A microfluidic platform to investigate the role of mechanical constraints on tissue reorganization.

Mechanical constraints have a high impact on development processes, and there is a need for new tools to investigate the role of mechanosensitive pathways in tissue reorganization during development. We present here experiments in which embryonic cell aggregates are aspired through constrictions in microfluidic channels, generating highly heterogeneous flows and large cell deformations that can be imaged using two-photon microscopy. This approach provides a way to measure in situ local viscoelastic properties of 3D tissues and connect them to intracellular and intercellular events, such as cell shape changes and cell rearrangements. These methods could be applied to organoids to investigate and quantify rheological properties of tissues, and to understand how constraints affect development.

Nanomechanical Hallmarks of Helicobacter pylori Infection in Pediatric Patients.

BACKGROUND: the molecular mechanism of gastric cancer development related to Helicobacter pylori (H. pylori) infection has not been fully understood, and further studies are still needed. Information regarding nanomechanical aspects of pathophysiological events that occur during H. pylori infection can be crucial in the development of new prevention, treatment, and diagnostic measures against clinical consequences associated with H. pylori infection, including gastric ulcer, duodenal ulcer, and gastric cancer. METHODS: in this study, we assessed mechanical properties of children's healthy and H. pylori positive stomach tissues and the mechanical response of human gastric cells exposed to heat-treated H. pylori cells using atomic force microscopy (AFM NanoWizard 4 BioScience JPK Instruments Bruker). Elastic modulus (i.e., the Young's modulus) was derived from the Hertz-Sneddon model applied to force-indentation curves. Human tissue samples were evaluated using rapid urease tests to identify H. pylori positive samples, and the presence of H. pylori cells in those samples was confirmed using immunohistopathological staining. RESULTS AND CONCLUSION: collected data suggest that nanomechanical properties of infected tissue might be considered as markers indicated H. pylori presence since infected tissues are softer than uninfected ones. At the cellular level, this mechanical response is at least partially mediated by cell cytoskeleton remodeling indicating that gastric cells are able to tune their mechanical properties when subjected to the presence of H. pylori products. Persistent fluctuations of tissue mechanical properties in response to H. pylori infection might, in the long-term, promote induction of cancer development.

Rigidity percolation uncovers a structural basis for embryonic tissue phase transitions.

Embryo morphogenesis is impacted by dynamic changes in tissue material properties, which have been proposed to occur via processes akin to phase transitions (PTs). Here, we show that rigidity percolation provides a simple and robust theoretical framework to predict material/structural PTs of embryonic tissues from local cell connectivity. By using percolation theory, combined with directly monitoring dynamic changes in tissue rheology and cell contact mechanics, we demonstrate that the zebrafish blastoderm undergoes a genuine rigidity PT, brought about by a small reduction in adhesion-dependent cell connectivity below a critical value. We quantitatively predict and experimentally verify hallmarks of PTs, including power-law exponents and associated discontinuities of macroscopic observables. Finally, we show that this uniform PT depends on blastoderm cells undergoing meta-synchronous divisions causing random and, consequently, uniform changes in cell connectivity. Collectively, our theoretical and experimental findings reveal the structural basis of material PTs in an organismal context.

Nanomechanics and Histopathology as Diagnostic Tools to Characterize Freshly Removed Human Brain Tumors.

BACKGROUND: The tissue-mechanics environment plays a crucial role in human brain physiological development and the pathogenesis of different diseases, especially cancer. Assessment of alterations in brain mechanical  properties during cancer progression might provide important information about possible tissue abnormalities with clinical relevance. METHODS: With atomic force microscopy (AFM), the stiffness of freshly removed human brain tumor tissue was determined on various regions of the sample and compared to the stiffness of healthy human brain tissue that was removed during neurosurgery to gain access to tumor mass. An advantage of indentation measurement using AFM is the small volume of tissue required and high resolution at the single-cell level. RESULTS: Our results showed great heterogeneity of stiffness within metastatic cancer or primary high-grade gliomas compared to healthy tissue. That effect was not clearly visible in lower-grade tumors like meningioma. CONCLUSION: Collected data indicate that AFM might serve as a diagnostic tool in the assessment of human brain tissue stiffness in the process of recognizing tumors.

Differential tissue stiffness of body column facilitates locomotion of Hydra on solid substrates.

The bell-shaped members of the Cnidaria typically move around by swimming, whereas the Hydra polyp can perform locomotion on solid substrates in an aquatic environment. To address the biomechanics of locomotion on rigid substrates, we studied the 'somersaulting' locomotion in Hydra We applied atomic force microscopy to measure the local mechanical properties of Hydra's body column and identified the existence of differential Young's modulus between the shoulder region versus rest of the body column at 3:1 ratio. We show that somersaulting primarily depends on differential tissue stiffness of the body column and is explained by computational models that accurately recapitulate the mechanics involved in this process. We demonstrate that perturbation of the observed stiffness variation in the body column by modulating the extracellular matrix polymerization impairs the 'somersault' movement. These results provide a mechanistic basis for the evolutionary significance of differential extracellular matrix properties and tissue stiffness.

A unified rheological model for cells and cellularised materials.

The mechanical response of single cells and tissues exhibits a broad distribution of time-scales that often gives rise to a distinctive power-law rheology. Such complex behaviour cannot be easily captured by traditional rheological approaches, making material characterisation and predictive modelling very challenging. Here, we present a novel model combining conventional viscoelastic elements with fractional calculus that successfully captures the macroscopic relaxation response of epithelial monolayers. The parameters extracted from the fitting of the relaxation modulus allow prediction of the response of the same material to slow stretch and creep, indicating that the model captured intrinsic material properties. Two characteristic times, derived from the model parameters, delimit different regimes in the materials response. We compared the response of tissues with the behaviour of single cells as well as intra and extra-cellular components, and linked the power-law behaviour of the epithelium to the dynamics of the cell cortex. Such a unified model for the mechanical response of biological materials provides a novel and robust mathematical approach to consistently analyse experimental data and uncover similarities and differences in reported behaviour across experimental methods and research groups. It also sets the foundations for more accurate computational models of tissue mechanics.

Application of Mechanical Forces on Drosophila Embryos by Manipulation of Microinjected Magnetic Particles.

Cells generate mechanical forces to shape tissues during morphogenesis. These forces can activate several biochemical pathways and trigger diverse cellular responses by mechano-sensation, such as differentiation, division, migration and apoptosis. Assessing the mechano-responses of cells in living organisms requires tools to apply controlled local forces within biological tissues. For this, we have set up a method to generate controlled forces on a magnetic particle embedded within a chosen tissue of Drosophila embryos. We designed a protocol to inject an individual particle in early embryos and to position it, using a permanent magnet, within the tissue of our choice. Controlled forces in the range of pico to nanonewtons can be applied on the particle with the use of an electromagnet that has been previously calibrated. The bead displacement and the epithelial deformation upon force application can be followed with live imaging and further analyzed using simple analysis tools. This method has been successfully used to identify changes in mechanics in the blastoderm before gastrulation. This protocol provides the details, (i) for injecting a magnetic particle in Drosophila embryos, (ii) for calibrating an electromagnet and (iii) to apply controlled forces in living tissues.

Tissue rheology in embryonic organization.

Tissue morphogenesis in multicellular organisms is brought about by spatiotemporal coordination of mechanical and chemical signals. Extensive work on how mechanical forces together with the well-established morphogen signalling pathways can actively shape living tissues has revealed evolutionary conserved mechanochemical features of embryonic development. More recently, attention has been drawn to the description of tissue material properties and how they can influence certain morphogenetic processes. Interestingly, besides the role of tissue material properties in determining how much tissues deform in response to force application, there is increasing theoretical and experimental evidence, suggesting that tissue material properties can abruptly and drastically change in development. These changes resemble phase transitions, pointing at the intriguing possibility that important morphogenetic processes in development, such as symmetry breaking and self-organization, might be mediated by tissue phase transitions. In this review, we summarize recent findings on the regulation and role of tissue material properties in the context of the developing embryo. We posit that abrupt changes of tissue rheological properties may have important implications in maintaining the balance between robustness and adaptability during embryonic development.

Mechanical Characterization of Human Brain Tissue and Soft Dynamic Gels Exhibiting Electromechanical Neuro-Mimicry.

Synthetic hydrogels are an important class of materials in tissue engineering, drug delivery, and other biomedical fields. Their mechanical and electrical properties can be tuned to match those of biological tissues. In this work, hydrogels that exhibit both mechanical and electrical biomimicry are reported. The presented dual networks consist of supramolecular networks formed from 2:1 homoternary complexes of imidazolium-based guest molecules in cucubit[8]uril and covalent networks of oligoethylene glycol-(di)methacrylate. The viscoelastic properties of human brain tissues are also investigated. The mechanical properties of the dual network gels are benchmarked against the human tissue, and it is found that they both are neuro-mimetic and exhibit cytocompatibility in a neural stem cell model.

Glial Tissue Mechanics and Mechanosensing by Glial Cells.

Understanding the mechanical behavior of human brain is critical to interpret the role of physical stimuli in both normal and pathological processes that occur in CNS tissue, such as development, inflammation, neurodegeneration, aging, and most common brain tumors. Despite clear evidence that mechanical cues influence both normal and transformed brain tissue activity as well as normal and transformed brain cell behavior, little is known about the links between mechanical signals and their biochemical and medical consequences. A multi-level approach from whole organ rheology to single cell mechanics is needed to understand the physical aspects of human brain function and its pathologies. This review summarizes the latest achievements in the field.

A compact 0.5 T MR elastography device and its application for studying viscoelasticity changes in biological tissues during progressive formalin fixation.

PURPOSE: To develop a method of compact tabletop magnetic resonance elastography (MRE) for rheological tests of tissue samples and to measure changes in viscoelastic powerlaw constants of liver and brain tissue during progressive fixation. METHODS: A 10-mm bore, 0.5-T permanent-magnet-based MRI system was equipped with a gradient-amplifier-controlled piezo-actuator and motion-sensitive spin echo sequence for inducing and measuring harmonic shear vibrations in cylindrical samples. Shear modulus dispersion functions were acquired at 200-5700 Hz in animal tissues at different states of formalin fixation and fitted by the springpot powerlaw model to obtain shear modulus µ and powerlaw exponent α. RESULTS: In a frequency range of 300-1500 Hz, unfixed liver tissue was softer and less dispersive than brain tissue with µ = 1.68 ± 0.17 kPa and α = 0.51 ± 0.06 versus µ = 2.60 ± 0.68 kPa and α = 0.68 ± 0.03. Twenty-eight hours of formalin fixation yielded a 400-fold increase in liver µ, 25-fold increase in brain µ, and two-fold reduction in α of both tissues. CONCLUSION: Compact 0.5-T MRE facilitates automated measurement of shear modulus dispersion in biological tissue at low costs. Formalin fixation changes the viscoelastic properties of tissues from viscous-soft to elastic-stiff more markedly in liver than brain. Magn Reson Med 79:470-478, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Identification of rheological properties of human body surface tissue.

According to World Health Organization obesity is one of the greatest public health challenges of the 21st century. It has tripled since the 1980s and the numbers of those affected continue to rise at an alarming rate, especially among children. There are number of devices that act as a prevention measure to boost person's motivation for physical activity and its levels. The placement of these devices is not restricted thus the measurement errors that appear because of the body rheology, clothes, etc. cannot be eliminated. The main objective of this work is to introduce a tool that can be applied directly to process measured accelerations so human body surface tissue induced errors can be reduced. Both the modeling and experimental techniques are proposed to identify body tissue rheological properties and prelate them to body mass index. Multi-level computational model composed from measurement device model and human body surface tissue rheological model is developed. Human body surface tissue induced inaccuracies can increase the magnitude of measured accelerations up to 34% when accelerations of the magnitude of up to 27 m/s(2) are measured. Although the timeframe of those disruptions are short - up to 0.2 s - they still result in increased overall measurement error.