658 research outputs found
Starke Kinder- und Jugendparlamente
STARKE KINDER- UND JUGENDPARLAMENTE
Starke Kinder- und Jugendparlamente / Roth, Roland (Rights reserved) ( -
Thermorheology of living cells: impact of temperature variations on cell mechanics
Upon temperature changes, we observe a systematic shift of creep
compliance curves J (t) for single living breast epithelial cells. We use a
dual-beam laser trap (optical stretcher) to induce temperature jumps within
milliseconds, while simultaneously measuring the mechanical response of whole
cells to optical force. The cellular mechanical response was found to differ
between sudden temperature changes compared to slow, long-term changes
implying adaptation of cytoskeletal structure. Interpreting optically induced cell
deformation as a thermorheological experiment allows us to consistently explain
data on the basis of time–temperature superposition, well known from classical
polymer physics. Measured time shift factors give access to the activation
energy of the viscous flow of MCF-10A breast cells, which was determined
to be 80 kJ mol−1. The presented measurements highlight the fundamental
role that temperature plays for the deformability of cellular matter. We propose
thermorheology as a powerful concept to assess the inherent material properties
of living cells and to investigate cell regulatory responses upon environmental
changes
Actin and microtubule networks contribute differently to cell response for small and large strains
Cytoskeletal filaments provide cells with mechanical stability and organization. The main key players
are actin filaments and microtubules governing a cell’s response to mechanical stimuli. We
investigated the specific influences of these crucial components by deforming MCF-7 epithelial cells at
small(\u845% deformation) and large strains(>5% deformation). To understand specific contributions
of actin filaments and microtubules, we systematically studied cellular responses after treatment with
cytoskeleton influencing drugs. Quantification with the microfluidic optical stretcher allowed
capturing the relative deformation and relaxation of cells under different conditions. We separated
distinctive deformational and relaxational contributions to cell mechanics for actin and microtubule
networks for two orders of magnitude of drug dosages. Disrupting actin filaments via latrunculin A,
for instance, revealed a strain-independent softening. Stabilizing these filaments by treatment with
jasplakinolide yielded cell softening for small strains but showed no significant change at large strains.
In contrast, cells treated with nocodazole to disrupt microtubules displayed a softening at large strains
but remained unchanged at small strains. Stabilizing microtubules within the cells via paclitaxel
revealed no significant changes for deformations at small strains, but concentration-dependent
impact at large strains. This suggests that for suspended cells, the actin cortex is probed at small strains,
while at larger strains; the whole cell is probed with a significant contribution from the microtubule
Thermal instability of cell nuclei
DNA is known to be a mechanically and thermally stable structure. In its double
stranded form it is densely packed within the cell nucleus and is thermo-resistant
up to 70 °C. In contrast, we found a sudden loss of cell nuclei integrity at
relatively moderate temperatures ranging from 45 to 55 °C. In our study, suspended
cells held in an optical double beam trap were heated under controlled
conditions while monitoring the nuclear shape. At specific critical temperatures,
an irreversible sudden shape transition of the nuclei was observed. These temperature
induced transitions differ in abundance and intensity for various normal
and cancerous epithelial breast cells, which clearly characterizes different cell
types. Our results show that temperatures slightly higher than physiological
conditions are able to induce instabilities of nuclear structures, eventually
leading to cell death. This is a surprising finding since recent thermorheological
cell studies have shown that cells have a lower viscosity and are thus more
deformable upon temperature increase. Since the nucleus is tightly coupled to
the outer cell shape via the cytoskeleton, the force propagation of nuclear
reshaping to the cell membrane was investigated in combination with the
application of cytoskeletal drugs
Complex thermorheology of living cells
Temperature has a reliable and nearly instantaneous influence on mechanical responses of cells. As recently published, MCF-10A normal epithelial breast cells follow the time-temperature superposition (TTS) principle. Here, we measured thermorheological behaviour of eight common cell types within physiologically relevant temperatures and applied TTS to creep compliance curves. Our results showed that superposition is not universal and was seen in four of the eight investigated cell types. For the other cell types, transitions of thermorheological responses were observed at 36 °C. Activation energies (EA) were calculated for all cell types and ranged between 50 and 150 kJ mol-1. The scaling factors of the superposition of creep curves were used to group the cell lines into three categories. They were dependent on relaxation processes as well as structural composition of the cells in response to mechanical load and temperature increase. This study supports the view that temperature is a vital parameter for comparing cell rheological data and should be precisely controlled when designing experiments
Differences in cortical contractile properties between healthy epithelial and cancerous mesenchymal breast cells
Cell contractility is mainly imagined as a force dipole-like interaction based on actin stress fibers
that pull on cellular adhesion sites. Here, we present a different type of contractility based on
isotropic contractions within the actomyosin cortex. Measuring mechanosensitive cortical
contractility of suspended cells among various cell lines allowed us to exclude effects caused by
stress fibers. We found that epithelial cells display a higher cortical tension than mesenchymal cells,
directly contrasting to stress fiber-mediated contractility. These two types of contractility can even
be used to distinguish epithelial from mesenchymal cells. These findings from a single cell level
correlate to the rearrangement effects of actomyosin cortices within cells assembled in
multicellular aggregates. Epithelial cells form a collective contractile actin cortex surrounding
multicellular aggregates and further generate a high surface tension reminiscent of tissue
boundaries. Hence, we suggest this intercellular structure as to be crucial for epithelial tissue
integrity. In contrast, mesenchymal cells do not form collective actomyosin cortices reducing
multicellular cohesion and enabling cell escape from the aggregates
MRI assessment of changes in adipose tissue parameters after bariatric surgery
Bariatric surgery and other therapeutic options for obese patients are often evaluated by the loss of weight, reduction of comorbidities or improved quality of life. However, little is currently known about potential therapy-related changes in the adipose tissue of obese patients. The aim of this study was therefore to quantify fat fraction (FF) and T1 relaxation time by magnetic resonance imaging (MRI) after Roux-en-Y gastric bypass surgery and compare the resulting values with the preoperative ones. Corresponding MRI data were available from 23 patients (16 females and 7 males) that had undergone MRI before (M0) and one month after (M1) bariatric surgery. Patients were 22-59 years old (mean age 44.3 years) and their BMI ranged from 35.7-54.6 kg/m(2) (mean BMI 44.6 kg/m(2)) at M0. Total visceral AT volumes (VVAT-T, in L) were measured by semi-automatic segmentation of axial MRI images acquired between diaphragm and femoral heads. MRI FF and T1 relaxation times were measured in well-defined regions of visceral (VAT) and subcutaneous (SAT) adipose tissue using two custom-made analysis tools. Average BMI values were 45.4 kg/m(2) at time point M0 and 42.4 kg/m(2) at M1. Corresponding VVAT-T values were 5.94 L and 5.33 L. Intraindividual differences in both BMI and VVAT-T were highly significant (p<0.001). Average relaxation times T1 VAT were 303.7 ms at M0 and 316.9 ms at M1 (p<0.001). Corresponding T1(SAT) times were 283.2 ms and 280.7 ms (p = 0.137). Similarly, FFVAT differences (M0: 85.7%, M1: 83.4%) were significant (p <0.01) whereas FFSAT differences (M0: 86.1, M1: 85.9%) were not significant (p = 0.517). In conclusion, bariatric surgery is apparently not only related to a significant reduction in common parameters of adipose tissue distribution, here BMI and total visceral fat volume, but also significant changes in T1 relaxation time and fat fraction of visceral adipose tissue. Such quantitative MRI measures may potentially serve as independent biomarkers for longitudinal and cross-sectional measurements in obese patients
Annihilation of structural defects in chalcogenide absorber films for high-efficiency solar cells
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.In polycrystalline semiconductor absorbers for thin-film solar cells, structural defects may enhance electron-hole recombination and hence lower the resulting energy conversion efficiency. To be able to efficiently design and optimize fabrication processes that result in high-quality materials, knowledge of the nature of structural defects as well as their formation and annihilation during film growth is essential. Here we show that in co-evaporated Cu(In,Ga)Se-2 absorber films the density of defects is strongly influenced by the reaction path and substrate temperature during film growth. A combination of high-resolution electron microscopy, atomic force microscopy, scanning tunneling microscopy, and X-ray diffraction shows that Cu(In,Ga)Se-2 absorber films deposited at low temperature without a Cu-rich stage suffer from a high density of - partially electronically active - planar defects in the {112} planes. Real-time X-ray diffraction reveals that these faults are nearly completely annihilated during an intermediate Cu-rich process stage with [Cu]/([In] + [Ga]) > 1. Moreover, correlations between real-time diffraction and fluorescence analysis during Cu-Se deposition reveal that rapid defect annihilation starts shortly before the start of segregation of excess Cu-Se at the surface of the Cu(In,Ga)Se-2 film. The presented results hence provide direct insights into the dynamics of the film-quality-improving mechanism
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