195 research outputs found
Charge Conjugation from Space-Time Inversion
We show that the CPT group of the Dirac field emerges naturally from the PT
and P (or T) subgroups of the Lorentz group.Comment: 4 pages, no figure
Bundle Theory of Improper Spin Transformations
{\it We first give a geometrical description of the action of the parity
operator () on non relativistic spin Pauli spinors in
terms of bundle theory. The relevant bundle, , is a
non trivial extension of the universal covering group .
is the non relativistic limit of the corresponding Dirac matrix
operator and obeys . Then, from the direct
product of O(3) by , naturally induced by the structure of the galilean
group, we identify, in its double cover, the time reversal operator ()
acting on spinors, and its product with . Both, and
, generate the group . As in the case of parity,
is the non relativistic limit of the corresponding Dirac matrix
operator , and obeys .}Comment: 8 pages, Plaintex; titled changed, minor text modifications, one
reference complete
Remark on charge conjugation in the non relativistic limit
We study the non relativistic limit of the charge conjugation operation in the context of the Dirac equation coupled to an electromagnetic field.
The limit is well defined and, as in the relativistic case, ,
(parity) and (time reversal) are the generators of a matrix group
isomorphic to a semidirect sum of the dihedral group of eight elements and
. The existence of the limit is supported by an argument based in quantum
field theory. Also, and most important, the limit exists in the context of
galilean relativity. Finally, if one complexifies the Lorentz group and
therefore the galilean spacetime , then the explicit form of the matrix
for allows to interpret it, in this context, as the complex
conjugation of the spatial coordinates: . This result is
natural in a fiber bundle description.Comment: 8 page
Avaliação dos efeitos da eliminação da tensão através de ancoramento epineural: estudo experimental comparando resultados eletrofisiológicos e histomorfométricos após diferentes técnicas de reparo no nervo
Epineural stitches are a means to avoid tension in a nerve suture. We evaluate this technique, relative to interposed grafts and simple neurorraphy, in a rat model. METHOD: Twenty rats were allocated to four groups. For Group 1, sectioning of the sciatic nerve was performed, a segment 4 mm long discarded, and epineural suture with distal anchoring stitches were placed resulting in slight tension neurorraphy. For Group 2, a simple neurorraphy was performed. For Group 3, a 4 mm long graft was employed and Group 4 served as control. Ninety days after, reoperation, latency of motor action potentials recording and axonal counts were performed. Inter-group comparison was done by means of ANOVA and the non-parametric Kruskal-Wallis test. RESULTS: The mean motor latency for the simple suture (2.27Β±0.77 ms) was lower than for the other two surgical groups, but lower than among controls (1.69Β±0.56 ms). Similar values were founding in both group 1 (2.66Β±0.71 ms) and group 3 (2.64Β±0.6 ms). When fibers diameters were compared a significant difference was identified between groups 2 and 3 (p=0.048). CONCLUSION: Good results can be obtained when suturing a nerve employ with epineural anchoring stitches. However, more studies are needed before extrapolating results to human nerve sutures.A aproximação atravΓ©s de pontos epineurais Γ© uma forma de se reduzir a tensΓ£o numa neurorrafia. Neste estudo esta tΓ©cnica Γ© avaliada atravΓ©s da sua comparação com a interposição de enxertos e neurorrafia simples num modelo experimental utilizando o rato. MΓTODO: Vinte ratos foram utilizados e divididos em 4 grupos. No Grupo 1, apΓ³s a ressecção de 4 mm, os cotos do nervo foram aproximados atravΓ©s de pontos de ancoramento epineurais e suturados com tensΓ£o. No Grupo 2, uma neurorrafia simples foi realizada apΓ³s secção do nervo. No Grupo 3, um enxerto de 4 mm foi utilizado para o reparo e o Grupo 4 foi utilizado como controle. Noventa dias apΓ³s, os nervos foram novamente expostos e a medida da latΓͺncia do potencial de ação motor e a contagem axonal foram realizados. A comparação entre os grupos foi realizada atravΓ©s da comparação entre as mΓ©dias (ANOVA) e com o teste nΓ£o-paramΓ©trico de Kruskal-Wallis. RESULTADOS: A mΓ©dia da latΓͺncia motora na sutura simples (2,27Β±0,77 ms) foi menor em relação aos outros dois grupos onde o nervo foi seccionado e reparado e maior que o grupo controle (1,69Β±0,56 ms). Resultados semelhantes foram identificados nos grupos 1 (2,66Β±0,71 ms) e 3 (2,64Β±0,6 ms). Uma diferenΓ§a significativa diΓ’metros das fibras foi identificada quando comparados os grupos 2 e 3 (p=0,048). CONCLUSΓO: Resultados equiparΓ‘veis aos obtidos com enxerto podem ser obtidos quando a neurorrafia Γ© realizada com pontos epineurais de ancoramento com tensΓ£o, mas estudos adicionais sΓ£o necessΓ‘rios antes desses resultados serem extrapolados para o reparo de nervo em seres humanos
Quantum Mechanics and Leggett's Inequalities
We show that when the proper description of the behaviour of individual
photons or spin 1/2 particles in a spherically symmetric entangled pair is done
through the use of the density matrix, the Leggett's inequality is not violated
by quantum mechanics.Comment: 7 pages, no figures. A missing global sign in the r.h.s. of eq.
(4.10) in section 4 of version 1 (v1) invalidates the conclusion of that
particular section, which is then suppressed in the present version (v2
Negative Autoregulation by Fas Stabilizes Adult Erythropoiesis and Accelerates Its Stress Response
Erythropoiesis maintains a stable hematocrit and tissue oxygenation in the basal state, while mounting a stress response that accelerates red cell production in anemia, blood loss or high altitude. Thus, tissue hypoxia increases secretion of the hormone erythropoietin (Epo), stimulating an increase in erythroid progenitors and erythropoietic rate. Several cell divisions must elapse, however, before Epo-responsive progenitors mature into red cells. This inherent delay is expected to reduce the stability of erythropoiesis and to slow its response to stress. Here we identify a mechanism that helps to offset these effects. We recently showed that splenic early erythroblasts, βEryAβ, negatively regulate their own survival by co-expressing the death receptor Fas, and its ligand, FasL. Here we studied mice mutant for either Fas or FasL, bred onto an immune-deficient background, in order to avoid an autoimmune syndrome associated with Fas deficiency. Mutant mice had a higher hematocrit, lower serum Epo, and an increased number of splenic erythroid progenitors, suggesting that Fas negatively regulates erythropoiesis at the level of the whole animal. In addition, Fas-mediated autoregulation stabilizes the size of the splenic early erythroblast pool, since mutant mice had a significantly more variable EryA pool than matched control mice. Unexpectedly, in spite of the loss of a negative regulator, the expansion of EryA and ProE progenitors in response to high Epo in vivo, as well as the increase in erythropoietic rate in mice injected with Epo or placed in a hypoxic environment, lagged significantly in the mutant mice. This suggests that Fas-mediated autoregulation accelerates the erythropoietic response to stress. Therefore, Fas-mediated negative autoregulation within splenic erythropoietic tissue optimizes key dynamic features in the operation of the erythropoietic network as a whole, helping to maintain erythroid homeostasis in the basal state, while accelerating the stress response
Planet Hunters. VI: An Independent Characterization of KOI-351 and Several Long Period Planet Candidates from the Kepler Archival Data
We report the discovery of 14 new transiting planet candidates in the Kepler
field from the Planet Hunters citizen science program. None of these candidates
overlapped with Kepler Objects of Interest (KOIs) at the time of submission. We
report the discovery of one more addition to the six planet candidate system
around KOI-351, making it the only seven planet candidate system from Kepler.
Additionally, KOI-351 bears some resemblance to our own solar system, with the
inner five planets ranging from Earth to mini-Neptune radii and the outer
planets being gas giants; however, this system is very compact, with all seven
planet candidates orbiting AU from their host star. A Hill
stability test and an orbital integration of the system shows that the system
is stable. Furthermore, we significantly add to the population of long period
transiting planets; periods range from 124-904 days, eight of them more than
one Earth year long. Seven of these 14 candidates reside in their host star's
habitable zone.Comment: 27 pages, 6 figures, 5 tables, Accepted to AJ (in press) (updated
title from original astro-ph submission
The Erythropoietin Receptor Stimulates Rapid Cycling and Formation of Larger Red Cells During Mouse and Human Erythropoiesis [preprint]
Erythroid terminal differentiation entails cell divisions that are coupled to progressive decreases in cell size. EpoR signaling is essential for the survival of erythroid precursors, but it is unclear whether it has other functions in these cells. Here we endowed mouse precursors that lack the EpoR with survival signaling, finding that this was sufficient to support their differentiation into enucleated red cells, but that the process was abnormal. Precursors underwent fewer and slower cell cycles and yet differentiated into smaller red cells. Surprisingly, EpoR further accelerated cycling of early erythroblasts, the fastest cycling cells in the bone marrow, while simultaneously increasing their cell size. EpoR-mediated formation of larger red cells was independent of the established pathway regulating red cell size by iron through Heme-regulated eIF2Ξ± kinase (HRI). We confirmed the effect of Epo on red cell size in human volunteers, whose mean corpuscular volume (MCV) increased following Epo administration. This increase persisted after Epo declined and was not the result of increased reticulocytes. Our work reveals a unique effect of EpoR signaling on the interaction between the cell cycle and cell growth. Further, it suggests new diagnostic interpretations for increased red cell volume, as reflecting high Epo and erythropoietic stress
A Key Commitment Step in Erythropoiesis Is Synchronized with the Cell Cycle Clock through Mutual Inhibition between PU.1 and S-Phase Progression
During red blood cell development, differentiation and cell cycle progression are intimately and uniquely linked through interdependent mechanisms involving the erythroid transcriptional suppressor PU.1 and the cyclin-dependent kinase inhibitor p57KIP2
Cytotoxic Effect of Poly-Dispersed Single Walled Carbon Nanotubes on Erythrocytes In Vitro and In Vivo
Single wall Carbon Nanotubes (SWCNTs) are hydrophobic and do not disperse in aqueous solvents. Acid functionalization of SWCNTs results in attachment of carboxy and sulfonate groups to carbon atoms and the resulting acid functionalized product (AF-SWCNTs) is negatively charged and disperses easily in water and buffers. In the present study, effect of AF-SWCNTs on blood erythrocytes was examined. Incubation of mouse erythrocytes with AF-SWCNTs and not with control SWCNTs, resulted in a dose and time dependent lysis of erythrocyte. Using fluorescence tagged AF-SWCNTs, binding of AF-SWCNTs with erythrocytes could be demonstrated. Confocal microscopy results indicated that AF-SWCNTs could enter the erythrocytes. Treatment with AF-SWCNTs resulted in exposure of hydrophobic patches on erythrocyte membrane that is indicative of membrane damage. A time and dose dependent increase in externalization of phosphatidylserine on erythrocyte membrane bilayer was also found. Administration of AF-SWCNTs through intravenous route resulted in a transient anemia as seen by a sharp decline in blood erythrocyte count accompanied with a significant drop in blood haemoglobin level. Administration of AF-SWCNTs through intratracheal administration also showed significant decline in RBC count while administration through other routes (gavage and intra-peritoneal) was not effective. By using a recently developed technique of a two step in vivo biotinylation of erythrocytes that enables simultaneous enumeration of young (age <10 days) and old (age>40 days) erythrocytes in mouse blood, it was found that the in vivo toxic effect of AF-SWCNTs was more pronounced on older subpopulation of erythrocytes. Subpopulation of old erythrocytes fell after treatment with AF-SWCNTs but recovered by third day after the intravenous administration of AF-SWCNTs. Taken together our results indicate that treatment with AF-SWCNTs results in acute membrane damage and eventual lysis of erythrocytes. Intravenous administration of AF-SWCNTs resulted in a transient anemia in which older erythrocytes are preferably lysed
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