Showing papers on "Metaphase published in 1989"
TL;DR: It is concluded that microtubules polymerize at kinetochores and depolymerize near the poles throughout metaphase, providing strong evidence for polewards flux in Kinetochore micro Tubules.
Abstract: I have synthesized a novel derivative of carboxyfluorescein that is nonfluorescent, but can be converted to a fluorescent form by exposure to 365-nm light. This photoactivable, fluorescent probe was covalently attached to tubulin and microinjected into mitotic tissue culture cells, where it incorporated into functional spindles. To generate a fluorescent bar across the mitotic spindle, metaphase cells were irradiated with a slit microbeam. This bar decreased in intensity over the first minute, presumably due to turnover of nonkinetochore microtubules. The remaining fluorescent zones, now presumably restricted to kinetochore microtubules, moved polewards at 0.3-0.7 microns/min. This result provides strong evidence for polewards flux in kinetochore microtubules. In conjunction with earlier biotin-tubulin incorporation experiments (Mitchison, T. J., L. Evans, E. Schulze, and M. Kirschner. 1986. Cell. 45:515-527), I conclude that microtubules polymerize at kinetochores and depolymerize near the poles throughout metaphase. The significance of this observation for spindle structure and function is discussed. Local photoactivation of fluorescence should be a generally useful method for following molecular dynamics inside living cells.
607 citations
TL;DR: The M‐phase‐specific protein kinase from starfish oocytes at first meiotic metaphase is purified to near homogeneity using an improved procedure based on affinity chromatography on the immobilized yeast protein suc1, identical to MPF, the cytoplasmic factor that controls entry of eukaryotic cells into M‐ phase.
Abstract: We have purified to near homogeneity the M-phase-specific protein kinase from starfish oocytes at first meiotic metaphase, using an improved procedure based on affinity chromatography on the immobilized yeast protein suc1. As already reported, this is identical to MPF, the cytoplasmic factor that controls entry of eukaryotic cells into M-phase. MPF is a complex formed by the stoichiometric association of a 34-kd polypeptide previously identified as cdc2 with a polypeptide that migrates with the same mobility as starfish cyclin in SDS-PAGE (apparent mol. wt 47 kd). A cDNA clone encoding starfish cyclin B has been isolated and its sequence determined. It contains a single open reading frame encoding a predicted 43 729-dalton protein. Partial microsequencing of the 47-kd polypeptide component of MPF allowed its identification as the starfish cyclin. Since the apparent mol. wt of native starfish MPF was found to be less than 100 kd, it is a heterodimer comprising one molecule of cdc2 and one molecule of cyclin B.
521 citations
TL;DR: A single cell was removed from each of 30 human embryos at the 6-10 cell cleavage stage three days after in-vitro fertilisation, and each male embryo was sexed from the DNA by amplification of a repeated sequence specific for the Y chromosome.
456 citations
TL;DR: It is shown that in intact cells, two distinct types of ribosomal chromatin coexist in Friend cells, one that contains nucleosomes and represents the inactive copies and one that lacks a repeating structure and corresponds to the transcribed genes.
376 citations
TL;DR: In cows, protein synthesis is required for germinal vesicle breakdown (GVBD) and the nuclear changes in the bovine oocyte during 24 h of culture are examined more closely.
Abstract: In cows, protein synthesis is required for germinal vesicle breakdown (GVBD). This study examines more closely the need for protein synthesis and the nuclear changes in the bovine oocyte during 24 h of culture. Bovine oocytes with compact and complete cumulus were washed and incubated in groups of 10 for up to 24 h in SO-pd drops of TCM-199 supplemented with follicle-stimulating hormone (NIAMADD, 0.5 �.Lg/ml), luteinizing hormone (LH) NJAMADD, 5 �.tg/ml), estradiol-17fi (1 �.tg/ml), pyruvate (20 pM), and 10% heattreated fetal calf serum. Medium was overlaid with paraffin oil. Oocytes (n = 891) were fixed at the end of each 3-h interval from 0 to 24 h of culture, or at 24 h after addition of cycloheximide (10 pg/ml at 10 different times during maturation (0, 1, 2, 3, 6, 9, 12, 15, 18, 21 h; n = 175). At each time point, the chromosomal status of oocytes was evaluated, frequencies were computed, and the time spent on each step was determined. The germinal vesicle (GV) was present from 0 to 6.6 h, GVBD at 6.6 to 8.0 h, chromatin condensation at 8.0 to 10.3 h, metaphase I at 10.3 to 15.4 h, anaphase I at 15.4 to 16.6, telophase I at 16.6 to 18.0 h, and metaphase II at 18.0 to 24 h. Cycloheximide blocked oocyte maturation at GVBD, if added from 0 to 3 h; at chromatin condensation, 4fpresent from 6 to 24 h; and at metaphase 1, when present from 9 to 12 h. When cycloheximide was present from 12 to 24 h an increasing number of oocytes reached metaphase II (19% for 12 h, 36% for 15 h, 51% for 18 h, 90% for 21 h, 94% for 24 h), but many abnormalities were noted:
306 citations
TL;DR: The activation and inactivation of maturation promoting factor and cyclin are discussed, saying very little about how it executes its downstream functions.
241 citations
TL;DR: The Ki-67 antigen is preserved in nuclear matrix preparations obtained after in situ fractionation of interphase cells and when mitotic cells were exposed to such treatments, the obtained fluorescence data suggested that the antigen may be part of the chromosome scaffold.
Abstract: In interphase cells the proliferation-associated antigen recognized by monoclonal antibody Ki-67 is almost exclusively located in the nucleoli. When cells at several stages of mitosis were examined for the localization of the Ki-67 antigen, a striking redistribution could be observed. During prophase the distinct nucleolar Ki-67 fluorescence changed to a bright irregular meshwork throughout the nucleoplasm. At metaphase the antigen appeared to be distributed in a reticulate structure surrounding the condensed chromosomes, while at late telophase a punctated staining of the entire nucleoplasm was observed, which preceded the typical nucleolar localization pattern in each of the two daughter cells. Immunolabelling with Ki-67 of metaphase chromosome spreads revealed a circumferential staining of the individual chromosomes. The Ki-67 antigen is preserved in nuclear matrix preparations obtained after in situ fractionation of interphase cells. When mitotic cells were exposed to such treatments, the obtained fluorescence data suggested that the antigen may be part of the chromosome scaffold. Quantification of the Ki-67 fluorescence signal using flow cytometry revealed the highest staining intensities in mitotic cells. Furthermore, it was shown that nutritionally deprived cells became negative for Ki-67.
240 citations
TL;DR: Depending on the type of activating stimulus, oocytes exhibit the capacity for full activation at different ages, and the oocyte arrest in M III is similar to M II and can be released by subsequent activation.
223 citations
TL;DR: Early seed development in crosses between the hexaploid wheat genotype 'Chinese Spring' and the maize genotypes was studied to determine the timing of elimination of the maize chromosomes, suggesting all maize chromosomes were lost during the first three cell-division cycles in most embryos.
Abstract: Early seed development in crosses between the hexaploid wheat genotype 'Chinese Spring' and the maize genotype 'Seneca 60' was studied to determine the timing of elimination of the maize chromosomes. Elimination of one or more maize chromosomes occurred in about 70% of zygotic mitoses. Metaphase nuclei from two-celled embryos had 5 to 10 maize chromosomes, most of which were lost during the second cell division. About half the metaphase nuclei from four-celled embryos had no maize chromosomes, and the remainder had one to five. Anaphase or telophase nuclei from four-celled embryos showed no maize chromosomes in about half the cells and one or more pairs of lagging maize daughter chromosomes in the remainder. No maize chromosomes were seen in metaphase preparations from embryos with eight or more cells. These data strongly suggest that all maize chromosomes were lost during the first three cell-division cycles in most embryos. All embryos with four or more cells had micronuclei, showing that embryo develop...
148 citations
TL;DR: It is reported that antibodies raised against a highly purified and biochemically well characterized nonhistone "High-Mobility Group" protein, HMG-I, specifically localize this protein to the G-bands in mammalian metaphase chromosomes.
Abstract: Mammalian metaphase chromosomes can be identified by their characteristic banding pattern when stained with Giemsa dye after brief proteolytic digestion. The resulting G-bands are known to contain regions of DNA enriched in A/T residues and to be the principal location for the L1 (or Kpn 1) family of long interspersed repetitive sequences in human chromosomes. Here we report that antibodies raised against a highly purified and biochemically well characterized nonhistone "High-Mobility Group" protein, HMG-I, specifically localize this protein to the G-bands in mammalian metaphase chromosomes. In some preparations in which chromosomes are highly condensed, HMG-I appears to be located at the centromere and/or telomere regions of mammalian chromosomes as well. To our knowledge, this is the first well-characterized mammalian protein that localizes primarily to G-band regions of chromosomes.
133 citations
TL;DR: In a blind study, chromosome aberrations in tumor cells were analyzed by conventional cytogenetic techniques (G banding) and nonradioactive in situ hybridization with chromosome-specific probes and gave reliable results and allowed detection of cell subpopulations that were not detected by analyzing metaphase chromosomes.
TL;DR: The early appearance and stability of mouse MPF in Metaphase I oocytes depend on protein phosphorylation rather than on protein synthesis, and protein synthesis is necessary to maintain the condensation of the chromosomes in metaphase II oocytes.
TL;DR: Anaphase in mammalian fibroblasts and epithelial cells is not triggered by brief calcium transients, and anaphase is a calcium-modulated event, usually accompanied by a sustained elevation of Cai above 50 nM, which is dependent upon an intact spindle.
Abstract: Swiss 3T3 fibroblasts and LLC-PK epithelial cells in prometaphase or metaphase were either injected with fura-2 or loaded with the acetoxymethyl ester derivative of fura-2 (fura-2 AM) and monitored by microspectrofluorimetry. With both methods of loading, we observed two aspects of intracellular free calcium (Cai) metabolism. (a) Most fibroblasts and epithelial cells exhibited a gradual rise from 75 nM in metaphase to 185 nM during cleavage, returning to baseline by early G1. (b) Mitotic Swiss 3T3 cells exhibited rapid transient Cai changes, similar to those previously reported [Poenie, M., J. Alderton, R. Y. Tsien, R. A. Steinhardt. 1985. Nature (Lond.). 315:147-149; Poenie, M., J. Alderton, R. Steinhardt, and R. Tsien. 1986. Science (Wash. DC). 233:886-889; Ratan, R., and M. L. Shelanski. 1988. J. Cell Biol. 107:993]. These Cai transients occurred repetitively, often beginning in metaphase and continuing long after daughter cell formation. Eliminating serum or calcium from the medium abolished the transients, but delayed neither the gradual Cai elevation nor anaphase onset. Co-injection of EGTA or 1,2-bis-(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) with fura-2 in calcium-free medium, but not in calcium containing medium, blocked both anaphase and the sustained Cai elevation in almost all cases. Blocked cells were rescued by returning calcium to the medium, whereupon Cai slowly but steadily rose as the cell entered anaphase. Spindle microtubules persisted through the EGTA block. Depolymerization of spindle microtubules by nocodazole also reversibly blocked anaphase onset and the sustained Cai elevation, but did not block transients. This study has revealed the following: (a) anaphase in mammalian fibroblasts and epithelial cells is not triggered by brief calcium transients; (b) anaphase is a calcium-modulated event, usually accompanied by a sustained elevation of Cai above 50 nM; (c) the elevation of Cai is dependent upon an intact spindle; and (d) fibroblasts progress through mitosis by drawing upon either intracellular or extracellular sources of calcium.
TL;DR: The possibility that alphoid DNA repeats are a key component of kinetochore structure is suggested, as a method was developed to carry out in situ hybridization of DNA and indirect immunofluorescent staining of antigen on the same cell preparation.
TL;DR: Cell electroporation is used with great success to permeabilize Chinese hamster ovary cells for the introduction of restriction enzymes, resulting in extremely high frequencies (greater than 90%) of aberrant metaphase cells and also a dramatic decrease in cell survival, as measured by subsequent colony formation.
Abstract: Restriction endonucleases that make either blunt- or cohesive-end DNA double-strand breaks can induce chromosome aberrations. We have used cell electroporation with great success to permeabilize Chinese hamster ovary cells for the introduction of restriction enzymes. The introduction of restriction enzymes by this method resulted in extremely high frequencies (> 90%) of aberrant metaphase cells and also a dramatic decrease in cell survival, as measured by subsequent colony formation. Cell electroporation by itself caused no increase in aberrant chromosomes and had only a slight effect on cell survival.
TL;DR: It is suggested that the IE1 acidic nuclear phosphoprotein displays some properties similar to those of the EBNA-1 protein of Epstein-Barr virus and may potentially play a role in maintenance of the latent state of HCMV DNA.
Journal Article•
TL;DR: Results demonstrate that NVB is as active on mitotic microtubules and less active on axonal micro Tubules than vincristine and vinblastine, and can be related to the potent antitumor effect of the drug with minor neurotoxicity.
TL;DR: A genetic locus rough deal is described in Drosophila melanogaster, identified by mutations that interfere with the faithful transmission of chromosomes to daughter cells during mitosis, suggesting that the reduced fidelity of chromosome transmission to the daughter cells is due to a failure in a mechanism involved in assuring the proper release of sister chromatids.
Abstract: We describe a genetic locus rough deal (rod) in Drosophila melanogaster, identified by mutations that interfere with the faithful transmission of chromosomes to daughter cells during mitosis. Five mutant alleles were isolated, each associated with a similar set of mitotic abnormalities in the dividing neuroblasts of homozygous mutant larvae: high frequencies of aneuploid cells and abnormal anaphase figures, in which chromatids may lag, form bridges, or completely fail to separate. Surviving homozygous adults are sterile, and show cuticular defects associated with cell death, i.e., roughened eyes, sparse abdominal bristles, and notched wing margins. The morphological process of spermatogenesis is largely unaffected and motile sperm are produced, but meiocyte aneuploidy is common. The nature of the observed abnormalities in mitotic cells suggests that the reduced fidelity of chromosome transmission to the daughter cells is due to a failure in a mechanism involved in assuring the proper release of sister chromatids.
TL;DR: Nuclear transplantation showed that 16-cell stage porcine nuclei, which are lamin A/C negative, acquired the A/ C epitope after transfer to an enucleated metaphase II oocyte, suggesting that the A-C epitope is developmentally regulated.
Abstract: The nuclear lamins, proteins that reside on the inner face of the nuclear envelope, are thought to provide attachment sites for anchoring the chromatin to the nuclear envelope, thus facilitating the overall organization of the nucleus. The composition of the nuclear lamin proteins changes during differentiation and development in a variety of mammalian and nonmammalian tissues. Bovine and porcine oocytes and early embryos were prepared for immunocytochemical detection of nuclear lamins using three different antibodies (recognizing lamin B, lamins A/B/C, or lamins A/C). In both species, germinal vesicle nuclei and early cleavage stage nuclei react positively with the antibodies. However, on nuclei of bovine embryos, the A/C epitope was not detectable at the 16-cell stage, compact morula, spherical blastocyst, or the chorionic cell nuclei of a Day 35 conceptus, but was detectable on both amniotic and embryonic ectodermal cell nuclei of a Day 35 conceptus. All three antibodies reacted with nuclei from two bovine tissue culture cell lines (bovine embryonic cells and Madin-Darby bovine kidney cells) and one porcine kidney cell line. Nuclei in porcine embryos followed a similar pattern, except the loss of the A/C epitope occurred at the 8-cell stage and the epitope was absent from compact morula and spherical blastocyst stage nuclei. All interphase nuclei in both species reacted with both anti-lamin A/B/C and anti-lamin B antibodies, whereas metaphase chromosomes did not react with any of the lamin antibodies tested. The change in recognizing the lamin epitope occurred one cell cycle after the expected transition from maternal control to zygotic control of development. Nuclear transplantation showed that 16-cell stage porcine nuclei, which are lamin A/C negative, acquired the A/C epitope after transfer to an enucleated metaphase II oocyte. These results suggest that the A/C epitope is developmentally regulated.
TL;DR: The onset of dephosphorylation of the centrosome coincided with the onset of anaphase and continued in the absence of chromosome movement.
Abstract: The cyclical phosphorylation and dephosphorylation of the centrosome during mitosis was analyzed by immunofluorescence methods using the MPM-2 antibody, which reacts with a subset of mitotic phosphoproteins. Quantification of MPM-reactivity indicated that centrosomal phosphorylation attained a maximal level just prior to anaphase onset. This level was maintained in metaphase cells blocked from further mitotic progression with the microtubule depolymerizing agent nocodazole. However, when nocodazole was added to cells that had just initiated anaphase, the level of centrosomal phosphorylation decreased rapidly as in untreated anaphase cells. We conclude that the onset of dephosphorylation of the centrosome coincided with the onset of anaphase and continued in the absence of chromosome movement. Dephosphorylation of MPM-2 reactive phosphoproteins may be taken as a biochemical indicator of anaphase onset.
TL;DR: Quantitative analysis with a charge coupled device image sensor revealed that the bleached zones in anaphase cells showed no fluorescence recovery, suggesting that these kinetochore fiber microtubules of metaphase cells do not turn over.
Abstract: In previous work we injected mitotic cells with fluorescent tubulin and photobleached them to mark domains on the spindle microtubules. We concluded that chromosomes move poleward along kinetochore fiber microtubules that remain stationary with respect to the pole while depolymerizing at the kinetochore. In those experiments, bleached zones in anaphase spindles showed some recovery of fluorescence with time. We wished to determine the nature of this recovery. Was it due to turnover of kinetochore fiber microtubules or of nonkinetochore microtubules or both? We also wished to investigate the question of turnover of kinetochore microtubules in metaphase. We microinjected cells with x-rhodamine tubulin (x-rh tubulin) and photobleached spindles in anaphase and metaphase. At various times after photobleaching, cells were detergent lysed in a cold buffer containing 80 microM calcium, conditions that led to the disassembly of almost all nonkinetochore microtubules. Quantitative analysis with a charge coupled device image sensor revealed that the bleached zones in anaphase cells showed no fluorescence recovery, suggesting that these kinetochore fiber microtubules do not turn over. Thus, the partial fluorescence recovery seen in our earlier anaphase experiments was likely due to turnover of nonkinetochore microtubules. In contrast fluorescence in metaphase cells recovered to approximately 70% the control level within 7 min suggesting that many, but perhaps not all, kinetochore fiber microtubules of metaphase cells do turn over. Analysis of the movements of metaphase bleached zones suggested that a slow poleward translocation of kinetochore microtubules occurred. However, within the variation of the data (0.12 +/- 0.24 micron/min), it could not be determined whether the apparent movement was real or artifactual.
TL;DR: It is concluded that the B chromosomes of C. capillaris are isochromosomes with active rRNA genes located near both telomeres, and both darkly-staining nucleolar constrictions of chromosomes No. 3 and silver deposits at the telomere of Bs.
Abstract: A non-radioactive in situ hybridization method using biotin-labelled rDNA has made it possible to localize rRNA genes not only at the secondary constriction in both homologous chromosomes No. 3 of Crepis capillaris but also in the B chromosomes occurring in the plants employed. Very clear dot-like rDNA signals at the telomeres of both arms were observed in all B chromosomes. Histochemical silver staining, which is indicative of transcriptional activity of rRNA gene clusters, resulted in both darkly-staining nucleolar constrictions of chromosomes No. 3 and silver deposits at the telomeres of Bs. We conclude that the B chromosomes of C. capillaris are isochromosomes with active rRNA genes located near both telomeres.
TL;DR: A technique of in situ hybridization on metaphase chromosomes with biotinylated DNA probes was used to localize unique DNA sequences on chromosomes and allowed a localization of two probes 1.8 and 1.3 kb long.
Abstract: A technique of in situ hybridization on metaphase chromosomes with biotinylated DNA probes is described. This technique was used to localize unique DNA sequences on chromosomes and allowed a localization of two probes 1.8 and 1.3 kb long. The hybridization signal appears like two, twin, spots on the two sister chromatids, allowing a clear distinction from the background. Moreover a chromosomal localization is possible by counting a relatively small number of mitoses compared with the technique using 3H-labeled DNA probes.
TL;DR: Centromere arrangement in interphase and metaphase cells of two human tumour cell lines was analysed using anti-kinetochore antibodies as immunofluorescent probes and centromeres showed a strong tendency to associate with each other.
Abstract: Centromere arrangement in interphase and metaphase cells of two human tumour cell lines was analysed using anti-kinetochore antibodies as immunofluorescent probes. In GLC1 interphase nuclei, kinetochores were non-randomly positioned around the nucleolus and close to the nuclear membrane. During S and early G2 phase, necklace-like strands of kinetochores were formed in the centre of the nucleus. The duplication of sister kinetochores during the G2 phase was not synchronized. At late G2 phase, a relatively random topological distribution of centromeres was observed with short linear arrays of sister kinetochores. Carefully spread metaphase plates of MDA-MB231 cells generally exhibited a linear alignment of centromeres and large centromeric clusters. In completely pulverized MDA-MB231 cells, centromeres showed a strong tendency to associate with each other.
TL;DR: The nature of oocyte cytoplasmic factors controlling the development of the male pronucleus was investigated by inseminating human, zona-free oocytes at metaphase of the 1st and 2nd meiotic division, and the later stages of pronuclear development were particularly sensitive to polyspermy.
Abstract: The nature of oocyte cytoplasmic factors controlling the development of the male pronucleus was investigated by inseminating human, zona-free oocytes at metaphase of the 1st and 2nd meiotic division. Oocytes at metaphase of the 2nd meiotic division could support the full structural and functional development of male pronuclei, whereas the vast majority of those at metaphase of the 1st meiotic division failed to do so. This suggests that oocyte cytoplasmic factors required for male pronuclear formation do not develop fully until the oocyte reaches the 2nd meiotic metaphase. When increasing numbers of spermatozoa entered one oocyte, the transformation of sperm nuclei into pronuclei was impaired progressively. The later stages of pronuclear development were particularly sensitive to polyspermy. These factors should be taken into consideration in the development of techniques of micromanipulation-assisted insemination.
Journal Article•
TL;DR: The fine structure distribution of polypeptides associated with these RNP complexes during interphase and mitosis in mammalian tissue culture cells using immunoelectron microscopy is determined.
TL;DR: It is demonstrated that trisomic mosaicism may not be a rare event in human pre-embryos, and further evidence is provided that mitotic non-disjunction is important for the production of aberrations in humanPre-Embryos.
Abstract: Previous studies have reported chromosome aberrations in human pre-embryos after in-vitro fertilization (IVF). Although the reason for these abnormalities is not clear, there is evidence that they can arise during gametogenesis, fertilization or cleavage. The present study has examined further the incidence of chromosome abnormalities in human pre-embryos after IVF, using oocytes recovered from normal volunteer women and from women undergoing infertility treatment in an embryo-replacement programme. Chromosome preparations were performed for 75 pre-embryos. Of these 35 (47%) gave at least one metaphase in which analysis was possible. The overall incidence of abnormal pre-embryos was 40% (14/35). The absolute frequency of aberrations was 9% for trisomies, 3% for polyploidies, 26% for structural anomalies and 3% for hypodiploidies. Five pre-embryos were found to be mosaics, three of which had each one trisomic metaphase. In five of the pre-embryos multiple anomalies were found. In 13 of the 14 abnormal pre-embryos the aberrations were found in only one metaphase. The present study demonstrates that trisomic mosaicism may not be a rare event in human pre-embryos. Further evidence is provided that mitotic non-disjunction is important for the production of aberrations in human pre-embryos.
TL;DR: The gene for human complement component C9 has been mapped to chromosome 5 by using a novel application of the polymerase chain reaction to amplify specifically the human C9 gene on a background of rodent DNA in somatic cell hybrids.
TL;DR: This histopathological and cytogenetic study of 13 patients with NHL shows that the higher numbers of interphase NORs in the high‐grade tumours is not necessarily a reflection of increased numbers of NOR‐bearing chromosomes.
Abstract: The ribosomal genes (located on the acrocentric chromosomes 13-15, 21-22) may be identified by their silver stained gene products, i.e. NOR related proteins. The NOR bearing chromosome activity can be observed at metaphase with the potential for all ten chromosomes to be positively stained. On the other hand, during interphase they fuse so that eventually only a single silver positive structure is seen in resting normal cells. Investigations of histopathological sections of non-Hodgkin's lymphoma (NHL) have demonstrated a correlation between the numbers of interphase NORs and the grade of tumour. There is generally a higher number of interphase in high-grade, and a lower number in low-grade tumours. This histopathological and cytogenetic study of 13 patients with NHL shows that the higher numbers of interphase NORs in the high-grade tumours is not necessarily a reflection of increased numbers of NOR-bearing chromosomes. Examples were found of high-grade neoplasms, showing the expected high numbers of interphase NORs, but not an increased number of NOR-bearing chromosomes. Conversely, some low-grade tumours, with the expected low number of interphase NORs, had increased numbers of NOR-bearing chromosomes. Our conclusion is that the interphase NOR number is related to factors other than chromosome numbers. We suggest that NOR numbers at interphase may be related to cell turnover. This is supported by previous investigations using DNA flow cytometry and the monoclonal antibody Ki67.