Convection and diffusion in tissues and tissue cultures

Design of a versatile microfluidic device for imaging precision-cut-tissue slices

Precision-cut-tissues (PCTs), which preserve many aspects of a tissue's microenvironment, are typically imaged using conventional sample dishes and chambers. These can require large amounts of reagent and, when used for flow-through experiments, the shear forces applied on the tissues are often ill-defined. Their physical design also makes it difficult to image large volumes and repetitively image smaller regions of interest in the living slice. We report here on the design of a versatile microfluidic device capable of holding mouse or human pancreas PCTs for 3D fluorescence imaging using confocal and selective plane illumination microscopy (SPIM). Our design positions PCTs within a 5 × 5 mm × 140µm deep chamber fitted with 150µm tall channels to facilitate media exchange. Shear stress in the device is localized to small regions on the surface of the tissue and can be easily controlled. This design allows for media exchange at flowrates ∼10-fold lower than those required for conventional chambers. Finally, this design allows for imaging the same immunofluorescently labeled PCT with high resolution on a confocal and with large field of view on a SPIM, without adversely affecting image quality.

A Quick and Reliable Method to Decellularize a Gracilis Flap: A Crucial Step Toward Building a Muscle

INTRODUCTION

Tissue loss as a consequence of congenital anomalies, trauma, malignancy, or gangrene represents a major health care problem in the United States. Because younger individuals are disproportionately affected, the costs are magnified over time and the resultant individual and societal effects are tremendous. The currently available options to restore soft tissue defects are associated with donor site morbidities. Vascularized composite allotransplantation may provide form, function, and esthetics without a donor site; however, it comes with the significant risk associated with toxic immunosuppression (Biomaterials. 2015;61:246-256, Ann Plast Surg. 2015;75(1):112-116, Transplantation. 2009;88(2):203-210). Engineered tissues offer promise in finding viable alternatives to allograft and autologous tissues. In this study, we present our simple and quick method to decellularize a muscle without disrupting the vascular network integrity or the extracellular matrix. Optimizing the decellularization process is a crucial step toward creating an "off-the-shelf" flap that can be used for soft tissue reconstruction.

METHODS

The superficial gracilis muscle of 20 rats were harvested on their circulation and decellularized using perfusion with Krebs-Henseleit buffer and sodium dodecyl sulfate for 6 hours. These flaps were evaluated by gross morphology, histology, DNA quantification, integrity of the vascular network, scanning electron microscopy, and transmission electron microscopy.

RESULTS

All samples were decellularized successfully as determined by DNA content and histological analysis for cellular content. The vascular network was preserved in all samples.

CONCLUSIONS

We present a quick, simple, and affordable method to decellularize a muscle flap through the vascular network. Our proposed method is efficient and can be completed in a significantly shorter time when compared with other methods. It is also safe and does not affect integrity of tissue, and this is essential for a reliable recellularization.

Plans and detours

At various stages of my career, I made and followed plans for progress and growth. However, owing to several unexpected challenges, I had to make detours into unplanned vistas. When I look around at the careers of my former students and younger colleagues, I see fewer long-term, stable positions and frequent changes both in employers and in functions every ten or twenty years. Our curriculum prepares our students for manufacturing careers in the United States, but the current trend of corporate merger and outsourcing can suddenly turn their work into marketing or finance in the Far East. A fox that knows many tricks may be better suited to changing environments than a hedgehog that knows only one single trick. Perhaps future education should not only teach few subjects in depth for immediate career needs but also teach adaptability to unrelated challenges.

A novel alginate hollow fiber bioreactor process for cellular therapy applications

Gel-matrix culture environments provide tissue engineering scaffolds and cues that guide cell differentiation. For many cellular therapy applications such as for the production of islet-like clusters to treat Type 1 diabetes, the need for large-scale production can be anticipated. The throughput of the commonly used nozzle-based devices for cell encapsulation is limited by the rate of droplet formation to approximately 0.5 L/h. This work describes a novel process for larger-scale batch immobilization of mammalian cells in alginate-filled hollow fiber bioreactors (AHFBRs). A methodology was developed whereby (1) alginate obstruction of the intra-capillary space medium flow was negligible, (2) extra-capillary alginate gelling was complete and (3) 83 +/- 4% of the cells seeded and immobilized were recovered from the bioreactor. Chinese hamster ovary (CHO) cells were used as a model aggregate-forming cell line that grew from mostly single cells to pancreatic islet-sized spheroids in 8 days of AHFBR culture. CHO cell growth and metabolic rates in the AHFBR were comparable to small-scale alginate slab controls. Then, the process was applied successfully to the culture of primary neonatal pancreatic porcine cells, without significant differences in cell viability compared with slab controls. As expected, alginate-immobilized culture in the AHFBR increased the insulin content of these cells compared with suspension culture. The AHFBR process could be refined by adding matrix components or adapted to other reversible gels and cell types, providing a practical means for gel-matrix assisted cultures for cellular therapy.

Mechanisms of interstitial flow-induced remodeling of fibroblast-collagen cultures

Interstitial fluid flow, critical for macromolecular transport, was recently shown to drive fibroblast differentiation and perpendicular cell and matrix alignment in 3D collagen cultures. Here we explore the mechanisms underlying this flow-induced cell and collagen alignment. Cell and matrix alignment was assessed from 3D confocal reflectance stacks using a Fast Fourier Transform method. We found that human dermal and lung fibroblasts align perpendicular to flow in the range of 5-13 mum/s (0.1-0.3 dyn/cm(2)) in collagen; however, neither cells nor matrix fibers align in fibrin cultures, which unlike collagen, is covalently cross-linked and generally degraded by cell fibrinolysis. We also found that even acellular collagen matrices align weakly upon exposure to flow. Matrix alignment begins within 12 h of flow onset and continues, along with cell alignment, over 48 h. Together, these data suggest that interstitial flow first induces collagen fiber alignment, providing contact guidance for the cells to orient along the aligned matrix; later, the aligned cells further remodel and align their surrounding matrix fibers. These findings help elucidate the effects of interstitial flow on cells in matrices and have relevance physiologically in tissue remodeling and in tissue engineering applications.

Engineering principles of clinical cell-based tissue engineering

Tissue engineering is a rapidly evolving discipline that seeks to repair, replace, or regenerate specific tissues or organs by translating fundamental knowledge in physics, chemistry, and biology into practical and effective materials, devices, systems, and clinical strategies. Stem cells and progenitors that are capable of forming new tissue with one or more connective tissue phenotypes are available from many adult tissues and are defined as connective tissue progenitors. There are four major cell-based tissue-engineering strategies: (1) targeting local connective tissue progenitors where new tissue is desired, (2) transplanting autogenous connective tissue progenitors, (3) transplanting culture-expanded or modified connective tissue progenitors, and (4) transplanting fully formed tissue generated in vitro or in vivo. Stem cell function is controlled by changes in stem cell activation and self-renewal or by changes in the proliferation, migration, differentiation, or survival of the progeny of stem cell activation, the downstream progenitor cells. Three-dimensional porous scaffolds promote new tissue formation by providing a surface and void volume that promotes the attachment, migration, proliferation, and desired differentiation of connective tissue progenitors throughout the region where new tissue is needed. Critical variables in scaffold design and function include the bulk material or materials from which it is made, the three-dimensional architecture, the surface chemistry, the mechanical properties, the initial environment in the area of the scaffold, and the late scaffold environment, which is often determined by degradation characteristics. Local presentation or delivery of bioactive molecules can change the function of connective tissue progenitors (activation, proliferation, migration, differentiation, or survival) in a manner that results in new or enhanced local tissue formation. All cells require access to substrate molecules (oxygen, glucose, and amino acids). A balance between consumption and local delivery of these substrates is needed if cells are to survive. Transplanted cells are particularly vulnerable. Theoretical calculations can be used to explore the relationships among cell density, diffusion distance, and cell viability within a graft and to design improved strategies for transplantation of connective tissue progenitors. Rational strategies for tissue engineering seek to optimize new tissue formation through the logical selection of conditions that modulate the performance of connective tissue progenitors in a graft site to produce a desired tissue. This increasingly involves strategies that combine cells, matrices, inductive stimuli, and techniques that enhance the survival and performance of local or transplanted connective tissue progenitors.

Immobilized mammalian cell cultivation in hollow fiber bioreactors

Cultivation of animal cells for the production of recombinant proteins is an important method for manufacturing complex proteins requiring posttranslational processing. One of the often considered methods for cultivation is by immobilization of the cells in hollow fiber bioreactors (HFBRs). These systems allow the cells to grow to high densities in a shear protected environment; furthermore the product can be accumulated in high concentration in the case of ultrafiltration HFBRs. Operation and scale-up are constrained by nutrient and product transport with oxygen transfer to growing cells being the most critical parameter. Mathematical models describing HFBRs have proved to be useful in quantitating and understanding the constraints and guiding the scale-up of this approach to animal cell cultivation.

Evidence of transcellular permeability pathway in microvessels

We used video fluorescence microscopy of the vascular bed in the cremaster muscle of rat and mouse to study the transfer of plasmalemma vesicles (caveolae) across the microvessel barrier in situ. The water-soluble styryl pyridinium dye RH414, which adsorbs to and fluoresces at the membrane-water interface, was used as a marker for vesicular traffic through endothelial cells. Fluorescein isothiocyanate (FITC), similar in molecular size to the styryl pyridinium probe, was used to mark for dye transfer by the paracellular pathway. Transcellular dye flux was determined by comparing the fluorescence intensities of RH414 and FITC on either side of the vessel wall (i.e., in microvessel lumen and in muscle tissue at various distances from the microvessel wall). We observed that RH414 accumulated in the interstitium more rapidly than FITC. We next studied the role of the 60-kDa albumin-binding glycoprotein gp60, hypothesized to activate transcellular permeability, in stimulating the transcellular vesicle traffic. Introduction of anti-gp60 antibody into the microvessel to cross-link and activate gp60 markedly increased the transvascular flux of RH414. Control isotype-matched antibody had no effect on the RH414 flux. The sterol-binding agent filipin, which disassembles caveolae, inhibited the RH414 flux induced by gp60 cross-linking. The transfer of styryl pyridinium dyes in intact microvessels suggests that plasmalemmal membrane traffic across the skeletal muscle microvessel barrier is a constitutively active process. The results indicate that the gp60-dependent pathway is important in regulating endothelial permeability in situ via a transcellular mechanism.

Oxygen transfer in a diffusion-limited hollow fiber bioartificial liver

A mathematical model was developed to predict oxygen transport in a hollow fiber bioartificial liver device. Model parameters were taken from the Hepatix ELAD configuration; a blood perfused hollow fiber cartridge with hepatocytes seeded in the extracapillary space. Cellular oxygen uptake is based on Michaelis-Menten kinetics, and nonlinear oxygen transport in the blood is considered. The effect of modulating three important parameters is investigated, namely, the Michaelis-Menten constants Vm (volumetric oxygen consumption of the hepatocytes) and Km (half-saturation constant), and hollow fiber oxygen permeability. A computer implementation of the model is used to assess whether a given cell mass could be maintained within such a device. The results suggest that liver cell lines possessing low rates of oxygen consumption could be maintained if membranes of sufficiently high oxygen permeability are used. For primary hepatocytes, which have much higher oxygen demands, radial transport of oxygen is rate limiting, and the axial-flow hollow fiber cartridge is thus an inappropriate design for use as a bioartificial liver with primary hepatocytes.

Erythrocyte aggregation and erythrocyte deformability modify the permeability of erythrocyte enriched fibrin network

Intravascular thrombus formed under low shear conditions consists of red cells enmeshed within a fibrin network. Since red cells reduce the permeability of fibrin network by surface drag and by volume occupancy the significance of red cell aggregability and deformability in network permeability needs examination. In this study networks were developed by the addition of thrombin to washed red cells suspended in platelet free plasma. The effects of the polymers polyvinylpyrrolidone (PVP) and poloxamer 188 on network permeability were compared to gauge the influence of red cell aggregation. Both polymers increase network permeability by an action on fibrin polymerisation but PVP alone enhances red cell aggregation. PVP was found to increase network permeability significantly both by increasing the permeability of the fibrin component of the network and by increasing red cell aggregation and thus reducing red cell surface drag. In separate experiments red cells were pre-treated with heat, glutaraldehyde, or diamide to reduce cell deformability. Decreased cell deformability caused significant reductions in network permeability. This was ascribed to the reduced aggregability of hardened red cells. Red cell aggregation during coagulation enhances molecular transport through modifying the network. This may have implications for the penetration of fibrinolytic agents.

Effect of feed flow-rate, antigen concentration and antibody density on immunoaffinity purification of coagulation factor IX

A simple physical model of immunoaffinity chromatography (IAC) demonstrates that immobilized monoclonal antibody (MAb) capacity in IAC purification will be a function of many parameters, including feed flow-rate and antigen concentration, and MAb density (mg MAb immobilized/ml resin). We studied IAC of factor IX, and examined the effect of parameter variation on MAb capacity. MAb capacity (1) was not affected by feed flow-rate or antigen concentration, and (2) decreased as MAb density increased. (1) Suggested that diffusion of factor IX into the resin bead was not limiting. Characteristic diffusion, convection and reaction times were calculated and used in dimensional analysis to compare their relative magnitudes. If MAb was assumed to be localized to the outer 10% of the bead volume, this analysis concluded that diffusion was not limiting, consistent with the suggestions of our experimental data. (2) Suggests that high MAb densities make MAb less accessible.

A mechanism for the regulation of ligament width based on the resonance frequency of ion concentration waves

A model is developed for a mechanism for the regulation of the width of ligament spaces and of other tissue spaces bounded by calcified surfaces. The proposed mechanism involves the transmission, detection, and retransmission of ion concentration waves by cells located on the calcified surfaces. It is assumed that these cells can use the information regarding ligament width contained in the resonance frequency of the cell-concentration wave system. The assumptions of the proposed mechanism are supported by recent experimental evidence concerning the effect of electrical signals on bone cells, the use of frequency-encoded information by cells, and the production of low frequency K+ pulses by osteoblast-like cells. The relation between resonance frequency and ligament width is derived, and the resonance frequencies corresponding to measured ligament widths are shown to occur in the same frequency range as occur in the K+ pulses emitted by bone cells. The model suggests definite experimental tests that involve investigating the effect in vitro of ion concentration wave frequency on bone cell activity and hormone receptors.