Tissue Fluidity Research Articles

Magnetic resonance elastography quantification of the solid-to-fluid transition of liver tissue due to decellularization

Maintenance of tissue extracellular matrix (ECM) and its biomechanical properties for tissue engineering is one of the substantial challenges in the field of decellularization and recellularization. Preservation of the organ-specific biomatrix is crucial for successful recellularization to support cell survival, proliferation, and functionality. However, understanding ECM properties with and without its inhabiting cells as well as the transition between the two states lacks appropriate test methods capable of quantifying bulk viscoelastic parameters in soft tissues.We used compact magnetic resonance elastography (MRE) with 400, 500, and 600 Hz driving frequency to investigate rat liver specimens for quantification of viscoelastic property changes resulting from decellularization. Tissue structures in native and decellularized livers were characterized by collagen and elastin quantification, histological analysis, and scanning electron microscopy.Decellularization did not affect the integrity of microanatomy and structural composition of liver ECM but was found to be associated with increases in the relative amounts of collagen by 83-fold (37.4 ± 17.5 vs. 0.5 ± 0.01 μg/mg, p = 0.0002) and elastin by approx. 3-fold (404.1 ± 139.6 vs. 151.0 ± 132.3 μg/mg, p = 0.0046). Decellularization reduced storage modulus by approx. 9-fold (from 4.9 ± 0.8 kPa to 0.5 ± 0.5 kPa, p < 0.0001) and loss modulus by approx. 7-fold (3.6 kPa to 0.5 kPa, p < 0.0001), indicating a marked loss of global tissue rigidity as well as a property shift from solid towards more fluid tissue behavior (p = 0.0097).Our results suggest that the rigidity of liver tissue is largely determined by cellular components, which are replaced by fluid-filled spaces when cells are removed. This leads to an overall increase in tissue fluidity and a viscous drag within the relatively sparse remaining ECM. Compact MRE is an excellent tool for quantifying the mechanical properties of decellularized biological tissue and a promising candidate for useful applications in tissue engineering.

How tissue fluidity influences brain tumor progression

Mechanical properties of biological tissues and, above all, their solid or fluid behavior influence the spread of malignant tumors. While it is known that solid tumors tend to have higher mechanical rigidity, allowing them to aggressively invade and spread in solid surrounding healthy tissue, it is unknown how softer tumors can grow within a more rigid environment such as the brain. Here, we use in vivo magnetic resonance elastography (MRE) to elucidate the role of anomalous fluidity for the invasive growth of soft brain tumors, showing that aggressive glioblastomas (GBMs) have higher water content while behaving like solids. Conversely, our data show that benign meningiomas (MENs), which contain less water than brain tissue, are characterized by fluid-like behavior. The fact that the 2 tumor entities do not differ in their soft properties suggests that fluidity plays an important role for a tumor's aggressiveness and infiltrative potential. Using tissue-mimicking phantoms, we show that the anomalous fluidity of neurotumors physically enables GBMs to penetrate surrounding tissue, a phenomenon similar to Saffman-Taylor viscous-fingering instabilities, which occur at moving interfaces between fluids of different viscosity. Thus, targeting tissue fluidity of malignant tumors might open horizons for the diagnosis and treatment of cancer.

Tomoelastography Distinguishes Noninvasively between Benign and Malignant Liver Lesions.

Patients with increased liver stiffness have a higher risk of developing cancer, however, the role of fluid-solid tissue interactions and their contribution to liver tumor malignancy remains elusive. Tomoelastography is a novel imaging method for mapping quantitatively the solid-fluid tissue properties of soft tissues in vivo. It provides high resolution and thus has clear clinical applications. In this work we used tomoelastography in 77 participants, with a total of 141 focal liver lesions of different etiologies, to investigate the contributions of tissue stiffness and fluidity to the malignancy of liver tumors. Shear-wave speed (c) as surrogate for tissue stiffness and phase-angle (φ) of the complex shear modulus reflecting tissue fluidity were abnormally high in malignant tumors and allowed them to be distinguished from nontumorous liver tissue with high accuracy [c: AUC = 0.88 with 95% confidence interval (CI) = 0.83-0.94; φ: AUC = 0.95, 95% CI = 0.92-0.98]. Benign focal nodular hyperplasia and hepatocellular adenoma could be distinguished from malignant lesions on the basis of tumor stiffness (AUC = 0.85, 95% CI = 0.72-0.98; sensitivity = 94%, 95% CI = 89-100; and specificity = 85%, 95% CI = 62-100), tumor fluidity (AUC = 0.86, 95% CI = 0.77-0.96; sensitivity = 83%, 95% CI = 72-93; and specificity = 92%, 95% CI = 77-100) and liver stiffness (AUC = 0.84, 95% CI = 0.74-0.94; sensitivity = 72%, 95% CI = 59-83; and specificity = 88%, 95% CI = 69-100), but not on the basis of liver fluidity. Together, hepatic malignancies are characterized by stiff, yet fluid tissue properties, whereas surrounding nontumorous tissue is dominated by solid properties. Tomoelastography can inform noninvasively on the malignancy of suspicious liver lesions by differentiating between benign and malignant lesions with high sensitivity based on stiffness and with high specificity based on fluidity. SIGNIFICANCE: Solid-fluid tissue properties measured by tomoelastography can distinguish malignant from benign masses with high accuracy and provide quantitative noninvasive imaging biomarkers for liver tumors.

Brain maturation is associated with increasing tissue stiffness and decreasing tissue fluidity

Biomechanical cues guide proliferation, growth and maturation of neurons. Yet the molecules that shape the brain’s biomechanical properties are unidentified and the relationship between neural development and viscoelasticity of brain tissue remains elusive.Here we combined novel in-vivo tomoelastography and ex-vivo proteomics to investigate whether viscoelasticity of the mouse brain correlates with protein alterations within the critical phase of brain maturation. For the first time, high-resolution atlases of viscoelasticity of the mouse brain were generated, revealing that (i) brain stiffness increased alongside progressive accumulation of microtubular structures, myelination, cytoskeleton linkage and cell-matrix attachment, and that (ii) viscosity-related tissue fluidity decreased alongside downregulated actin crosslinking and axonal organization.Taken together, our results show that brain maturation is associated with a shift of brain mechanical properties towards a more solid-rigid behavior consistent with reduced tissue fluidity. This shift appears to be driven by several molecular processes associated with myelination, cytoskeletal crosslinking and axonal organization. Statement of SignificanceThe viscoelastic properties of brain tissue shape the environment in which neurons proliferate, grow, and mature. In the present study, novel tomoelastography was used to spatially map tissue mechanical properties of the in-vivo mouse brain during maturation. In vivo tomoelastography was also combined with ex vivo mass spectrometry proteomic analysis to identify the molecules which shape the biomechanical properties of brain tissue. With the combined technique, we observed that brain maturation is associated with a shift of brain mechanical properties towards a more solid-rigid behavior consistent with reduced tissue fluidity which is driven by multiple molecular processes. We believe that this shift of brain mechanical properties discovered in our study reflects a fundamental biophysical signature of brain maturation.

Tissue Fluidity Promotes Epithelial Wound Healing.

SummaryThe collective behaviour of cells in epithelial tissues is dependent on their mechanical properties. However, the contribution of tissue mechanics to wound healing in vivo remains poorly understood. Here we investigate the relationship between tissue mechanics and wound healing in live Drosophila wing imaginal discs and show that by tuning epithelial cell junctional tension, we can systematically alter the rate of wound healing. Coincident with the contraction of an actomyosin purse string, we observe cells flowing past each other at the wound edge by intercalating, reminiscent of molecules in a fluid, resulting in seamless wound closure. Using a cell-based physical model, we predict that a reduction in junctional tension fluidises the tissue through an increase in intercalation rate and corresponding reduction in bulk viscosity, in the manner of an unjamming transition. The resultant fluidisation of the tissue accelerates wound healing. Accordingly, when we experimentally reduce tissue tension in wing discs, intercalation rate increases and wounds repair in less time.

Mesenchymal proteases and tissue fluidity remodel the extracellular matrix during airway epithelial branching in the embryonic avian lung.

Reciprocal epithelial-mesenchymal signaling is essential for morphogenesis, including branching of the lung. In the mouse, mesenchymal cells differentiate into airway smooth muscle that wraps around epithelial branches, but this contractile tissue is absent from the early avian lung. Here, we have found that branching morphogenesis in the embryonic chicken lung requires extracellular matrix (ECM) remodeling driven by reciprocal interactions between the epithelium and mesenchyme. Before branching, the basement membrane wraps the airway epithelium as a spatially uniform sheath. After branch initiation, however, the basement membrane thins at branch tips; this remodeling requires mesenchymal expression of matrix metalloproteinase 2, which is necessary for branch extension but for not branch initiation. As branches extend, tenascin C (TNC) accumulates in the mesenchyme several cell diameters away from the epithelium. Despite its pattern of accumulation, TNC is expressed exclusively by epithelial cells. Branch extension coincides with deformation of adjacent mesenchymal cells, which correlates with an increase in mesenchymal fluidity at branch tips that may transport TNC away from the epithelium. These data reveal novel epithelial-mesenchymal interactions that direct ECM remodeling during airway branching morphogenesis.

The same but different: cell intercalation as a driver of tissue deformation and fluidity.

The ability of cells to exchange neighbours, termed intercalation, is a key feature of epithelial tissues. Intercalation is predominantly associated with tissue deformations that drive morphogenesis. More recently, however, intercalation that is not associated with large-scale tissue deformations has been described both during animal development and in mature epithelial tissues. This latter form of intercalation appears to contribute to an emerging phenomenon that we refer to as tissue fluidity-the ability of cells to exchange neighbours without changing the overall dimensions of the tissue. Here, we discuss the contribution of junctional dynamics to intercalation governing both morphogenesis and tissue fluidity. In particular, we focus on the relative roles of junctional contractility and cell-cell adhesion as the driving forces behind intercalation. These two contributors to junctional mechanics can be used to simulate cellular intercalation in mechanical computational models, to test how junctional cell behaviours might regulate tissue fluidity and contribute to the maintenance of tissue integrity and the onset of disease.This article is part of the Theo Murphy meeting issue 'Mechanics of development'.

In vivo collective cell migration requires an LPAR2-dependent increase in tissue fluidity.

Collective cell migration (CCM) and epithelial-mesenchymal transition (EMT) are common to cancer and morphogenesis, and are often considered to be mutually exclusive in spite of the fact that many cancer and embryonic cells that have gone through EMT still cooperate to migrate collectively. Here we use neural crest (NC) cells to address the question of how cells that have down-regulated cell-cell adhesions can migrate collectively. NC cell dissociation relies on a qualitative and quantitative change of the cadherin repertoire. We found that the level of cell-cell adhesion is precisely regulated by internalization of N-cadherin downstream of lysophosphatidic acid (LPA) receptor 2. Rather than promoting the generation of single, fully mesenchymal cells, this reduction of membrane N-cadherin only triggers a partial mesenchymal phenotype. This intermediate phenotype is characterized by an increase in tissue fluidity akin to a solid-like-to-fluid-like transition. This change of plasticity allows cells to migrate under physical constraints without abolishing cell cooperation required for collectiveness.

Mechanics of Epithelial Tissue Homeostasis and Morphogenesis

Epithelia are robust tissues that support the structure of embryos and organs and serve as effective barriers against pathogens. Epithelia also chemically separate different physiological environments. These vital functions require tight association between cells through the assembly of junctions that mechanically stabilize the tissue. Remarkably, epithelia are also dynamic and can display a fluid behavior. Cells continuously die or divide, thereby allowing functional tissue homeostasis. Epithelial cells can change shape or intercalate as tissues deform during morphogenesis. We review the mechanical basis of tissue robustness and fluidity, with an emphasis on the pivotal role of junction dynamics. Tissue fluidity emerges from local active stresses acting at cell interfaces and allows the maintenance of epithelial organization during morphogenesis and tissue renewal.

Regulated tissue fluidity steers zebrafish body elongation

The tailbud is the posterior leading edge of the growing vertebrate embryo and consists of motile progenitors of the axial skeleton, musculature and spinal cord. We measure the 3D cell flow field of the zebrafish tailbud and identify changes in tissue fluidity revealed by reductions in the coherence of cell motion without alteration of cell velocities. We find a directed posterior flow wherein the polarization between individual cell motion is high, reflecting ordered collective migration. At the posterior tip of the tailbud, this flow makes sharp bilateral turns facilitated by extensive cell mixing due to increased directional variability of individual cell motions. Inhibition of Wnt or Fgf signaling or cadherin 2 function reduces the coherence of the flow but has different consequences for trunk and tail extension. Modeling and additional data analyses suggest that the balance between the coherence and rate of cell flow determines whether body elongation is linear or whether congestion forms within the flow and the body axis becomes contorted.

Cancer metastasis and in vivo dissemination of tissue-dwelling pathogens: Extrapolation of mechanisms and exchange of treatment strategies thereof

Most cancer cells would not result in devastating tumours if it were not for their ability to metastasize. The process of cancer metastasis involves significant cell shape, motility, and adhesive changes of pre-cancerous cells, and the remodelling of the extracellular matrix, as well as cognate properties of neighbouring normal cells. Such changes will be hereafter referred to as "tissue fluidity changes". A number of pathogens are known to disseminate to distant organs from sites of infection within a few days. A compromise on the ability to disseminate rapidly could be deleterious to the pathogen (e.g. the pathogen might be cleared before it reaches immuno-privileged sites within its host). Several ways of dissemination could be envisioned - and some are known to occur - ranging from rather passive such as outgrowth and lysis of tissues, residence in the bloodstream, "hitch-hiking" on migratory cells of the immune and lymphatic systems to an active dissemination process involving tissue fluidity changes similar to those that cancer cells invoke to be able to metastasize. The latter is particularly expected to be an important mechanism for the in vivo dissemination of tissue-dwelling pathogens. The mechanisms behind metastasis can, therefore, be viewed as part of the unifying features between cancer cells and pathogens other than their characteristic high proliferation index (at least in one form in the case of digenetic parasites). The current paper presents a synthesis of the hitherto reported but rather scattered data that broadly reinforce the premise of unifying metastasis processes. The overwhelming research outcome in cancer metastasis might therefore serve as a spring board for facilitating the studies of pathogen metastasis and, importantly, relevant cancer treatment strategies can be adopted to combat infectious diseases.

Organ printing: computer-aided jet-based 3D tissue engineering

Tissue engineering technology promises to solve the organ transplantation crisis. However, assembly of vascularized 3D soft organs remains a big challenge. Organ printing, which we define as computer-aided, jet-based 3D tissue-engineering of living human organs, offers a possible solution. Organ printing involves three sequential steps: pre-processing or development of ‘blueprints’ for organs; processing or actual organ printing; and postprocessing or organ conditioning and accelerated organ maturation. A cell printer that can print gels, single cells and cell aggregates has been developed. Layer-by-layer sequentially placed and solidified thin layers of a thermo-reversible gel could serve as ‘printing paper’. Combination of an engineering approach with the developmental biology concept of embryonic tissue fluidity enables the creation of a new rapid prototyping 3D organ printing technology, which will dramatically accelerate and optimize tissue and organ assembly.

Search for mycoplasma in synovial fluids from patients with rheumatoid arthritis.

<h3>Abstract</h3> Tight junctions are pivotal components of epithelial tissues connecting neighboring cells to provide protective barriers. However, explicit knowledge of their role during other crucial biological processes, such as collective cell migration, remains sparse. Here, the importance of the tight junction proteins ZO1 and ZO2 for epithelial migration is investigated employing video microscopy in conjunction with velocimetry, segmentation, cell tracking, and atomic force microscopy/spectroscopy. The results indicate that ZO proteins are necessary for fast and coherent migration. In particular, ZO1 and 2 loss induces actomyosin remodeling away from the central cortex towards the periphery of individual cells, resulting in altered viscoelastic properties. A tug-of-war emerges between two cell populations with distinct morphological and mechanical properties: 1) smaller and highly contractile cells with an outwards bulged apical membrane, and 2) larger, flattened cells, which, due to tensile stress, display a higher proliferation rate. In response, the cell density increases, leading to crowding- induced jamming and more small cells over time. These smaller cells are particularly immobile and therefore drive jamming. Knockout of only ZO1 induces a similar but less pronounced behavior. This study shows that ZO proteins are necessary for efficient collective cell migration by maintaining tissue fluidity and controlling proliferation.

Probable use of atomic power

Most cancer cells would not result in devastating tumours if it were not for their ability to metastasize. The process of cancer metastasis involves significant cell shape, motility, and adhesive changes of pre-cancerous cells, and the remodelling of the extracellular matrix, as well as cognate properties of neighbouring normal cells. Such changes will be hereafter referred to as “tissue fluidity changes”. A number of pathogens are known to disseminate to distant organs from sites of infection within a few days. A compromise on the ability to disseminate rapidly could be deleterious to the pathogen (e.g. the pathogen might be cleared before it reaches immuno-privileged sites within its host). Several ways of dissemination could be envisioned – and some are known to occur – ranging from rather passive such as outgrowth and lysis of tissues, residence in the bloodstream, “hitch-hiking” on migratory cells of the immune and lymphatic systems to an active dissemination process involving tissue fluidity changes similar to those that cancer cells invoke to be able to metastasize. The latter is particularly expected to be an important mechanism for the in vivo dissemination of tissue-dwelling pathogens. The mechanisms behind metastasis can, therefore, be viewed as part of the unifying features between cancer cells and pathogens other than their characteristic high proliferation index (at least in one form in the case of digenetic parasites). The current paper presents a synthesis of the hitherto reported but rather scattered data that broadly reinforce the premise of unifying metastasis processes. The overwhelming research outcome in cancer metastasis might therefore serve as a spring board for facilitating the studies of pathogen metastasis and, importantly, relevant cancer treatment strategies can be adopted to combat infectious diseases.