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PubMed1990

Gas release in comet nuclei.

The evolution of a comet nucleus is investigated, taking into account the crystallization process by which the gas trapped in the ice is released to flow through the porous ice matrix. The equations of conservation of the energy and of the masses of ice and gas are solved throughout the nucleus, to obtain the evolution of the temperature, gas pressure and density profiles. A spherical nucleus composed of cold, porous amorphous ice, with 10% of CO trapped in it, serves as initial model. Several values of density (porosity) and pore size are considered. For each combination of parameters the model is evolved for 20-30 revolutions in comet P/Halley's orbit. Two aspects of the release of gas upon crystallization are analyzed and discussed: (a) the resulting continuous outward flux with high peaks at the time of crystallization, which is a cyclic process in the low-density models and sporadic in the high-density ones; (b) the internal pressures obtained down to depths of a few tens to approximately 200 m (depending on parameters), that are found to exceed the compressional strength of cometary ice. As a result, both cracking and explosions of the overlying ice layer and ejection of gas and ice/dust grains are expected to follow crystallization. They should appear as outbursts or sudden brightening of the comet. The model of 0.2 g cm-3 density is found to reproduce quite well many of the light-curve and activity characteristics of comet P/Halley.

Astronomical Phenomena↗
PubMed2003

Myosin motors and not actin comets are mediators of the actin-based Golgi-to-endoplasmic reticulum protein transport.

We have previously reported that actin filaments are involved in protein transport from the Golgi complex to the endoplasmic reticulum. Herein, we examined whether myosin motors or actin comets mediate this transport. To address this issue we have used, on one hand, a combination of specific inhibitors such as 2,3-butanedione monoxime (BDM) and 1-[5-isoquinoline sulfonyl]-2-methyl piperazine (ML7), which inhibit myosin and the phosphorylation of myosin II by the myosin light chain kinase, respectively; and a mutant of the nonmuscle myosin II regulatory light chain, which cannot be phosphorylated (MRLC2(AA)). On the other hand, actin comet tails were induced by the overexpression of phosphatidylinositol phosphate 5-kinase. Cells treated with BDM/ML7 or those that express the MRLC2(AA) mutant revealed a significant reduction in the brefeldin A (BFA)-induced fusion of Golgi enzymes with the endoplasmic reticulum (ER). This delay was not caused by an alteration in the formation of the BFA-induced tubules from the Golgi complex. In addition, the Shiga toxin fragment B transport from the Golgi complex to the ER was also altered. This impairment in the retrograde protein transport was not due to depletion of intracellular calcium stores or to the activation of Rho kinase. Neither the reassembly of the Golgi complex after BFA removal nor VSV-G transport from ER to the Golgi was altered in cells treated with BDM/ML7 or expressing MRLC2(AA). Finally, transport carriers containing Shiga toxin did not move into the cytosol at the tips of comet tails of polymerizing actin. Collectively, the results indicate that 1) myosin motors move to transport carriers from the Golgi complex to the ER along actin filaments; 2) nonmuscle myosin II mediates in this process; and 3) actin comets are not involved in retrograde transport.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1996

Analysis of DNA alkylation damage and repair in mammalian cells by the comet assay.

The single cell gel electrophoresis (SCGE) or comet assay, which measures DNA strand breaks in individual cells, was used to analyse DNA damage and repair induced by the SN1-type alkylating carcinogens N-ethyl-N'-nitro-N-nitrosoguanidine and N-ethyl-N-nitrosourea in CHO cells. The comet assay was comparable in sensitivity to the alkaline elution assay. The alkyl-adducts detected as DNA single-strand breaks (ssb) by this technique were completely repaired within 24 h after treatment. These data indicate that long-lived lesions, such as alkylphosphotriesters, are not converted into ssb under the standard SCGE alkaline conditions (pH 13.5). The lesions revealed by the comet assay are mainly apurinic/apyrimidinic (AP) sites and breaks formed as intermediates in the base excision repair process of N-alkylpurines. When SCGE was performed at pH 12.5 instead of pH 13.5 a lower level of ssb was detected and these breaks were completely resealed within 2 h after treatment. These data suggest that different subsets of lesions are detected under different pH conditions. The SCGE combined with inclusion within the cells of endonuclease III revealed that a high portion of AP sites induced by alkylation damage were not converted into ssb by alkali. The level of endonuclease III-sensitive sites decreased as a function of the repair time and by 24 h after treatment no sites were left on the DNA. The use of this modified SCGE assay allows the estimation of the total amount of unrepaired AP sites present on DNA. Alkylation-induced ssb as detected by the comet assay should be regarded as an indicator of repair rate and balance more than a measure of actual DNA damage.

Alkylation↗
PubMed1996

Cytogenetic studies of PCB77 on brown trout (Salmo trutta fario) using the micronucleus test and the alkaline comet assay.

Polychlorinated biphenyls (PCBs) are stable pollutants, which can be found in almost every compartment of terrestrial and aquatic ecosystems. They are very lipophilic and therefore have the potency of accumulating in the fat stores of animals. The mechanisms by which PCBs exert their adverse effects are still unclear. It is known that PCBs induce some important biotransformation enzymes, but their mutagenic properties are still controversial. The DNA breakage and clastogenic potency of a planar PCB77 (3,3',4,4'-tetrachlorobiphenyl) was determined in vivo in fish, using the single cell gel electrophoresis or comet assay and the micronucleus test, on erythrocytes of the brown trout exposed for 3, 9 and 14 days to initial PCB concentrations of 780 and 918 pg/ml, dissolved in the water. Blood was taken by a caudal puncture and the erythrocytes were either deposited in an agarose gel (0.6%) for the comet assay or smeared directly on slides for the micronucleus test. Five fish were studied per treatment and 50 and 2000 erythrocytes per concentration and per animal were analysed for the comet assay and the micronucleus test respectively. Ethyl methanesulphonate (EMS) at a concentration of 25 mg/l water was used as a positive control. Although EMS induced a statistically significant increase of single strand breaks in the comet assay, in neither of the two tests used, were mutagenic effects due to PCB exposure observed.

Animals↗
PubMed2002

Germ cell and dose-dependent DNA damage measured by the comet assay in murine spermatozoaa after testicular X-irradiation.

The single-cell gel electrophoresis (Comet) assay has been widely used to measure DNA damage in human sperm in a variety of physiological and pathological conditions. We investigated the effects of in vivo radiation, a known genotoxin, on spermatogenic cells of the mouse testis and examined sperm collected from the vas deferens using the neutral Comet assay. Irradiation of differentiating spermatogonia with 0.25-4 Gy X-rays produced a dose-related increase in DNA damage in sperm collected 45 days later. Increases were found when measuring Comet tail length and percentage of tail DNA, but the greatest changes were in tail moment (a product of tail length and tail DNA). Spermatids, spermatocytes, differentiating spermatogonia, and stem cell spermatogonia were also irradiated in vivo with 4 Gy X-rays. DNA damage was indirectly deduced to occur at all stages. The maximum increase was seen in differentiating spermatogonia. DNA damaged cells were, surprisingly, still detected 120 days after stem cell spermatogonia had been irradiated. The distribution of DNA damage among individual sperm cells after irradiation was heterogeneous. This was seen most clearly when changes in the Comet tail length were measured when there were discrete undamaged and damaged populations. After increasing doses of irradiation, an increasing proportion of cells were found in the damaged population. Because a proportion of undamaged sperm cells remains after all but the highest dose, the possibility of normal fertility remains. However, fertilization with a spermatozoa carrying high amounts of DNA damage could lead to effects as diverse as embryonic death and cancer susceptibility in the offspring.

Animals↗
PubMed1997

Antibodies against vaccinia virus do not neutralize extracellular enveloped virus but prevent virus release from infected cells and comet formation.

Vaccinia virus (VV) produces two antigenically and structurally distinct infectious virions, intracellular mature virus (IMV) and extracellular enveloped virus (EEV). EEV is important for the efficient dissemination of virus both in vivo and in vitro where it causes formation of comet-shaped virus plaques. Here, we show that EEV, in contrast to IMV, is resistant to neutralization by antibodies bound to its surface. However, antibodies against EEV can prevent comet formation in cell culture. To explain this apparent paradox, we investigated the mechanism by which antibodies inhibit comet formation and demonstrated that antibodies prevent EEV release from infected cells, and consequently comet formation, by agglutination of the virus on the cell surface. Two complementary observations allow this conclusion: first, electron microscopy showed that infected cells incubated with medium containing anti-vaccinia virus antibodies have virus aggregates on their surface; second, culture medium from these cells contained a 4 log10 fold reduction in the physical particle/ml titre in comparison with control culture. A mechanism by which antibodies to EEV proteins provide immunological protection is thus restriction of EEV release rather than neutralization of free EEV particles.

Agglutination↗
PubMed2001

Organic composition of C/1999 S4 (LINEAR): a comet formed near Jupiter?

In the current paradigm, Oort cloud comets formed in the giant planets' region of the solar nebula, where temperatures and other conditions varied greatly. The measured compositions of four such comets (Halley, Hyakutake, Hale-Bopp, and Lee) are consistent with formation from interstellar ices in the cold nebular region beyond Uranus. The composition of comet C/1999 S4 (LINEAR) differs greatly, which suggests that its ices condensed from processed nebular gas, probably in the Jupiter-Saturn region. Its unusual organic composition may require reevaluation of the prebiotic organic material delivered to the young Earth by comets.

Journal Article↗
PubMed2002

X-ray emission from comets.

The discovery of x-ray emission from comet Hyakutake was surprising given that comets are known to be cold. Observations by x-ray satellites such as the Röntgen Satellite (ROSAT) indicate that x-rays are produced by almost all comets. Theoretical and observational work has demonstrated that charge-exchange collisions of highly charged solar wind ions with cometary neutral species can explain this emission. X-ray observations of comets and other solar system objects may be used to determine the structure and dynamics of the solar wind.

Meteoroids↗
PubMed2003

Doubly ionized carbon observed in the plasma tail of comet Kudo-Fujikawa.

Comet C/2002 X5 (Kudo-Fujikawa) was observed near its perihelion of 0.19 astronomical unit by the Ultraviolet Coronagraph Spectrometer aboard the Solar and Heliospheric Observatory spacecraft. Images of the comet reconstructed from high-resolution spectra reveal a quasi-spherical cloud of neutral hydrogen and a variable tail of C+ and C2+ that disconnects from the comet and subsequently regenerates. The high abundance of C2+ and C+, at least 24% relative to water, cannot be explained by photodissociation of carbon monoxide and is instead attributed to the evaporation and subsequent photoionization of atomic carbon from organic refractory compounds present in the cometary dust grains. This result serves to strengthen the connection between comets and the material from which the Solar System formed.

Carbon↗
PubMed2005

The Kuiper belt and the solar system's comet disk.

Our planetary system is embedded in a small-body disk of asteroids and comets, vestigial remnants of the original planetesimal population that formed the planets. Once formed, those planets dispersed most of the remaining small bodies. Outside of Neptune, this process has left our Kuiper belt and built the Oort cloud, as well as emplacing comets into several other identifiable structures. The orbits in these structures indicate that our outer solar system's comet disk was shaped by a variety of different physical processes, which teach us about how the giant planets formed. Recent work has shown that the scattered disk is the most likely source of short-period comets. Moreover, a growing body of evidence indicates that the sculpting of the Kuiper belt region may have involved large-scale planetary migration, the presence of other rogue planetary objects in the disk, and/or the close passage of other stars in the Sun's birth cluster.

Journal Article↗
PubMed2006

Comet 81P/Wild 2 under a microscope.

The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.

Journal Article↗
PubMed1997

Photometric behavior of comet Hale-Bopp (C/1995 O1) before perihelion.

Narrowband photometric observations of comet Hale-Bopp (C/1995 O1) between 25 July 1995 and 15 February 1997 indicated gas and dust production rates of 20 and 100 times greater, respectively, than observed at the same heliocentric distances for comet P/Halley in 1985. Hale-Bopp produced dust at a rate greater than has been observed for any other comet at any distance since at least 1977. On the basis of the observed production rate of the hydroxyl molecule, the calculated minimum effective diameter of Hale-Bopp's nucleus is 17 kilometers, but the actual diameter of the nucleus is likely to be at least two to three times larger. The chemical composition of Hale-Bopp is consistent with that of other long-period comets originating from the Oort Cloud.

Carbon↗
PubMed1997

Radar detection of the nucleus and coma of comet hyakutake

Radar observations of comet Hyakutake (C/1996 B2) made at the Goldstone Deep Space Communications Complex in California have detected echoes from the nucleus and from large grains in the inner coma. The nucleus of this bright comet was estimated to be only 2 to 3 kilometers in diameter. Models of the coma echo indicate backscatter from porous, centimeter-size grains ejected anisotropically at velocities of tens of meters per second. The radar observations suggest that a comet's activity may be a poor indicator of its size and provide evidence that large grains constitute an important component of the mass loss from a typical active comet.

Journal Article↗
PubMed1990

Comet-tail artifact from cholesterol crystals: observations in the postlithotripsy gallbladder and an in vitro model.

The "comet tail" is a well-known ultrasound artifact that appears as a series of parallel bands radiating from a source. This artifact was observed in the gallbladder lumen in eight of 10 patients after extracorporeal shock wave lithotripsy of radiolucent calculi. To investigate the cause of the comet tail artifact, cholesterol crystals were studied in an in vitro model. The comet-tail appearance was reproduced in vitro and was found to be directly related to the interrogating frequency and the size of the crystal aggregates. The authors conclude that cholesterol crystals can serve as the source of the comet-tail artifact secondary to reverberation within the crystals.

Cholelithiasis↗
PubMed2003

Transthoracic sonography of diffuse parenchymal lung disease: the role of comet tail artifacts.

OBJECTIVE: The value of transthoracic sonography in diffuse parenchymal lung disease is not established. This prospective study was conducted to analyze pleural and parenchymal alterations in patients with diffuse parenchymal lung disease by transthoracic sonography and to compare the results with the findings of a control group. METHODS: Fifty-three patients with diffuse parenchymal lung disease and 35 control subjects underwent transthoracic sonography for assessment of (1) basal pleural effusion, (2) the presence and number of comet tail artifacts, (3) thickening, (4) an irregular, fragmented pleural line, and (5) subpleural alterations. RESULTS: Basal pleural effusion was revealed in 37.7% of the patients with diffuse parenchymal lung disease (0% of the control subjects; P < .001), and 98.1% had multiple (>6 per scan) comet tail artifacts (14.3% of the control subjects; P < .001). Only a few artifacts (< or = 6 per scan; P < .001) were shown in 85.7% of the control subjects. All the control subjects had strongly localized comet tail artifacts and no thickened pleural line (84.9% of patients; P < .001). In 98.1 % of the patients, an irregular, fragmented pleural surface could be detected (82.8% of the control subjects; P < .05). Subpleural alterations were found in 37.7% of the patients and in 8.6% of the control subjects (P < .001). CONCLUSIONS: Diffuse parenchymal lung disease should be considered if multiple comet tail artifacts distributed over the whole surface of the lung together with a thickened and irregular, fragmented pleural line are visible. Transthoracic sonography may reflect the distribution of pleural involvement and may show subpleural alterations.

Artifacts↗
PubMed1996

Use of the comet assay to identify cells sensitive to tirapazamine in multicell spheroids and tumors in mice.

Tirapazamine, a bioreductive drug preferentially toxic to hypoxic cells, produces significant numbers of DNA single-strand breaks that can be detected using the alkaline comet assay. Our goal was to determine whether single-strand breaks measured using this assay could act as a surrogate end point for cell killing in multicell spheroids and solid tumors in mice. In spheroids composed of Chinese hamster V79 cells, WiDr human colon carcinoma cells, or SiHa human cervical carcinoma cells, histograms of tail moments (indicators of DNA damage in the comet assay) could be used to identify the percentage of cells that sustained sufficient DNA damage to cause cell death after treatment with tirapazamine. The proportion of comets with tail moments </= 20 (i.e., with damage comparable to that produced by about 10 Gy) correlated with cell survival irrespective of cell type, dose of tirapazamine, time of treatment, or position of cell in the spheroid. Single-cell suspensions from squamous cell carcinoma VII tumors in C3H mice or SiHa xenografts in severe combined immunodeficient mice were also analyzed for clonogenicity and DNA damage. Again, the percentage of comets with tail moments </= 20 was found to be a good predictor of cell killing for both tumor types, providing tumor samples were obtained no more than 1 h after i.p. drug administration. Because tirapazamine is currently undergoing clinical trials, application of this procedure could provide an early indicator of tumors likely to contain hypoxic, susceptible cells and also the extent of the response.

Animals↗
PubMed1997

The cell cycle positions influence DNA migration as measured with the alkaline comet assay in stimulated human lymphocytes.

Virtually any eukaryotic cell can be processed for analysis of DNA damage using the comet assay. The most commonly examined human cells are lymphocyte populations. However, many parameters can affect the response of lymphocytes to the assay in terms of the ability to detect damage. The response of cultivated lymphocytes in the comet assay indicated cycle-dependent differences. The cell cycle position has been shown to affect the results obtained using both the alkaline and neutral assays. This is primarily a reflection of the complications of including S-phase DNA. In the alkaline assay, replicating structures are interpreted as strand breaks when denatured, increasing the level of detectable damage. We performed the alkaline comet assay to detect differences in the extent of DNA in stimulated human lymphocytes collected at different sample times after mitogen stimulation. Our results clearly indicate that proliferating lymphocytes have a greater migration of DNA (measured with the comet assay as DNA damage) than quiescent lymphocytes. The lymphocytes collected at 36, 42, and 48 h after mitogen stimulation showed a significantly increased extent of DNA migration in comparison to the lymphocytes collected at 0 and 24 h after stimulation. It, probably, can be explained by the higher frequency of the S phase cells in lymphocyte populations collected at 36, 42, and 48 h. Sites of active DNA replication during the S phase behave like single-strand breaks when denatured in alkali, and their presence may result in a significant increase of the tail moments in S phase cells.

Adult↗
PubMed1997

[Ultrasonographic anomalies of intrahepatic biliary ducts: contribution of the comet-tail image].

OBJECTIVES: The aim of this study was to evaluate the sensitivity and specificity of the ultrasound intrahepatic bile duct wall comet-tail artifact for the diagnosis of biliary disease. METHODS: All subjects who underwent upper abdominal ultrasound examination were included in the study over an 18-month period. The presence or absence of the comet-tail artifact (that is hyperechoic pattern with several parallels lines, constant, lining on the intrahepatic biliary ducts) was noted. RESULTS: Fourteen thousand four hundred and ninety-eight patients were included. The comet-tail artifact was only found in 10 of the 14 211 patients without biliary tract disease (99.9% specificity), but in 35 of the 287 patients with biliary disease (12.2% sensitivity). The positive predictive value for the diagnosis of biliary tract disease was 77.7%. The artifact was associated with an angiocholitis in 83% of the patients. CONCLUSION: Despite its low sensitivity, the sonographic comet-tail artifact of the intrahepatic bile duct wall was strongly correlated with the presence of biliary disease. This artifact may be related to an enlargement of the bile duct wall.

Bile Duct Diseases↗