4,188 research outputs found
Octonic Electrodynamics
In this paper we present eight-component values "octons", generating
associative noncommutative algebra. It is shown that the electromagnetic field
in a vacuum can be described by a generalized octonic equation, which leads
both to the wave equations for potentials and fields and to the system of
Maxwell's equations. The octonic algebra allows one to perform compact combined
calculations simultaneously with scalars, vectors, pseudoscalars and
pseudovectors. Examples of such calculations are demonstrated by deriving the
relations for energy, momentum and Lorentz invariants of the electromagnetic
field. The generalized octonic equation for electromagnetic field in a matter
is formulated.Comment: 12 pages, 1 figur
The 4D geometric quantities versus the usual 3D quantities. The resolution of Jackson's paradox
In this paper we present definitions of different four-dimensional (4D)
geometric quantities (Clifford multivectors). New decompositions of the torque
N and the angular momentum M (bivectors) into 1-vectors N_{s}, N_{t} and M_{s},
M_{t} respectively are given. The torques N_{s}, N_{t} (the angular momentums
M_{s}, M_{t}), taken together, contain the same physical information as the
bivector N (the bivector M). The usual approaches that deal with the 3D
quantities , , , ,
, etc. and their transformations are objected from the viewpoint of
the invariant special relativity (ISR). In the ISR it is considered that 4D
geometric quantities are well-defined both theoretically and
\emph{experimentally} in the 4D spacetime. This is not the case with the usual
3D quantities. It is shown that there is no apparent electrodynamic paradox
with the torque, and that the principle of relativity is naturally satisfied,
when the 4D geometric quantities are used instead of the 3D quantities.Comment: 13 pages, revte
Nitric Oxide Synthesis Is Increased in Cybrid Cells with m.3243A > G Mutation
Nitric oxide (NO) is a free radical and a signaling molecule in several pathways, produced by nitric oxide synthase (NOS) from the conversion of L-arginine to citrulline. Supplementation of L-arginine has been used to treat MELAS (mitochondrial encephalopathy with lactic acidosis and stroke like syndrome), a mitochondrial disease caused by the m. 3243A>G mutation. Low levels of serum arginine and endothelium dysfunction have been reported in MELAS and this treatment may increase NO in endothelial cells and promote vasodilation, decreasing cerebral ischemia and strokes. Although clinical benefits have been reported, little is known about NO synthesis in MELAS. in this study we found that osteosarcoma derived cybrid cells with high levels of m. 3243A>G had increased nitrite, an NO metabolite, and increased intracellular NO, demonstrated by an NO fluorescent probe (DAF-FM). Muscle vessels from patients with the same mutation had increased staining in NADPH diaphorase, suggestive of increased NOS. These results indicate increased production of NO in cells harboring the m. 3243A>G, however no nitrated protein was detected by Western blotting. Further studies are necessary to clarify the exact mechanisms of L-arginine effect to determine the appropriate clinical use of this drug therapy.Fundacao de Amparo a Pesquisa de SĂŁo PauloCoordenação de Aperfeiçoamento de Pessoal de NĂvel Superior (CAPES)Universidade Federal de SĂŁo Paulo, Escola Paulista Med, Dept Neurol & Neurosurg, BR-04039032 SĂŁo Paulo, BrazilUniv Miami, Miller Sch Med, Dept Neurol & Cell Biol, Miami, FL 33101 USAUniversidade Federal de SĂŁo Paulo, Escola Paulista Med, Dept Neurol & Neurosurg, BR-04039032 SĂŁo Paulo, BrazilWeb of Scienc
Congenital pancreatoblastoma: a case report
The literature describes 15 cases of congenital pancreatoblastoma (PB): 5 had prenatal diagnosis, none
had metastases at diagnosis, 7 were associated with BeckwitheWiedemann syndrome (BWS). In 13 cases
resection was radical, while in 2 there were macroscopic residues. Only one patient underwent
chemotherapy after distant recurrence. All children are alive except one who died because of problems
related to BWS. Our goal is to describe the approach adopted in an infant with congenital PB treated in
our center. After a prenatal third semester diagnosis of abdominal anechoic lesion, the radiological investigations
(ultrasound, MRI) performed at birth described a cystic lesion of unclear nature. We proceeded
to laparoscopic exploration, transformed into open approach after the detection of a lesion
located in the body of the pancreas; this lesion was resected, preserving the head and tail of pancreas.
The histological diagnosis showed a completely excised PB. After excluding metastatic lesions, we
decided to perform only careful follow-up without chemotherapy. The follow-up at 12 months is
negative. Although PB is a malignant tumor that requires a multidisciplinary treatment, the congenital
cases seem to have a less aggressive biological behavior. The treatment, therefore, in case of complete
resection, could be only surgical, followed by a careful follow-up. These forms are often associated with
congenital BWS, but in our case the patient did not have the typical characteristics of the syndrome
Spatial signal amplification in cell biology: a lattice-gas model for self-tuned phase ordering
Experiments show that the movement of eukaryotic cells is regulated by a
process of phase separation of two competing enzymes on the cell membrane, that
effectively amplifies shallow external gradients of chemical attractant.
Notably, the cell is able to self-tune the final enzyme concentrations to an
equilibrium state of phase coexistence, for a wide range of the average
attractant concentration. We propose a simple lattice model in which, together
with a short-range attraction between enzymes, a long-range repulsion naturally
arises from physical considerations, that easily explains such observed
behavior
Bayesian sequential integration within a preclinical pharmacokinetic and pharmacodynamic modeling framework:Lessons learned
The present manuscript aims to discuss the implications of sequential knowledge integration of small preclinical trials in a Bayesian pharmacokinetic and pharmacodynamic (PK-PD) framework. While, at first sight, a Bayesian PK-PD framework seems to be a natural framework to allow for sequential knowledge integration, the scope of this paper is to highlight some often-overlooked challenges while at the same time providing some guidances in the many and overwhelming choices that need to be made. Challenges as well as opportunities will be discussed that are related to the impact of (1) the prior specification, (2) the choice of random effects, (3) the type of sequential integration method. In addition, it will be shown how the success of a sequential integration strategy is highly dependent on a carefully chosen experimental design when small trials are analyzed
Universal features of cell polarization processes
Cell polarization plays a central role in the development of complex
organisms. It has been recently shown that cell polarization may follow from
the proximity to a phase separation instability in a bistable network of
chemical reactions. An example which has been thoroughly studied is the
formation of signaling domains during eukaryotic chemotaxis. In this case, the
process of domain growth may be described by the use of a constrained
time-dependent Landau-Ginzburg equation, admitting scale-invariant solutions
{\textit{\`a la}} Lifshitz and Slyozov. The constraint results here from a
mechanism of fast cycling of molecules between a cytosolic, inactive state and
a membrane-bound, active state, which dynamically tunes the chemical potential
for membrane binding to a value corresponding to the coexistence of different
phases on the cell membrane. We provide here a universal description of this
process both in the presence and absence of a gradient in the external
activation field. Universal power laws are derived for the time needed for the
cell to polarize in a chemotactic gradient, and for the value of the smallest
detectable gradient. We also describe a concrete realization of our scheme
based on the analysis of available biochemical and biophysical data.Comment: Submitted to Journal of Statistical Mechanics -Theory and Experiment
Percolation, Morphogenesis, and Burgers Dynamics in Blood Vessels Formation
Experiments of in vitro formation of blood vessels show that cells randomly
spread on a gel matrix autonomously organize to form a connected vascular
network. We propose a simple model which reproduces many features of the
biological system. We show that both the model and the real system exhibit a
fractal behavior at small scales, due to the process of migration and dynamical
aggregation, followed at large scale by a random percolation behavior due to
the coalescence of aggregates. The results are in good agreement with the
analysis performed on the experimental data.Comment: 4 pages, 11 eps figure
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