66 research outputs found
Hybrid kinematic control for rigid body pose stabilization using dual quaternions
In this paper, we address the rigid body pose stabilization problem using dual quaternion formalism. We propose a hybrid control strategy to design a switching control law with hysteresis in such a way that the global asymptotic stability of the closed-loop system is guaranteed and such that the global attractivity of the stabilization pose does not exhibit chattering, a problem that is present in all discontinuous-based feedback controllers. Using numerical simulations, we illustrate the problems that arise from existing results in the literature—as unwinding and chattering—and verify the effectiveness of the proposed controller to solve the robust global pose stability problem
Comparison of isoprene chemical mechanisms under atmospheric night-time conditions in chamber experiments : Evidence of hydroperoxy aldehydes and epoxy products from NO3 oxidation
The gas-phase reaction of isoprene with the nitrate radical (NO3) was investigated in experiments in the outdoor SAPHIR chamber under atmospherically relevant conditions specifically with respect to the chemical lifetime and fate of nitrato-organic peroxy radicals (RO2). Observations of organic products were compared to concentrations expected from different chemical mechanisms: (1) the Master Chemical Mechanism, which simplifies the NO3 isoprene chemistry by only considering one RO2 isomer; (2) the chemical mechanism derived from experiments in the Caltech chamber, which considers different RO2 isomers; and (3) the FZJ-NO3 isoprene mechanism derived from quantum chemical calculations, which in addition to the Caltech mechanism includes equilibrium reactions of RO2 isomers, unimolecular reactions of nitrate RO2 radicals and epoxidation reactions of nitrate alkoxy radicals. Measurements using mass spectrometer instruments give evidence that the new reactions pathways predicted by quantum chemical calculations play a role in the NO3 oxidation of isoprene. Hydroperoxy aldehyde (HPALD) species, which are specific to unimolecular reactions of nitrate RO2, were detected even in the presence of an OH scavenger, excluding the possibility that concurrent oxidation by hydroxyl radicals (OH) is responsible for their formation. In addition, ion signals at masses that can be attributed to epoxy compounds, which are specific to the epoxidation reaction of nitrate alkoxy radicals, were detected. Measurements of methyl vinyl ketone (MVK) and methacrolein (MACR) concentrations confirm that the decomposition of nitrate alkoxy radicals implemented in the Caltech mechanism cannot compete with the ring-closure reactions predicted by quantum chemical calculations. The validity of the FZJ-NO3 isoprene mechanism is further supported by a good agreement between measured and simulated hydroxyl radical (OH) reactivity. Nevertheless, the FZJ-NO3 isoprene mechanism needs further investigations with respect to the absolute importance of unimolecular reactions of nitrate RO2 and epoxidation reactions of nitrate alkoxy radicals. Absolute concentrations of specific organic nitrates such as nitrate hydroperoxides would be required to experimentally determine product yields and branching ratios of reactions but could not be measured in the chamber experiments due to the lack of calibration standards for these compounds. The temporal evolution of mass traces attributed to product species such as nitrate hydroperoxides, nitrate carbonyl and nitrate alcohols as well as hydroperoxy aldehydes observed by the mass spectrometer instruments demonstrates that further oxidation by the nitrate radical and ozone at atmospheric concentrations is small on the timescale of one night (12gh) for typical oxidant concentrations. However, oxidation by hydroxyl radicals present at night and potentially also produced from the decomposition of nitrate alkoxy radicals can contribute to their nocturnal chemical loss
Efeito da enrofloxacina sobre a interação blastocisto endométrio e seu reflexo no desenvolvimento placentário e fetal em ratas
Vascularização arterial da região do nó sinoatrial em corações suínos: origem, distribuição e quantificação
Adaptive remodelling of intestinal epithelium assessed using stereology: correlation of single cell and whole organ data with nutrient transport
Adaptation in the intestinal epithelium
depends on cell number and the properties of individual
cells but these responses operate within different time
frames. Changes in number take days to accomplish but
those in behaviour may occur within hours. This article
reviews the value of stereology for characterising
structural features of the average enterocyte and the
entire organ (mammalian small intestine or avian lower
intestine) during adaptation. Stereological data are
correlated with the physiology and molecular biology of
glucose and Na+ transpon. In small intestine, account is
taken of vertical (crypt-villus) and longitudinal
(craniocaudal) gradients and of adaptations to
chemically-induced diabetes and diet. Results show that
longer-term adaptation depends critically on epithelial
renewal. In diabetic small intestine, changes in glucose
transport are accompanied by changes in the number, but
not morphology, of villous enterocytes. In avian lower
intestine, increased Na+ transport requires changes in
cell number and the extent of their apical, but not
basolateral, membrane surfaces. These changes allow
opportunities to incorporate more (or more active)
transport sites in apical and basolateral membrane
domains of individual cells and of whole organs
Epithelia1 integrity, cell death and cell loss in mammalian small intestine
In recent years, the different mechanisms of
epithelial cell loss which occur in mammalian and avian
small intestine have been re-investigated. Information is
now available for a variety of mammalian types and
mechanisms can be divided into two major classes: [i]
those preserving epithelial integrity by maintaining
intercellular tight junctions throughout early-to-late
stages of cell extrusion; and [ii] those which compromise
integrity by introducing breaches in epithelial continuity.
Both classes are associated with the activity andtor
proximity of non-epithelia1 cells (mainly lymphocytes
and mononuclear phagocytes) located in the epithelium
or underlying lamina propria. Intraepithelial lymphocytes
may be involved in enterocyte targetting and
killing whilst lamina propria (LP) macrophages
sequester cell debris. Where epithelial integrity is
maintained, two types of loss can be identified. In the
first (type l), complete cells are extruded into the lumen.
In the second (type 2), only anucleate apical cell
fragments pass into the lumen . There are two variants of
type 2 loss distinguishable by the fate of the nucleated
basal portions of cells. One variant (type 2a) creates
large intercellular spaces extending from the preserved
apical cap to the basal lamina and containing enterocyte
debris for phagocytosis. The second (type 2b) involves
the gradual shrinkage of individual cells (which become
more electron-dense) and in situ degeneration of their
nucleated subapical portions in increasingly narrower intercellular spaces between adjacent healthy
enterocytes. The mechanism of removal of these
fragments is unclear but may be via macrophages or
surrounding enterocytes. Apoptosis has been implicated
in both type 1 and type 2 extrusion. In contrast, type 3
loss involves morphological changes in enterocytes
which are reminiscent of those seen in necrosis and is
accompanied by breaks in epithelial continuity following
cell swelling, a decrease in cell electron density and total or subtotal degradation of organelles and membranes. It
ends in loss of either an abnormal cell apex (with
subsequent exposure of the degraded cell contents and
their spillage into the lumen) or a complete cell remnant
(extruded into the lumen before total disintegration of
plasma membranes)
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