Functional Epithelium Remodeling in Response to Applied Stress under In Vitro Conditions

Alginate Gel-Based Carriers for Encapsulation of Carotenoids: On Challenges and Applications

Sodium alginate is one of the most interesting and the most investigated and applied biopolymers due to its advantageous properties. Among them, easy, simple, mild, rapid, non-toxic gelation by divalent cations is the most important. In addition, it is abundant, low-cost, eco-friendly, bio-compatible, bio-adhesive, biodegradable, stable, etc. All those properties were systematically considered within this review. Carotenoids are functional components in the human diet with plenty of health benefits. However, their sensitivity to environmental and process stresses, chemical instability, easy oxidation, low water solubility, and bioavailability limit their food and pharmaceutical applications. Encapsulation may help in overcoming these limitations and within this review, the role of alginate-based encapsulation systems in improving the stability and bioavailability of carotenoids is explored. It may be concluded that all alginate-based systems increase carotenoid stability, but only those of micro- and nano-size, as well as emulsion-based, may improve their low bioaccessibility. In addition, the incorporation of other biopolymers may further improve encapsulation system properties. Furthermore, the main techniques for evaluating the encapsulation are briefly considered. This review critically and profoundly explains the role of alginates in improving the encapsulation process of carotenoids, suggesting the best alternatives for those systems. Moreover, it provides a comprehensive cover of recent advances in this field.

The rearrangement of co-cultured cellular model systems via collective cell migration

Cancer invasion through the surrounding epithelium and extracellular matrix (ECM) is the one of the main characteristics of cancer progression. While significant effort has been made to predict cancer cells response under various drug therapies, much less attention has been paid to understand the physical interactions between cancer cells and their microenvironment, which are essential for cancer invasion. Considering these physical interactions on various co-cultured in vitro model systems by emphasizing the role of viscoelasticity, the tissue surface tension, solid stress, and their inter-relations is a prerequisite for establishing the main factors that influence cancer cell spread and develop an efficient strategy to suppress it. This review focuses on the role of viscoelasticity caused by collective cell migration (CCM) in the context of mono-cultured and co-cultured cancer systems, and on the modeling approaches aimed at reproducing and understanding these biological systems. In this context, we do not only review previously-published biophysics models for collective cell migration, but also propose new extensions of those models to include solid stress accumulated within the spheroid core region and cell residual stress accumulation caused by CCM.

The role of viscoelasticity in long time cell rearrangement

Cell rearrangement caused by collective cell migration (CCM) during free expansion of epithelial monolayers has become a landmark in our current understanding of fundamental biological processes such as tissue development, regeneration, wound healing or cancer invasion. Cell spreading causes formation of mechanical waves which has a feedback effect on cell rearrangement and can lead to the cell jamming state. The mechanical waves describe oscillatory changes in cell velocity, as well as, the rheological parameters that affect them. The velocity oscillations, obtained at a time scale of hours, are in the form of forward and backward flows. Collision of forward and backward flows can induce an increase in the cell compressive stress accompanied with cell packing density which have a feedback impact on cell mobility, tissue viscoelasticity and alters the tissue stiffness. The tissue stiffness depends on the cell packing density and the active/passive (i.e. migrating/resting) state of single cells and can be used as an indicator of cell jamming state transition. Since cell stiffness can be measured it may directly show in which state the multicellular system is. In this work a review of existing modeling approaches is given along with assortment of published experimental findings, in order to invite experimentalists to test given theoretical considerations in multicellular systems.

Viscoelasticity, Like Forces, Plays a Role in Mechanotransduction

Viscoelasticity and its alteration in time and space has turned out to act as a key element in fundamental biological processes in living systems, such as morphogenesis and motility. Based on experimental and theoretical findings it can be proposed that viscoelasticity of cells, spheroids and tissues seems to be a collective characteristic that demands macromolecular, intracellular component and intercellular interactions. A major challenge is to couple the alterations in the macroscopic structural or material characteristics of cells, spheroids and tissues, such as cell and tissue phase transitions, to the microscopic interferences of their elements. Therefore, the biophysical technologies need to be improved, advanced and connected to classical biological assays. In this review, the viscoelastic nature of cytoskeletal, extracellular and cellular networks is presented and discussed. Viscoelasticity is conceptualized as a major contributor to cell migration and invasion and it is discussed whether it can serve as a biomarker for the cells’ migratory capacity in several biological contexts. It can be hypothesized that the statistical mechanics of intra- and extracellular networks may be applied in the future as a powerful tool to explore quantitatively the biomechanical foundation of viscoelasticity over a broad range of time and length scales. Finally, the importance of the cellular viscoelasticity is illustrated in identifying and characterizing multiple disorders, such as cancer, tissue injuries, acute or chronic inflammations or fibrotic diseases.

Multiscale nature of cell rearrangement caused by collective cell migration

Collective cell migration (CCM), a highly coordinated and fine-tuned migratory mode, is involved in a plethora of biological processes, such as embryogenesis, tissue repair and cancer invasion. Although a good comprehension of how cells collectively migrate by following molecular rules has been generated, the impact of cellular rearrangements on collective migration remains less understood. Thus, considering CCM from a multi-scale quantitative approach could result in a powerful tool to address the contribution of cellular rearrangements in CCM and help to understand this important but still controversial topic. In this work, a review of existing literature in CCM modeling at different scales is given along with assortment of published experimental findings, to invite experimentalists to test given theoretical considerations in multicellular systems. In addition, three different time and space scales (free or weakly connected cells, cluster of cells and collision fronts of different cells clusters) are considered and the multi-scale nature of those processes was discussed with special emphasis of jamming and unjamming states.

Collective cell migration and residual stress accumulation: Rheological consideration

Stress generation during collective cell migration represents one of the key factors which influence the configuration of migrating cells, viscoelasticity of multicellular systems and their inter-relation. Local generation of stress (normal and shear) is significant even in 2D. Normal stress is primarily accumulated within a core region of migrating cell clusters during their movement through the dense environment and during the collisions of migrating cell clusters caused by uncorrelated motility. Shear stress can be significant within perturbed boundary layers around migrating clusters. Cells are more sensitive to the action of shear stress compared with normal stress. Shear stress of a few Pa significantly influences cell state. Prior studies have shown that collectively migrating cells move in such a way to minimize this stress, but it has not yet been determined how cells effectively minimize it. Deeper insight into possible cell mechanisms for minimizing undesirable shear stress would be of great importance because it may help to direct morphogenesis, accelerate wound healing or prevent cancer invasion. In the proposed model three primary mechanisms in which cells may reduce shear are given: decreasing speed, tissue thickening, and/or reducing slip effects. Suggestions obtained from the proposed model indicate a need for further experimental studies that will reveal whether the heterogeneity in the cell-cell adhesion types correlates well with the stiffness inhomogeneity, or changes in the adhesion clustering, cytoskeletal linkage or some other modification to the adhesion complex (adherens junctions or tight junctions) are occurring to influence overall adhesive strength.

Jamming state transition and collective cell migration

Jamming state transition has been used in literature to describe migrating-to-resting cell state transition during collective cell migration without proper rheological confirmation. Yield stress often has been used as an indicator of a jamming state. Yield stress points to the liquid-to-solid state transition, but not a priori to jamming state transition. Various solid states such as elastic solid and viscoelastic solids can be considered in the context of their ability to relax. The relaxation time for (1) an elastic solid tends to zero, (2) Kelvin-Voigt viscoelastic solid is finite, and (3) jamming state tends to infinity.

In order to clarify the meaning of jamming state from the rheological standpoint we formulated the constitutive model of this state based on following conditions (1) migration of the system constituents is much damped such that the diffusion coefficient tends to zero, (2) relaxation time tends to infinity, (3) storage and loss moduli satisfy the condition G^′(ω)/G^"(ω) = const > 1. Jamming state represents the non-linear viscoelastic solid state. The main characteristic of this state is that the system cannot relax.

Jamming state transition of multicellular systems caused by collective cell migration is discussed on a model system such as cell aggregate rounding after uni-axial compression between parallel plates based on the data from the literature. Cell aggregate rounding occurs via successive relaxation cycles. Every cycle corresponds to a different scenario of cell migration. Three scenarios were established depending on the magnitude of mechanical and biochemical perturbations (1) ordered scenario with reduced perturbations corresponds to the case that most of the cells migrate, (2) disordered scenario corresponds to the case that some cell groups migrate while the others (at the same time) stay in resting state (corresponds to medium perturbations), and (3) highly suppressed cell migration under large perturbations corresponds to the viscoelastic solid under jamming state. If cells reach the jamming state in one cycle, they are able to overcome this undesirable state and start migrating again in the next cycle by achieving the first or second scenarios again.