SEARCH · PubMed Health

Results for “Segregation”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 181 records · Page 10Linked to original sources

PubMed2004

Potential roles for centromere pairing in meiotic chromosome segregation.

One of the key differences between mitosis and meiosis is the necessity for exchange between homologous chromosomes. Crossing-over between homologous chromosomes is essential for proper meiotic chromosome segregation in most organisms, serving the purpose of linking chromosomes to their homologous partners until they segregate from one another at anaphase I. In several organisms it has been shown that occasional pairs of chromosomes that have failed to experience exchange segregate with reduced fidelity compared to exchange chromosomes, but do not segregate randomly. Such observations support the notion that there are mechanisms, beyond exchange, that contribute to meiotic segregation fidelity. Recent findings indicate that active centromere pairing is important for proper kinetochore orientation and consequently, segregation of non-exchange chromosomes. Here we discuss the implications of these findings for the behavior of meiotic chromosomes.

Animals↗

PubMed1998

Scatter factor induces segregation of multinuclear cells into several discrete motile domains.

The effects of scatter factor, HGF/SF, on multinuclear MDCK epitheliocytes were examined. Multinuclear cells were obtained by blocking cytokinesis by low concentration of cytochalasin D; these large cells had discoid shape and did not move much on the substrate. Incubation of these cells with HGF/SF induced their profound reorganization: their cytoplasm was reversibly segregated into several individually moving motile flattened domains, termed lamelloplasts and connected with one another by cylindrical domains termed cables. One or several nuclei were present in many lamelloplasts, but some lamelloplasts were anuclear. Nuclei were absent from the cables. Lamelloplasts continuously formed actin-rich ruffles at their edges; their cytoplasm contained small actin bundles and numerous focal adhesions. In contrast, cable, had no ruffles or focal adhesions. Dense networks of vimentin and keratin intermediate filaments were present in lamelloplasts; bundles of filaments of both types were seen in the cables. Segregation was accompanied by redistribution of centrosomes from perinuclear zone into lamelloplasts. As a result each lamelloplast in segregated cell acquired individual complex of centrosome and radiating microtubules. The cables contained numerous parallel microtubules but never had centrosomes. This reorganization of microtubular system was essential for segregation as alterations of shape and actin cytoskeleton were prevented by microtubule specific drugs: colcemid and Taxol (paclitaxel). It is suggested that mechanism of segregation is based on activation of two types of opposite actin reorganization: formation of actin networks in lamelloplasts and their dismantlement in the cables. Spatial distribution of the domains in which these opposite types of reorganizations occur may be regulated by microtubular system. It is also suggested that mechanisms of HGF/SF-induced segregation may be closely related to the mechanisms of important physiological reorganizations of cells, such as polarization of pseudopodial activities in motile cells and cytokinesis.

Animals↗

PubMed1988

Role of visual experience in promoting segregation of eye dominance patches in the visual cortex of the cat.

Transneuronal autoradiography was used to study the role of visual experience in the development of ocular dominance patches in the cat. In order to assess quantitatively the effects of visual deprivation, image analysis was used to measure the profiles of grain density in layer IV. Fourier power spectra of these profiles were computed to give a measure of the amplitudes and frequencies of the fluctuations in grain density that were present. Deprivation of normal patterned vision by binocular lid suture or by rearing in total darkness from shortly after birth abolished the dominant periodicity (of about 1.1 mm) in the distribution of left and right eye afferents in layer IV of area 17. A dominant periodicity of about 2.2 mm was, however, present in area 18 of both normal and dark-reared animals. Visual deprivation was not able to reverse segregation. One animal reared normally for 6 weeks was placed in the dark for a further 28 weeks and showed normal periodicities in the distribution of geniculate inputs to area 17. Another animal given 128 hours of experience and kept in the dark for the rest of the time until it was 12 weeks old also showed normal segregation. To determine the minimum amount of visual experience necessary for segregation to occur, four animals were given 8-, 24-, 48-, and 128-hour periods of visual experience and were studied at 12 weeks of age. Eight hours of experience had no detectable effect on segregation; periodicities of intermediate amplitude were present in animals that received 24 and 48 hours of experience, while 128 hours of experience resulted in periodicities of normal amplitude. Recovery from visual deprivation was studied by rearing kittens from birth in the dark for varying periods and then returning them to the normally lit colony room for periods of 6 to 22 weeks. Recovery from 6 weeks of dark rearing was found to be complete; much less recovery occurred following periods of 8 to 25 weeks of initial deprivation, and no recovery at all occurred after 30 weeks of deprivation. It is concluded that the spontaneous activity present in the geniculocortical afferents of dark-reared and lid-sutured cats is not adequate to drive normal periodic segregation in area 17, though it can do so in area 18. Between 48 and 128 hours of visual experience, given before 8 weeks of age, appears to be necessary and sufficient for normal periodic segregation of geniculate afferents in area 17 of the cat.

Animals↗

PubMed1991

Loss of grain boundary segregant during ion milling.

It is shown that material segregated to grain boundaries can be lost during ion milling. This specimen preparation artifact has been studied in the case of bismuth in copper and has also been observed for phosphorus in stainless steel. The loss is associated with specimen heating during ion milling and can be alleviated by good clamping and cooling of the specimen during milling. Specimen heating permits grain boundary diffusion of the segregating element to the specimen surfaces with subsequent loss of segregant from the specimen by evaporation or sputtering during ion milling. Loss of bismuth during in situ heating to 200-300 degrees C is demonstrated. Therefore, care must be taken in specimen preparation for analytical electron microscopy measurement of such segregation. Similar effects may occur during ion milling of other materials, especially those where low thermal conductivity will result in high beam heating. In these cases, care must be taken to avoid loss of segregant during specimen preparation. Additional tests showed that no significant loss of segregant was observed during X-ray microanalysis, even at nominal room temperature and probe currents five-fold higher than that normally used for microanalysis.

Bismuth↗

PubMed1998

Evolution of segregation distortion: potential for a high degree of polymorphism.

By means of a population genetical model, we study the evolution of segregation distortion. Most models of segregation distortion focus on a single distorter allele. In contrast, we consider the competition between a large number of distorters. Motivated by systems as the t complex of the house mouse or the Sd complex of Drosophila melanogaster, we assume that there is some "complementation" between distorter alleles, i.e. that the fitness of individuals heterozygous for two distorter alleles is higher than the fitness of homozygous individuals. In the presence of complementation, the most efficient distorter allele with the highest segregation ratio often does not outcompete less efficient distorters. In fact, our results show that coexistence of a large number of distorter alleles is more typical than the competitive exclusion of less efficient distorters by a single superior allele. We first consider the analytically tractable system where all distorters show the same amount of complementation. In this case, all distorters with a segregation ratio higher than a certain critical value will persist, resulting in a polymorphic population where the average segregation ratio is only slightly larger than 0.5. If the degree of complementation varies, there may be more than one stable equilibrium, and the outcome of competition may depend on the initial conditions. Motivated by empirical examples, we also consider the case that the distorting ability of an allele is negatively related to its effects on individual fitness. Interestingly, the outcome of competition depends crucially on details of such a trade-off. We conclude that verbal arguments are insufficient to predict the evolution of segregation distortion.

Alleles↗

PubMedDate not supplied

Mitochondrial gene segregation in mammals: is the bottleneck always narrow?

The segregation of a heteroplasmic silent polymorphism in the mitochondrial ND6 gene has been followed in a human maternal lineage comprising eight individuals and spanning three generations. Heteroplasmy persisted in all eight maternally related family members. More importantly, the frequencies of the two alleles showed relatively little variation among individuals or between generations. In contrast to the findings in other mammalian lineages, the present results indicate relatively slow mitochondrial gene segregation. A narrow bottleneck in the number of mitochondrial DNA (mtDNA) molecules, which occurs at some stage of oogenesis, has been advanced to explain rapid mammalian mitochondrial gene segregation. It is suggested here that the segregation of mitochondrial genes may be more complex than initially envisaged, and that models need to be developed that account for both rapid and slow segregation. One possibility, which reconciles both physical and genetic studies of mammalian mtDNA, is that the unit of mitochondrial segregation is the organelle itself, each containing multiple mtDNA molecules.

Alleles↗

PubMed1988

Human allospecific TLCs generated against HLA antigens associated with DR1 through DRw8. III. Family segregation analyses.

We have studied the complexity and fine specificity of the HLA-D region using a panel of T lymphocyte clones generated against alloantigens associated with HLA-DR1 through DRw8. After extensive testing in population studies, 89 clones were tested in proliferation assays with 14 families. Segregation patterns were analyzed for haplotype associations by calculating sequential lod scores to test the likelihood that genes encoding epitopes detected by TLCs were linked to HLA genes. Four general categories were identified: (1) clonal responses that segregated with the same HLA-D region haplotype in all informative pedigrees; (2) clonal responses that segregated with HLA in all pedigrees but not always with the same haplotype; (3) clonal responses that segregated with HLA in some families but failed to segregate in others or produced equivocal results; (4) clonal responses that did not segregate with HLA haplotypes.

Cell Line↗

PubMed2003

Transgressive segregation of erucic acid content in Brassica carinata A. Braun.

Two Ethiopian mustard ( Brassica carinata A. Braun) lines with low (about 10%) and zero erucic acid (C22:1) have been obtained. The low C22:1 mutant line L-2890 was isolated after a chemical-mutagen treatment of C-101 seeds (about 40% C22:1). The zero C22:1 line L-25X-1 was obtained by interspecific crossing. Our objective was to determine the genetic control of low and zero C22:1 contents in these lines and the relationship between the loci controlling these traits. Reciprocal crosses between L-2890, L-25X-1 and high C22:1 lines, and between L-2890 and L-25X-1, were made. The F(1), F(2) and BC(1) F(1) generations were obtained. No maternal or cytoplasmic effects for C22:1 content were observed in any of the crosses. The analysis of the fatty acid composition in the segregating populations from the crosses of L-2890 with the high C22:1 lines C-101 and L-1630 indicated that the segregation patterns fitted a model of two alleles at two loci, M1 and M2, with partial (near complete) dominance for high concentration. The segregation patterns in the cross of the zero C22:1 line L-25X-1 with the high C22:1 line L-1630, were explained on the basis of two genes, E1 and E2, with additive gene action. The F(1) and segregating generations of the crosses L-2890 x L-25X-1 showed a strong transgressive segregation with C22:1 values of up to 50.0%, four-fold higher than those of L-2890. The analyses of the F(2), BC(1)F(1) and F(3) generations indicated that the combination of alleles at four loci, M(1) and M(2) in L-2890 and E(1) and E(2) in L-25X-1, controlled the transgressive segregation for C22:1. The proposed genotypes (C22:1 content) for each parent were as follows: L-2890 (10% C22:1) = m(1) m(1) m(2) m(2) E(1) E(1) E(2) E(2); L-25X-1 (0% C22:1) = M(1) M(1) M(2) M(2) e(1) e(1) e(2) e(2); and C-101 (45% C22:1) = M(1) M(1) M(2) M(2) E(1) E(1) E(2) E(2).

Brassica↗

PubMed2002

Cellular autophagic capacity changes during azaserine-induced tumour progression in the rat pancreas. Up-regulation in all premalignant stages and down-regulation with loss of cycloheximide sensitivity of segregation along with malignant transformation.

The knowledge of alterations in regulation of autophagy during tumorigenesis may also help our understanding of its normal control. We established an experimental system and reported recently that autophagic capacity, measured as the cell's capability of increasing segregation (formation of autophagosomes) and subsequent degradation of cytoplasmic quanta were highly increased in premalignant nodule cells 6 months after initiation by azaserine in the rat pancreas in vivo. In the present study, we followed changes of these autophagic functions throughout the tumour progression. We carried out electron-microscopic morphometrical analysis of the expansion of autophagic vacuole compartment and subcompartments induced by vinblastine (an in vivo segregation enhancer), as well as their regression upon segregation-inhibitor cycloheximide post-treatment. Premalignant tumour samples were taken at month 5, month 8 (nodules), month 10 and month 15 (adenomas) after initiation. In all these stages, a highly increased and varying autophagic capacity was found compared with the host tissue. The basal (non-stimulated) autophagic compartment was measurable only at month 5 and month 15, and its regression upon cycloheximide was consistent with increased basal autophagic activity. Compared with the host tissue, autophagic capacity profoundly decreased in the differentiated and anaplastic adenocarcinomas at month 20, when, surprisingly, cycloheximide was unable to inhibit segregation. Our conclusion is that down-regulation of the cycloheximide sensitive segregation and a partly compensatory up-regulation of an alternative pathway of segregation might occur along with malignant transformation.

Adenocarcinoma↗

PubMed1988

Polarization of ooplasmic segregation and dorsal-ventral axis determination in ascidian embryos.

During ooplasmic segregation in ascidians, the myoplasm moves from its original location in the periphery of the unfertilized egg to the vegetal pole of the zygote. The vegetal cap of myoplasm marks the future site of gastrulation and the dorsal side of the embryo. The purpose of this investigation was to determine the mechanism for polarizing the myoplasm during ooplasmic segregation. To test the possibility that the myoplasm moves toward the sperm and that the vegetal pole is the exclusive site of sperm entry, we examined fertilization in egg fragments of the ascidians Styela plicata and Ciona savignyi. Similar frequencies of fertilization were exhibited by various egg fragments, including animal and vegetal fragments or multiple fragments prepared from the same egg. These results indicate that sperm do not enter the egg exclusively at the vegetal pole. Experiments with egg fragments and constricted eggs, combined with chalk marking of the animal pole, demonstrated that after fertilization the myoplasm segregates parallel to the animal-vegetal axis, usually toward the vegetal end of the cell. Activation of primary oocytes with the Ca2+ ionophore A23187 caused the myoplasm to segregate independently of the animal-vegetal axis. This confirms previous experiments in which eggs aligned along a glass fiber coated with A23187 segregated their myoplasm toward the fiber (W.R. Jeffery, 1982, Science 216, 545-547) and suggests that the intrinsic cue for polarization is a release of sequestered Ca2+ at fertilization. Therefore, it appears that ooplasmic segregation and the dorsal-ventral axis are polarized by maternal factors distributed in a concentration gradient along the animal-vegetal axis of the ascidian egg.

Animals↗

PubMed1996

Perceived texture segregation in chromatic element-arrangement patterns: high intensity interference.

An element-arrangement pattern is composed of two types of elements that differ in the ways in which they are arranged in different regions of the pattern. We report experiments on the perceived segregation of chromatic element-arrangement patterns composed of equal-size red and blue squares as the luminances of the surround, the interspaces and the background (surround plus interspaces) are varied. Perceived segregation was markedly reduced by increasing the luminance of the interspaces. Perceived segregation was approximately constant for constant ratios of interspace luminance to square luminance and increased with the contrast ratio of the squares. Unlike achromatic element-arrangement patterns composed of squares differing in lightness [Beck et al (1991). Vision Research, 32, 719-743] perceived segregation did not decrease when the luminance of the interspaces was below that of the squares. Similar results were obtained for red and yellow, red and green, green and yellow, green and blue, and blue and yellow squares. Perceived segregation based on edge alignment was not interfered with by high intensity interspaces. Stereoscopic cues that caused the squares composing the element-arrangement pattern to be seen in front of the interspaces did not greatly improve perceived segregation. One explanation of the results is in terms of inhibitory interactions among achromatic and chromatic cortical cells tuned to spatial frequency and orientation. Alternately, the results may be explained in terms of how the luminance of the interspaces affects the grouping of the squares for encoding surface representations. Neither explanation accounts fully for the data and both mechanisms may be involved.

Color Perception↗

PubMed2005

Racial residential segregation and geographic heterogeneity in black/white disparity in poor self-rated health in the US: a multilevel statistical analysis.

Existing evidence demonstrating a relationship between racial residential segregation and health has been based on aggregate analysis. Using a multilevel analytical framework, we assess the extent of geographic variation in black/white disparities in self-rated health across US metropolitan areas, and whether racial residential segregation accounts for such variation. We estimated multilevel regression models of poor self-rated health among 51,316 non-Hispanic white and non-Hispanic black adults nested within 207 metropolitan areas to assess the multilevel relationship between segregation and racial disparities in health. We found statistically significant variation in the black/white disparity in poor self-rated health across metropolitan areas, after controlling for individual level factors (age, sex, marital status, education and income) and residential segregation. High black isolation was associated with increased odds of reporting poor health among blacks (p<0.05). While a similar pattern was observed for white/black dissimilarity and white isolation, they were not statistically significant. Our multilevel analysis only partially supports the previously reported aggregate findings linking segregation to health. Additional multilevel statistical investigations across different health outcomes are required to draw firmer conclusions regarding the adverse effects of segregation on health.

Black or African American↗

PubMed2004

Figure-ground segregation can rely on differences in motion direction.

If the elements within a figure move synchronously while those in the surround move at a different time, the figure is easily segregated from the surround and thus perceived. Lee and Blake (1999) [Visual form created solely from temporal structure. Science, 284, 1165-1168] demonstrated that this figure-ground separation may be based not only on time differences between motion onsets, but also on the differences between reversals of motion direction. However, Farid and Adelson (2001) [Synchrony does not promote grouping in temporally structured displays. Nature Neuroscience, 4, 875-876] argued that figure-ground segregation in the motion-reversal experiment might have been based on a contrast artefact and concluded that (a)synchrony as such was 'not responsible for the perception of form in these or earlier displays'. Here, we present experiments that avoid contrast artefacts but still produce figure-ground segregation based on purely temporal cues. Our results show that subjects can segregate figure from ground even though being unable to use motion reversals as such. Subjects detect the figure when either (i) motion stops (leading to contrast artefacts), or (ii) motion directions differ between figure and ground. Segregation requires minimum delays of about 15 ms. We argue that whatever the underlying cues and mechanisms, a second stage beyond motion detection is required to globally compare the outputs of local motion detectors and to segregate figure from ground. Since analogous changes take place in both figure and ground in rapid succession, this second stage has to detect the asynchrony with high temporal precision.

Artifacts↗

PubMed2002

Segregation of photosystems in thylakoid membranes as a critical phenomenon.

The distribution of the two photosystems, PSI and PSII, in grana and stroma lamellae of the chloroplast membranes is not uniform. PSII are mainly concentrated in grana and PSI in stroma thylakoids. The dynamics and factors controlling the spatial segregation of PSI and PSII are generally not well understood, and here we address the segregation of photosystems in thylakoid membranes by means of a molecular dynamics method. The lateral segregation of photosystems was studied assuming a model comprising a two-dimensional (in-plane), two-component, many-body system with periodic boundary conditions and competing interactions between the photosystems in the thylakoid membrane. PSI and PSII are represented by particles with different values of negative charge. The pair interactions between particles include a screened Coulomb repulsive part and an exponentially decaying attractive part. The modeling results suggest a complicated phase behavior of the system, including quasi-crystalline phase of randomly distributed complexes of PSII and PSI at low ionic screening, well defined clustered state of segregated complexes at high screening, and in addition, an intermediate agglomerate phase where the photosystems tend to aggregate together without segregation. The calculations demonstrated that the ordering of photosystems within the membrane was the result of interplay between electrostatic and lipid-mediated interactions. At some values of the model parameters the segregation can be represented visually as well as by analyzing the correlation functions of the configuration.

Biophysical Phenomena↗

PubMed2003

Feature-specific electrophysiological correlates of texture segregation.

Discrimination between a figure and its surround is an important first step of pattern recognition. This discrimination usually relies, as a first step, on the detection of borders between a figure and its surround, for example based on spatial gradients in luminance, colour, or texture. There is evidence that neurones in the visual cortex are specifically activated by segregation between textures, but the relation between segregation based on different types of features such as colour, luminance, and motion is unclear. Evoked EEG potentials specific to texture segregation were investigated in 17 observers in two separate experiments and by means of functional magnetic resonance imaging in a separate study (Fahle et al., in preparation). Differences in either luminance, colour, line orientation, motion, or stereoscopic depth defined a checkerboard pattern. Patterns defined by each of these features elicited segregation-specific potentials. In contrast to earlier reports (Vision Research 37 (1997) 1409), however, we find pronounced differences between the segregation-specific potentials evoked through different features, especially regarding their peak latencies. The topographical distribution of the activity evoked reveals different polarities and partly specific locations for different stimulus features, indicating the existence of different processors for texture segregation based on different features.

Adult↗

PubMed2001

Subnanometer three-dimensional atom-probe investigation of segregation at MgO/Cu ceramic/metal heterophase interfaces.

Three-dimensional atom-probe (3DAP) microscopy has been applied to the study of segregation at ceramic/metal (C/M) interfaces. In this article, results on the MgO/Cu(X) (where X = Ag or Sb) systems are summarized. Nanometer-size MgO precipitates with atomically clean and atomically sharp interfaces were prepared in these systems by internal oxidation. Segregation of the ternary component (Ag or Sb) at the MgO/Cu heterophase interface was enhanced by extended low-temperature anneals. Magnesia precipitates in the 3DAP reconstructions were delineated as isoconcentration surfaces, and segregation of each ternary component at the C/M interfaces was analyzed with the proximity histogram method developed at Northwestern University. This method allows the direct extraction of the Gibbsian interfacial excess of solute at the C/M interfaces from the experimental data. A value of (3.2+/-2.0) x 10(17)m(-2) at 500 degrees C is obtained for the segregation of Ag at a MgO/Cu(Ag) interface, while a value of (2.9+/-0.9) x 10(18) m(-2) at 500 degrees C is obtained for the segregation of Sb at a MgO/Cu(Sb) interface. The larger Gibbsian excess for Sb segregation at this ceramic/metal heterophase interface is most likely due to the so-called pdeltaV effect.

Journal Article↗

PubMed1993

Parental nucleosomes segregated to newly replicated chromatin are underacetylated relative to those assembled de novo.

Antibodies specific for acetylated histone H4 were used to examine the acetylation state of parental histones that segregate to newly replicated DNA. To generate newly replicated chromatin containing only segregated parental nucleosomes, isolated nuclei were labeled with [3H]TTP in vitro; alternatively, whole cells were labeled with [3H]thymidine in the presence of cycloheximide. Soluble chromatin was prepared by micrococcal nuclease digestion, and subjected to immunoprecipitation with "penta" antibodies (Lin et al., 1989). In sharp contrast to nucleosomes containing newly synthesized, diacetylated H4 (Perry et al., 1993), chromatin replicated in vitro was only marginally susceptible to immunoprecipitation. Control experiments established that bona fide acetylated chromatin was selectively immunoprecipitated by the same techniques and that segregated nucleosomes were not disassembled during treatment with "penta" antibodies. When replication was coupled to an in vitro histone acetylation system, the enrichment for segregated nucleosomes in the immunopellet increased approximately 3-fold, demonstrating that changes in the acetylation state of segregated histones can be detected immunologically and that parental histones on new DNA are accessible to acetyltransferases during, or immediately after, DNA replication. In vivo pulse-chase experiments, performed in the presence of cycloheximide, confirmed these results. Uptake experiments further established that concurrent histone acetylation did not alter the rate of DNA synthesis in vitro. Our results provide evidence that replication-competent chromatin is not obligatorily acetylated, and indicate that the acetylation status of segregated histones may be maintained during chromatin replication. The possible significance of this, with respect to the regulation of chromatin higher order structures during DNA replication, and the propagation of transcriptionally active vs inactive chromatin structures, is discussed.

Acetylation↗

PubMed2004

High frequency, synchronized bursting drives eye-specific segregation of retinogeniculate projections.

Blockade of retinal waves prevents the segregation of retinogeniculate afferents into eye-specific layers in the visual thalamus. However, the key features of retinal waves that drive this refinement are controversial. Some manipulations of retinal waves lead to normal eye-specific segregation but others do not. By comparing retinal spiking patterns in several mutant mice with differing levels of eye-specific segregation, we show that the presence of high-frequency bursts synchronized across neighboring retinal ganglion cells correlates with robust eye-specific segregation and that the presence of high levels of asynchronous spikes does not inhibit this segregation. These findings provide a possible resolution to previously described discrepancies regarding the role of retinal waves in retinogeniculate segregation.

Action Potentials↗