DNA methylation of ribosomal DNA and selected genes such as IGF2 and found no significant changes during development and aging. We asked if genetic components such as polymorphisms within DNA methyltransferase genes, DNMT1, DNMT3a, and DNMT3b, may contribute to observed changes in global DNA methylation status. The analysis of seven intragenic polymorphisms did not reveal any significant association with changes in global DNA methylation. Telomere shortage and a loss of pericentromeric heterochromatin during juvenile-to-aged period were also observed. Transcriptional rDNA activity, assessed as the number and size of nucleolar organizer regions, reflecting physiological state of the cell, and mitotic index were decreased with increasing horse donor age. Moreover, changes during juvenile-to-aged period and adult-to-aged period were compared and discussed. Taken together, changes in global DNA methylation status originating in development and affecting the stability of repetitive sequences may be associated with previously reported genomic instability during horse aging.

DNA hypomethylation and oxidative stress-mediated increase in genomic instability in equine sarcoid-derived fibroblasts

It is widely accepted that equine sarcoid disease, the most common skin associated neoplasm in equids, is induced by bovine papillomavirus (BPV-1). Although BPV-1 DNA has been found in almost all examined sarcoids so far, its detailed impact on the horse's host cell metabolism is largely unknown. We used equine fibroblast cell lines originating from sarcoid biopsies to study BPV-1-associated changes on DNA methylation status and oxidative stress parameters. Sarcoid-derived fibroblasts manifested increased proliferation in vitro, transcriptional rDNA activity (NORs expression) and DNA hypomethylation compared to control cells. Cells isolated from equine sarcoids suffered from oxidative stress: the expression of antioxidant enzymes was decreased and the superoxide production was increased. Moreover, increased ploidy, oxidative DNA damage and micronuclei formation was monitored in sarcoid cells. We postulate that both altered DNA methylation status and redox milieu may affect genomic stability in BPV-1-infected cells and in turn contribute to sarcoid pathology.

Cadmium-induced changes in genomic DNA-methylation status increase aneuploidy events in a pig Robertsonian translocation model

Although cadmium is a well-established human carcinogen, the mechanisms by which it induces cancer are poorly understood. It is suggested that cadmium-mediated carcinogenesis may include the modulation of gene expression and signal-transduction pathways, interference with antioxidant enzymes, inhibition of DNA repair and DNA methylation, and induction of apoptosis. Nevertheless, no predominant mechanism playing a role in metal-induced carcinogenesis has been reported. In the present study, we used a pig Robertsonian translocation model, which is a cross between a wild boar and domestic pig resulting in Robertsonian translocation (37,XX,der15;17 or 37,XY,der15;17), to determine the role of cadmium sulfate in the modulation of genomic DNA-methylation status and the induction of aneuploidy. We found a cadmium-mediated increase in aneuploidy within chromosome group A and C, but not within chromosome group D containing the translocated chromosome der15,17 which indicates that translocated chromosome is not more prone to chromosomal aberrations than are other chromosomes. We suggest that cadmium-induced aneuploidy (up to 5-μM concentration) may be mediated by global DNA hypermethylation as monitored with HPLC and 5-mdC immunostaining. In addition, the cyto- and genotoxic potential of cadmium was evaluated. Cadmium sulfate was able to induce apoptosis, inhibit cell-proliferative status and expression of nucleolar organizer regions (NORs), and increase oxidative DNA damage (8-oxoG content).

DNA replication initiation patterns and spatial dynamics of the human ribosomal RNA gene loci

Typically, only a fraction of the ≥600 ribosomal RNA (rRNA) gene copies in human cells are transcriptionally active. Expressed rRNA genes coalesce in specialized nuclear compartments – the nucleoli – and are believed to replicate during the first half of S phase. Paradoxically, attempts to visualize replicating rDNA during early S phase have failed. Here, I show that, in human (HeLa) cells, early-replicating rDNA is detectable at the nucleolar periphery and, more rarely, even outside nucleoli. Early-replicated rDNA relocates to the nucleolar interior and reassociates with the transcription factor UBF, implying that it predominantly represents expressed rDNA units. Contrary to the established model for active gene loci, replication initiates randomly throughout the early-replicating rDNA. By contrast, mostly silent rDNA copies replicate inside the nucleoli during mid and late S phase. At this stage, replication origins are fired preferentially within the non-transcribed intergenic spacers (NTSs), and ongoing rDNA transcription is required to maintain this specific initiation pattern. I propose that the unexpected spatial dynamics of the early-replicating rDNA repeats serve to ensure streamlined efficient replication of the most heavily transcribed genomic loci while simultaneously reducing the risk of chromosome breaks and rDNA hyper-recombination.

Expression of Fas and Fas-ligand and analysis of argyrophilic nucleolar organizer regions in squamous cell carcinoma: relationships with tumor stage and grade, and apoptosis

The aim of this study was to investigate whether levels of Fas and Fas-ligand (Fas-L) expression in oral and oropharyngeal squamous cell carcinomas (SCCs) are associated with tumor stage and grade, and to assess whether parameters related to argyrophilic nucleolar organizer regions (AgNORs) correlate with apoptosis in SCC cells and adjacent normal epithelium. Fifty-two specimens taken from the tumoral and neighboring normal tissues of 26 patients with oral and oropharyngeal SCC were analyzed for Fas/Fas-L expression, and 24 specimens from 12 patients for AgNOR parameters. Seventeen (65%) of the tumors were Fas and/or Fas-L-positive (by immunohistochemistry). A significant positive correlation was found between Fas/Fas-L expression and clinical tumor stage (P<0.01). Mean AgNOR number per nucleus, AgNOR size and the percentage area of each nucleus occupied by AgNORs (percent of nuclear area) were significantly increased in the SCC cells (4.49+/-1.28, 4.48+/-1.42, 5.56+/-1.22, respectively) when compared with the control neighboring squamous epithelial cells (2.58+/-0.61, 1.64+/-0.59 and 4.35+/-0.62%, respectively) (P<0.01). A significant positive correlation was found between the AgNOR parameters and Fas/Fas-L expression as apoptotic markers in the tumoral cells of SCC (P<0.05). There was also a significant positive correlation between the AgNOR parameters and the grading of tumors (P<0.05). In conclusion, AgNOR count was a strong proliferation marker in patients with SCC, and Fas and Fas-L staining was useful in tumor grading.

Modification of wheat rDNA loci by rye B chromosomes: a chromatin organization model

The rDNA loci, and their associated NORs, on chromosomes 1B and 6B of the hexaploid wheat cv. Lindström have been used as a chromatin marker to investigate the functional basis of the phenotype effects of introgressed supernumerary B chromosomes (Bs) of rye. The rye Bs themselves lack genes, other than those which determine their mitotic drive mechanism, and the way in which they can modulate characters determined by the A chromosome background has always been a puzzle. An isogenic line of Lindström plants carrying different numbers of Bs was used as the experimental system to see how different doses of Bs (from 0 to 6) affected the activity of the wheat NORs and the organization of their rDNA loci at interphase. Silver staining on metaphase chromosomes was used to evaluate the previous activity of the NORs, and to reveal variations in their size; and the pTa71 FISH probe from wheat was used to visualize structural modifications to the interphase rDNA loci. A single B had no measurable effect, but, as the B-number increased, there were significant changes in the physical dimensions of the metaphase NORs, reflecting reduced levels of their activity earlier in the cell cycle, and also in the condensation patterns of the interphase rDNA loci. In addition, the higher B-numbers caused a size heteromorphism between the homologous NORs. A model is discussed which interprets the phenotypic effects of Bs generically, in nucleotypic terms, based of their being 'genetically inert' but 'chromosomally active'.

The AgNOR proteins: qualitative and quantitative changes during the cell cycle

AgNOR proteins are a set of argyrophilic nucleolar proteins that accumulate in highly proliferating cells whereas their expression is very low in non-proliferating cells. Some of these proteins remain associated with the nucleolar organizer regions (NORs) during mitosis. In situ, the expression of AgNOR proteins is measured globally by quantification of the level of silver staining using morphometry and image analysis. To go deeper into the understanding of the relationship between the cell cycle and quantity of AgNOR proteins, it was necessary to determine the phases of cell cycle during which expression of AgNOR varies and what are the most variable proteins in each phase. To answer these questions, we set up the protocol permitting to detect and quantify AgNOR proteins on protein samples electrophoresed and transferred onto nitrocellulose membranes. This approach makes it possible to quantitatively evaluate individual AgNOR proteins and identify them, using nucleolar, nuclear and whole interphasic cell extracts, and chromosome-associated protein extracts. By this means, we identified nucleolin and protein B23 as the two major AgNOR proteins in the nucleolus during interphase and subunits of RNA polymerase I and transcription factor UBF as AgNOR proteins remaining associated with NORs during mitosis. We also observed that the increase in the level of nucleolin and protein B23 in rat liver seems to be linked with the cell cycle and not exclusively with stimulation of ribosomal gene (rDNA) transcription. Similarly in synchronized cells, the amount of nucleolin rapidly increases when cells enter the S phase (1.6-fold of the value of serum-deprived cells at 9 h, and 2.35-fold at 12 h after refeeding). The amount of protein B23 exhibits a lower and progressive increase with a maximum when the percentage of cells in G2 phase increased, i.e. after 24 h of cell cycle stimulation. We consider that the amount of AgNOR proteins can be a marker of proliferation, because this amount is related to cell cycle phases, schematically low for G1 phase and high for S-G2 phase. Thus, it is a measure of the relative proportion of cells in each phase, and consequently of the timing of each phase. The higher value indicates that the major part of the cells are in the S-G2 phase and correlatively few are in the G1 phase, and this characterizes a rapid cell cycle.

Proliferating cell nuclear antigen (PCNA) in non-Hodgkin's lymphomas: correlation with working formulation and Kiel classification in formalin-fixed paraffin-embedded material

PCNA is a 36-KD proliferating cell nuclear antigen associated with the cell cycle. The immunocytochemical detection of PCNA represents a useful tool for the study of tumor proliferation activity. This study documents the detection of PCNA, using antibody PC 10 in formalin-fixed, paraffin-embedded tissue, and correlates the proliferative activity of the non-Hodgkin's lymphomas (NHL) with histological grading assessed by the International Working Formulation (WF) and Kiel classification. In 92 cases of NHLs we found a strong correlation between the PCNA index and lymphoma grading. Statistically significant differences were also found between the proliferative index (PI) in low and high grade lymphomas according to the Kiel classification (t = 9.519; p < 0.001) and between low, intermediate and high grade lymphomas according to the WF classification (F = 79.01; p < 0.001). In the Kiel classification the mean of low grade lymphomas was 39.5% and of high grade 75.7%. In the WF the average of low grade lymphomas was 29.7%, intermediate 53.1% and high 75.1%. Although the differences among the groups had been significant, we found variations inside each histological subgroup in both classifications. The intermediate lymphomas were the most heterogeneous group, with PI inside the same histologic subtypes coincident with low and high grade lymphomas. Since PCNA may be used as a marker of cell proliferation in clinical studies to estimate the biological aggressiveness of lymphomas, its determination in intermediate grade NHL could be very useful to evaluate individual cases in this group and determine prognosis and probably the appropriate therapy.

A comparative assessment of proliferating cell nuclear antigen, c-myc p62, and nucleolar organizer region staining in non-Hodgkin's lymphomas: a histochemical and immunohistochemical study of 200 cases

Proliferating cell nuclear antigen (PCNA) and c-myc p62 oncoprotein are two nuclear proteins expressed in proliferating and transformed cells. They can be recognized immunohistochemically in paraffin sections by the monoclonal antibodies PC-10 and c-myc 1-9E10, respectively. On the other hand, nucleolar organizer regions (NORs) are loops of DNA that carry the r-RNA genes and can be visualized in paraffin sections as black dots (AgNORs) using a silver impregnation method. It has been suggested that the mean number of AgNORs may reflect the cellular kinetics of a tumor. We independently examined 200 cases of non-Hodgkin's lymphomas using the monoclonal antibodies PC-10 and c-myc 1-9E10, as well as the AgNOR method. Our study shows a very significant correlation between PCNA, c-myc expression, and AgNOR count on the one hand and histologic grade on the other (P < .001), although a significant overlap among the three grades exists. PC-10, c-myc 1-9E10, and AgNOR scores are all shown to be linearly related, even though significant discrepancies were observed, and the correlation is stronger between PCNA and AgNORs (PCNA v c-myc p62, r = .551; PCNA v AgNORs, r = .746; c-myc p62 v AgNORs, r = .529; P < .001). A remarkable finding is that the intermediate group of lymphomas is heterogeneous as far as the proliferative rate is concerned: diffuse large cell cleaved/non-cleaved lymphomas (category G of the Working Formulation) are characterized by a significantly higher proliferative index, as evidenced by the elevated PCNA, c-myc p62, and AgNOR scores, in comparison with the other types of intermediate-grade lymphomas (P < .001). However, the proliferative rate is lower than that of the high-grade lymphomas (PCNA, P < .05; c-myc p62, P < .001; AgNORs, P < .005). No significant difference exists between B-cell and T-cell lymphomas except for the higher expression of c-myc p62 in intermediate-grade B-cell lymphomas, obviously due to the higher proliferative rate of diffuse large cell lymphomas. Based on our findings, it appears that the combination of PCNA, c-myc p62, and AgNORs provides an accurate estimate of the proliferative rate of non-Hodgkin's lymphomas in paraffin sections. Clinical studies may show whether this information has prognostic value independent of histologic classification. In addition, our results suggest that category G (diffuse large cell) lymphomas may belong to a malignancy grade higher than the intermediate grade, a suggestion consistent with their more aggressive biologic behavior.

Temporal variation in nucleolar organizer region expression in bone marrow cells of individuals with leukemia

Silver staining was used to study nucleolar organizer region (NOR) expression in bone marrow cells obtained at two or, in one case, three time points from each of six leukemia patients. Using three measures of silver positivity, we observed that NOR expression was influenced by both metaphase stage and time. Silver positivity decreased significantly from one metaphase stage to the next, from prometaphase through late metaphase. When this variable was controlled for, significant changes in NOR activity were documented in comparisons between disease stages in the patients examined. However, patterns of NOR expression were not consistently associated with disease stage. These results indicate that in previous reports both the metaphase stage effect and the temporally changing nature of NOR activity have, as unrecognized variables, influenced observations of heterogeneity in NOR expression.

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.

Improvements in the premature chromosome condensation technique for cytogenetic analysis

In this paper we report a combination of procedures that serve to improve the usefulness of the technique of premature chromosome condensation in cytogenetic investigations. The mitotic inducer population was preincubated in high concentrations of BrdU, and the duration of fusion was increased to yield more discrete prematurely condensed chromosomes (PCC). After fusion, chromosome preparations were treated with a combination of G- or C-banding techniques and differential staining techniques. This combination of procedures allowed unequivocal distinction between the PCC and mitotic inducer chromosomes and yielded banded G1 and G2 PCC suitable for routine cytogenetic investigations.

Silver bands in chronic granulocytic leukemia. II. The Philadelphia chromosome

To test if rRNA gene activity in the Philadelphia (Ph) chromosome nucleolar organizing regions (NOR) differs from other NOR we scored silver banding patterns in leukemic cells of 32 patients (one patient was sampled twice) in the chronic phase of chronic granulocytic leukemia. The average number of bands per NOR in banded cells of these patients was 0.40 +/- 0.06 for the Ph NOR, not significantly different from the 0.41 +/- 0.02 per D chromosome NOR or the 0.41 +/- 0.04 per G. Across the 33 cases, the number of different Ph NOR banded in at least one cell (23 of 33, 70%) was also closely similar to the cumulative maximum number of chromosome D NOR banded per cell (137 of 198 banded, 69%) and G NOR banded (67 of 99, 67%). We conclude that the activity of rRNA genes in Ph NOR is closely similar to that of the other nine NOR in each CGL cell. In situ hybridization of 3H-labeled complementary rRNA to NOR of two patients revealed that the mean number of Ph NOR grains fell within the range of the other NOR. Also, grain distributions across the NOR of leukemic cells closely matched those of stimulated lymphocytes. Thus, a limited sampling has shown no evidence for consistent differences or changes in the rRNA gene number of Ph NOR.

On the position of nucleolus organizer regions (NORs) in interphase nuclei. Studies with a new, non-autoradiographic in situ hybridization method

The distribution of 18S and 28S ribosomal RNA (rRNA), i.e. the chromosomal nucleolus organizer regions (NORs) was visualized in interphases and metaphases of non-stimulated and phytohemagglutinin (PHA)-stimulated human lymphocytes with a recently developed non-autoradiographic in situ hybridization method. This procedure involves mercurated RNA as a probe and a sulfhydryl-trinitrophenyl-mercury binding ligand and FITC-labelled antibodies as detection system. Silver staining was used to visualize nucleoli in interphase. In the secondary constriction of all ten acrocentric chromosomes, varying amounts of rDNA were detected. In the interphase nuclei of most of the non-stimulated human lymphocytes, only one small nucleolus could be seen. The in situ hybridization, however, revealed several agglomerations of rDNA scattered over the whole nuclear area, clearly outnumbering the number of nucleoli in these cells. This means that not all of the NORs are transcriptionally active in non-stimulated lymphocytes and that these inactive NORs lie at a distinct distance from the active ones. With PHA stimulation (transforming the small lymphocytes from peripheral blood into large, lymphoblast-like cells) the number of nucleoli increased slightly, whereas the number of separable rDNA spots decreased. This means that in the course of PHA-induced cellular activation, formerly inactive NORs become transcriptionally active and tend to associate with one another. This indicates the occurrence of movements of the NORs within the nucleus, depending on their transcriptional activity.

Silver-stained nucleolar organizer regions in bone marrow cells and peripheral blood lymphocytes of Philadelphia chromosome-positive chronic myelocytic leukemia patients

Silver-stained nucleolar organizer regions (Ag-NOR) in bone marrow cells and/or phytohemagglutinin-stimulated peripheral blood lymphocytes were compared between six normal healthy persons as controls and 22 Philadelphia chromosome (Ph)-positive chronic myelocytic leukemia (CML) patients, to examine if any disease associated changes occur in the expression of Ag-NOR. Although the frequency of Ag-NOR-positive cells and the number of Ag-NOR per cell were generally greater in lymphocytes than in bone marrow cells in both controls and CML patients, the Ag-stainability of these cell types in CML patients was considerably heterogeneous, compared with that found in controls. The peripheral lymphocytes of CML patients in the chronic phase, but not in the blastic phase, exhibited a significantly lowered Ag-stainability when compared with those of controls. while no such difference was observed between bone marrow cells of controls and leukemia patients in both phases of CML. In the blastic phase, however, the occurrence of Ag-NOR on the Ph of CML bone marrow cells was significantly less than expected. The present findings are discussed in relation to the existing data on the Ag-NOR expression in both normal and neoplastic cells.

N-band proteins of nucleolar organizers: chromosomal mapping, subnucleolar localization and rDNA binding

The ribosomal DNA(rDNA)-containing chromatin in eukaryotes forms a unique architecture called the "secondary constriction" or "nucleolus organiser region (NOR)" on mitotic chromosomes. To gain more insight into non-histone chromosomal proteins (NHCP), termed "N-band proteins", that are specifically associated with the NOR in a wide variety of eukaryotes, we attempted to: identify the NHCP responsible for N-band staining; determine their stoichiometry; map them on metaphase chromosomes; determine their subnucleolar localization and examine their possible ability to bind rDNA. Based on several criteria, including chromosomal localization, solubility, association with chromatin, and intra-nuclear localization, two of the nucleolus-rich NHCP, termed component B of mol.wt 55,000 and component C of mol.wt. 41,000, were tentatively identified as N-band proteins. Immunological studies using a polyclonal, monospecific antibody raised against component C show that this protein is in fact associated with the chromosomal telomeres where NORs are located. In nucleoli, N-band proteins appear to be compartmentalized into a structure that probably corresponds to fibrillar components. Both components B and C are among several NHCPs that showed, under in vitro conditions, a strong affinity for rDNA cloned in lambda phage but not for calf thymus genomic DNA or phage vector DNA. The antibody against component C effectively suppressed in vitro transcription by RNA polymerase I in nucleoli and nucleolar chromatin. Component C appears to exist in the nucleus at 3.75-5.13 X 10(3) copies per rDNA unit or 0.09-0.13 copy per nucleotide. These findings support the hypothesis that the NOR is a chromosomal site, architecturally not only unique but also different from other chromatin regions in that constituent DNA, i.e., rDNA, is organized in a specific manner by interacting with specific NHCP, i.e., N-band proteins.

Comparison of metaphase and interphase nucleolar activity in Hela-CCL2 cells and PHA-stimulated human lymphocytes

Metaphase and interphase nucleolar activity in cultured Hela-CCL2 cells and PHA-stimulated human lymphocytes have been studied with silver nitrate staining. In metaphase, we examined the relationship between the actual number of active NORs (AgNORs) and total number of NOR-bearing acrocentrics. Interphase silver staining over the nucleus was analyzed cytodensitometrically and morphologically. From all investigations, Hela-CCL2 cells and lymphocytes were shown to have similar levels of nucleolar activity. Our results suggest that there is a form of regulation of nucleolar activity in malignant Hela-CCL2 cells as compared to PHA-stimulated human lymphocytes.

Relationship between the Ag-NOR proteins and ribosomal chromatin in situ during drug-induced RNA synthesis inhibition

In human TG tumor cells, the role of silver-NOR proteins was investigated by examining their relationship with the chromatin structure during inhibition of RNA synthesis by actionomycin-D treatment. This induced segregation of the nucleoli into four distinct zones and weakened the silver reaction. The fibrillar components were found to constitute the site of silver-stained proteins segregation. Feulgen-like osmium-ammine staining revealed that the DNA disappeared from the segregated nucleoli except for a network of nonnucleosomal filaments. When Ag-NOR protein detection was combined with chromatin visualization, we found constant overlapping of the silver reaction sites with the nonnucleosomal DNA filaments. Our results indicate that certain Ag-NOR proteins are not directly linked to active rRNA synthesis, but might rather affect the structure of ribosomal genes.

Silver bands in chronic granulocytic leukemia: I. Increased banding associated with blastic transformation

We have tested the proposal that the level of silver banding in leukemic cells of Ph1 + chronic granulocytic leukemia (CGL) patients increases as the disease progresses. Blood and/or bone marrow cells from 14 patients were cultured for 24 hr before banding. In all but one case, there were two populations of mitoses, those with silver bands on their nucleolar organizing regions (NORs) and those without. The percentage of cells that banded was higher, on average, in cultures from 7 patients in blastic transformation (80%) than in 8 chronic cases (36%) or in one accelerated phase (49%). Also, the mean number of NORs stained in banded cells was higher in blastic phase (6.9) compared with chronic phase cells (4.4). Hyperdiploid cell lines were present in four cases of myeloblastic transformation. All such cells were silver banded, and the mean fraction of NORs banded in them was relatively high. An increase in silver banding with time was shown in two of the patients. It seems that silver banding does increase in CGL cells as the disease progresses. This may arise either through an increase in the rate of ribosomal RNA synthesis in leukemic cells present in the blastic phase or possibly by a decrease in the rate of degradation (or processing) of newly synthesized rRNA.

Evidence for wheat-rye nucleolar competition (amphiplasty) in triticale by silver-staining procedure

Amphiplasty in hexaploid triticale, the artificial amphiploid of tetraploid wheat and diploid rye, is analyzed for the first time using a modified, highly reproducible, silver-staining procedure. A comparative analysis of metaphase somatic cells by phase contrast, C-banding and silver-staining of the hexaploid triticale cv. 'Cachirulo' and its parents, namely, the tetraploid durum wheat cv. 'Enano de Andujar' and the diploid rye cv. 'Petkus' has been made. Two silver-stained nucleolar organizer regions (Ag-NORs) (the chromosome pair 1 R) are observed in all rye plants analyzed, whereas four Ag-NORs (chromosome pairs 1 B and 6 B) are found both in the tetraploid wheat parent and in the triticale. The rye Ag-NORs are absent in the triticale. Since the Agstaining reaction of NORs can be considered as an indication for genetic activity, the silver procedure can be used to visualize gene functionality at the rDNA sites with conventional light microscopy and, consequently, the modified Ag-staining method described can be very useful in analyzing the amphiplasty phenomenon in natural or artificial hybrid combinations and derivatives in the Triticum group and its relatives.

Analysis of ring-shaped nucleoli in serially sectioned human lymphocytes

Serial section analysis has demonstrated that ring-shaped nucleoli of mature human lymphocytes are spherical structures consisting of a peripheral ribonucleoprotein shell that surrounds one large fibrillar center. The shell exhibits usually one or, less frequently, two openings. The fibrillar center is in contact with the nucleoplasm and perinucleolar condensed chromatin, which frequently appears as a pedicle-like structure. Several chromocenters are associated with the ring-shaped nucleolus.

Changes in nucleolar structure, number and size in cellular activation and inactivation. Observations in human phytohaemagglutinin-treated lymphocytes

Lymphocytes in human peripheral blood are small, relatively inactive cells. The addition of phytohaemagglutinin (PHA) to cultures of these cells causes a marked increase in cellular and nuclear size, DNA-synthesis and metabolic activity, which reaches a maximum three days after the onset of culturing. The cells then undergo an inactivation process over a period of approximately ten days by which time they have reverted to cells resembling small, inactive lymphocytes. Within the first three days, nucleoli increase in size and number, changing from ring-shaped to nucleolonema-exhibiting to compact nucleoli. In the course of the inactivation process the nucleoli decrease in size and change from compact nucleoli directly into ring-shaped nucleoli. Thus activation and inactivation pathways are different. There is an increase in the number of nucleoli during the inactivation phase up to the seventh day in culture, followed by a slight decrease until day 14. This suggests that nucleoli in metabolically active cells have a tendency to fuse, whereas those in inactive cells tend to fragment.

Stain intensity of human nucleolus organizer region reflects incorporation of uridine into mature ribosomal RNA

The stain intensity of the nucleolus organizer regions (NORs) of acrocentric chromosomes was correlated positively with incorporation of [3H]uridine into 18S rRNA and 28S rRNA from cultured diploid human skin fibroblasts. An analysis of these data from twins by a path model indicated that no other common genetic or environmental parameters were required to explain the relationship between NOR scores and uptake of [3H]uridine into mature rRNA species.

Molecular biology of cell division

Different domains of the SV40 A gene have different functions, such as viral DNA replication, cell DNA replication, and stimulation of cellular RNA synthesis. The sequences in the SV40 A gene that are critical for the induction of cell DNA synthesis lie on the map between nucleotide 4360 and nucleotide 4001, a stretch of 360 nucleotides coding for 120 of the 708 amino acids of the large T antigen. The sequences critical for stimulation of rRNA synthesis lie on the map further downstream, between nucleotides 3827 and 3506, thus indicating that the signals for growth in size and for cell DNA replication can be dissociated. Methylation of the SV40 A gene at multiple ECoRI* sites has no effect on its expression. However, methylation of the HSV-TK gene at one single ECoRI site 70 base pairs upstream from the cap site inhibits its expression. The results indicate that methylation of genes affects their expression, but only when methylation occurs at specific sites.

The number of nucleolar silver-stained granules of active nucleolus organizer regions in mitotic and interphase cells of rat Yoshida and Zajdela ascitic tumors

To provide more information on the number of silver-stained granules (SSGs) of active mitotic nucleolus organizer regions (NORs) as well as interphase nucleoli, rat Yoshida ascitic sarcoma and Zajdela ascitic hepatoma cells were studied by means of standardized one-step and two-step silver-staining procedures. The number of SSGs of mitotic active NORs was relatively constant and corresponded to the number of active NORs of animals bearing the tumors investigated. Some anaphases and telophases were "asymmetric", i.e. chromosomal figures (future nuclei) in one and the same cell contained different number of SSGs (active NORs). The incidence of such asymetric anaphases and telophases was higher in aneuploid (hypoploid) Zajdela hepatoma than in euploid (diploid) Yoshida sarcoma cells. In the interphase, the number of SSGs was low in small or large cells with distinct chromocenters or chromosomes condensation presumably representing postmitotic and premitotic cells. In contrast, the highest number of SSGs was noted in nucleoli of large cells which were usually characterized by a fine chromatin structure.

The morphological relationship in electron microscopy between NOR-silver proteins and intranucleolar chromatin

The relative distribution of NOR proteins and chromatin fibers in the nucleoli was visualized in human cell line. The chromatin was revealed by a Feulgen-like procedure using osmium-ammine as DNA tracer. This selective staining was combined with NOR-silver staining. We provide morphological evidence for constant overlapping of the silver deposit sites with dispersed intranucleolar chromatin fibers. Silver stained proteins were sometimes observed in contact with the chromatin fibers, suggesting that at least some of the Ag-NOR proteins might be closely connected with the dispersed nucleolar DNA.

Variations in the activity of nucleolar organizers in different tissues, demonstrated by silver staining of human normal and leukemic cells

A simple silver-staining technique that demonstrates those nucleolar organizing regions of metaphase chromosomes which are transcriptionally active during the preceding interphase (AgNORs) has been applied to cells obtained from the bone marrow and mitogen-stimulated peripheral blood lymphocyte cultures of hematologically normal individuals and patients with various forms of leukemia. In the majority of bone marrow cells from the normal controls and many of the patients, the number of cells with detectable AgNORs, and the staining intensities in those cells which were Ag+, were markedly reduced compared with the levels found in blood lymphocytes. The numbers of cells having satellite associations and the numbers of chromosomes participating in these associations also generally reflected the proportions of AgNORs present. When patterns of bone marrow silver staining were compared between patients with leukemia, distinct differences were found which could be correlated with cytology. It is suggested that different cell types have characteristic AgNOR staining profiles, reflecting specific regulation of ribosomal RNA synthesis in particular cell lineages. AgNOR staining may indicate, therefore, the predominant cell types that divide in the bone marrows of patients with different forms of leukemia.

The influence of the cell cycle on structure and number of nucleoli in cultured human lymphocytes

The nucleoli of lymphocytes undergo a typical sequence of structural changes after stimulation by phytohaemagglutinin. These changes are independent of the cell cycle. Neither the inhibition of DNA-synthesis (by adenosine and methotrexate), nor the elimination of postmitotic interphase nuclei (by a colchicine block of mitoses), nor the release from such blocks has a noticeable effect on nucleolar structure or on the sequence of nucleolar changes. The number of nucleoli per cell is clearly influenced by the cell cycle. Mitosis leads to a marked increase in the number of nucleoli, whereas in all stages of interphase a decrease occurs.

Rearrangement in rDNA-bearing chromosomes in cell lines from neoplastic cells

Cytological hybridization in situ and karyotypic analysis have been used to investigate positional changes in rDNA, and morphological changes in rDNA-bearing chromosomes, in a series of cell lines established from patients with various proliferative disorders. Characteristic features of chromosomes in these cell lines include extreme differences in the number of rRNA genes per locus, translocations involving rDNA, and distinct rearrangements of a large portion of the rDNA complex to a q-terminal position on acrocentric chromosomes.

Evidence for methylation of inactive human rRNA genes in amplified regions

In two unrelated families, the short arm of a 14p+ marker chromosome contains an increased number of copies of the 18S + 28S rRNA genes without a comparable increase in the transcriptional activity, as shown by silver staining. The DNA in this region is highly enriched in 5-methylcytosine, as shown by specific antibody binding. In contrast, the owl monkey and cat have a single major nucleolus organizer region (NOR) per haploid genome; these NORs contain about the same number of rRNA genes as the 14p+ chromosome but are not methylated. These findings suggest that most of the amplified human rRNA genes on the 14p+ chromosomes have been inactivated by a process involving DNA methylation.

Amplified ribosomal RNA genes in a rat hepatoma cell line are enriched in 5-methylcytosine

In a rat hepatoma cell line, H4-IIE-C3, a 10-fold excess of 18S and 28S rRNA genes has been found in amplified chromosome regions. Antibodies to 5-methylcytidine bound extensively to the DNA of these regions, indicating a high level of DNA methylation. Most of the amplified rRNA genes were transcriptionally inactive, as shown by their failure to stain with silver. DNAs from the tumor cells and control rat hepatocytes grown with L-[methyl-14C]methionine were digested with restriction endonuclease EcoRI; the DNA fragments were separated by agarose gel electrophoresis, denatured, transferred to nitrocellulose filters, and hybridized to 32P-labeled rRNA or cDNA. Fragments containing the 18S or 28S rRNA coding sequences occurred in three major size classes; all three were rich in 5-methylcytosine. Analysis of EcoRI fragments of DNA from the tumor and control cells after digestion with Hpa II or Msp I endonuclease indicated that the 5'-C-C-G-G-3' sequences in most of the amplified rRNA genes were methylated. Analysis of the fragments produced by digestion with Hha I endonuclease indicated a high degree of methylation within its recognition sequence in the amplified rRNA genes as well. The association of hypermethylation with restricted transcriptional activity suggests that DNA methylation may regulate the activity of the rRNA genes.

Nucleolar changes in human phytohaemagglutinin-stimulated lymphocytes

The nucleoli of lymphocytes from circulating peripheral blood and from phytohaemagglutinin (PHA)-stimulated cultures (from 2 h-96h) were studied using a silver method, RNA-specific fluorescent staining, and electron microscopy of ultrathin sections. In peripheral blood about 75% of the lymphocytes have one "ring-shaped" nucleolus composed of a distinct fibrillar centre surrounded by a dense pars fibrillaris and little granular material; the remaining lymphocytes showing two or more small "ring-shaped" nucleoli. With PHA stimulation, the number of cells with several nucleoli increases first (from 2 h--12 h). Next, cells containing one or, at most, two large nucleoli with nucleolonema devoid of fibrillar centers are seen (from 4 h on). 34 h after PHA, nucleoli of the "compact" type containing one or more fibrillar centres appear and comprise about 60% of the cells after 72 h. The appearance of more than one nucleolus per cell shortly after PHA administration suggests an activation of additional nucleolar organizer regions (NOR), which fuse to form one or two large nucleoli with nucleolonema. These are then transformed into "compact" nucleoli. The fibrillar centers stasin preferentially with silver. They contain nonchromosomal proteins and may serve as stores for nucleolar proteins. The fusion of activated NORs during the first cell cycle explains the relatively high frequency of satellite associations in first mitoses compared to later mitoses after stimulation.

Ultrastructural localization of Ag-NOR stained proteins in the nucleolus during the cell cycle and in other nucleolar structures

EM investigation of Ag-AS-NOR staining after short glutaraldehyde prefixation followed by Carnoy fixation maintained good ultrastructural preservation and reactive selectivity. This enables exact localization of silver deposits both in the fibrillar centers of typical or segregated nucleoli during interphase, and in chromosome NORs during mitosis. These results argue in favour of the possibility that fibrillar centers are the interphasic counterpart of chromosome NORs. Special structures such as nucleolar blobs and remnants usually considered to be of nucleolar origin, were also stained. - These findings seem to indicate a relationship between the distribution of the silver-stained proteins, the arrangement of the nucleolar structures and the degree of nucleolar activity resulting from the experimental conditions. These results are of interest at the time when the concept of the nucleolar matrix is gradually emerging.