A fibrin or collagen gel assay for tissue cell chemotaxis: assessment of fibroblast chemotaxis to GRGDSP

Chemotaxis Model for Breast Cancer Cells Based on Signal/Noise Ratio

Chemotaxis, a biased migration of cells under a chemical gradient, plays a significant role in diverse biological phenomena including cancer metastasis. Stromal cells release signaling proteins to induce chemotaxis, which leads to organ-specific metastasis. Epidermal growth factor (EGF) is an example of the chemical attractants, and its gradient stimulates metastasis of breast cancer cells. Hence, the interactions between EGF and breast cancer cells have long been a subject of interest for oncologists and clinicians. However, most current approaches do not systematically separate the effects of gradient and absolute concentration of EGF on chemotaxis of breast cancer cells. In this work, we develop a theoretical model based on signal/noise ratio to represent stochastic properties and report our microfluidic experiments to verify the analytical predictions from the model. The results demonstrate that even under the same EGF concentration gradients (0-50 or 0-150 ng/mL), breast cancer cells reveal a more evident chemotaxis pattern when the absolute EGF concentrations are low. Moreover, we found that reducing the number of EGF receptors (EGFRs) with addition of EGFR antibody (1 ng/mL) can promote chemotaxis at an EGF gradient of 0-1 ng/mL as shown by chemotaxis index (0.121 ± 0.037, reduced EGFRs vs. 0.003 ± 0.041, control). This counterintuitive finding suggests that EGFR-targeted therapy may stimulate metastasis of breast cancer because the partial suppression of the receptors makes the number of receptors close to the optimal one for chemotaxis. This analysis should be considered in anticancer drug design.

In Vitro Tissue-Engineered Skeletal Muscle Models for Studying Muscle Physiology and Disease

Healthy skeletal muscle possesses the extraordinary ability to regenerate in response to small-scale injuries; however, this self-repair capacity becomes overwhelmed with aging, genetic myopathies, and large muscle loss. The failure of small animal models to accurately replicate human muscle disease, injury and to predict clinically-relevant drug responses has driven the development of high fidelity in vitro skeletal muscle models. Herein, the progress made and challenges ahead in engineering biomimetic human skeletal muscle tissues that can recapitulate muscle development, genetic diseases, regeneration, and drug response is discussed. Bioengineering approaches used to improve engineered muscle structure and function as well as the functionality of satellite cells to allow modeling muscle regeneration in vitro are also highlighted. Next, a historical overview on the generation of skeletal muscle cells and tissues from human pluripotent stem cells, and a discussion on the potential of these approaches to model and treat genetic diseases such as Duchenne muscular dystrophy, is provided. Finally, the need to integrate multiorgan microphysiological systems to generate improved drug discovery technologies with the potential to complement or supersede current preclinical animal models of muscle disease is described.

A tuneable microfluidic system for long duration chemotaxis experiments in a 3D collagen matrix

In many cell types, migration can be oriented towards a chemical stimulus. In mammals, for example, embryonic cells migrate to follow developmental cues, immune cells migrate toward sites of inflammation, and cancer cells migrate away from the primary tumour and toward blood vessels during metastasis. Understanding how cells migrate in 3D environments in response to chemical cues is thus crucial to understanding directed migration in normal and disease states. To date, chemotaxis in mammalian cells has been primarily studied using 2D migration models. However, it is becoming increasingly clear that the mechanisms by which cells migrate in 2D and 3D environments dramatically differ, and cells in their native environments are confronted with a complex chemical milieu. To address these issues, we developed a microfluidic device to monitor the behaviour of cells embedded in a 3D collagen matrix in the presence of complex concentration fields of chemoattractants. This tuneable microsystem enables the generation of (1) homogeneous, stationary gradients set by a purely diffusive mechanism, or (2) spatially evolving, stationary gradients, set by a convection-diffusion mechanism. The device allows for stable gradients over several days and is large enough to study the behaviour of large cell aggregates. We observe that primary mature dendritic cells respond uniformly to homogeneous diffusion gradients, while cell behaviour is highly position-dependent in spatially variable convection-diffusion gradients. In addition, we demonstrate a directed response of cancer cells migrating away from tumour-like aggregates in the presence of soluble chemokine gradients. Together, this microfluidic device is a powerful system to observe the response of different cells and aggregates to tuneable chemical gradients.

Factors That Affect Tissue-Engineered Skeletal Muscle Function and Physiology

Tissue-engineered skeletal muscle has the promise to be a tool for studying physiology, screening muscle-active drugs, and clinical replacement of damaged muscle. To maximize the potential benefits of engineered muscle, it is important to understand the factors required for tissue formation and how these affect muscle function. In this review, we evaluate how biomaterials, cell source, media components, and bioreactor interventions impact muscle function and phenotype.

Highly efficient intracellular transduction in three-dimensional gradients for programming cell fate

Fundamental behaviour such as cell fate, growth and death are mediated through the control of key genetic transcriptional regulators. These regulators are activated or repressed by the integration of multiple signalling molecules in spatio-temporal gradients. Engineering these gradients is complex but considered key in controlling tissue formation in regenerative medicine approaches. Direct programming of cells using exogenously delivered transcription factors can by-pass growth factor complexity but there is still a requirement to deliver such activity spatio-temporally. We previously developed a technology termed GAG-binding enhanced transduction (GET) to efficiently deliver a variety of cargoes intracellularly using GAG-binding domains to promote cell targeting, and cell penetrating peptides (CPPs) to allow cell entry. Herein we demonstrate that GET can be used in a three dimensional (3D) hydrogel matrix to produce gradients of intracellular transduction of mammalian cells. Using a compartmentalised diffusion model with a source-gel-sink (So-G-Si) assembly, we created gradients of reporter proteins (mRFP1-tagged) and a transcription factor (TF, myogenic master regulator MyoD) and showed that GET can be used to deliver molecules into cells spatio-temporally by monitoring intracellular transduction and gene expression programming as a function of location and time. The ability to spatio-temporally control the intracellular delivery of functional proteins will allow the establishment of gradients of cell programming in hydrogels and approaches to direct cellular behaviour for many regenerative medicine applications.

STATEMENT OF SIGNIFICANCE

Regenerative medicine aims to reform functional biological tissues by controlling cell behaviour. Growth factors (GFs) are soluble cues presented to cells in spatio-temporal gradients and play important roles programming cell fate and gene expression. The efficient transduction of cells by GET (Glycosaminoglycan-enhanced transducing)-tagged transcription factors (TFs) can be used to by-pass GF-stimulation and directly program cells. For the first time we demonstrate diffusion of GET proteins generate stable protein transduction gradients. We demonstrated the feasibility of creating spatio-temporal gradients of GET-MyoD and show differential programing of myogenic differentiation. We believe that GET could provide a powerful tool to program cell behaviour using gradients of recombinant proteins that allow tissue generation directly by programming gene expression with TFs.

Production of Highly Aligned Collagen Scaffolds by Freeze-drying of Self-assembled, Fibrillar Collagen Gels

Matrix and cellular alignment are critical factors in the native function of many tissues, including muscle, nerve, and ligaments. Collagen is frequently a component of these aligned tissues, and collagen biomaterials are widely used in tissue engineering applications. However, the generation of aligned collagen scaffolds that maintain the native architecture of collagen fibrils has not been straightforward, with many methods requiring specialized equipment or technical procedures, extensive incubation times, or denaturing of the collagen. Herein, we present a simple, rapid method for fabrication of highly aligned collagen scaffolds. Collagen was assembled to form a fibrillar hydrogel in a cylindrical conduit with high aspect ratio and then frozen and lyophilized. The resulting collagen scaffolds demonstrated highly aligned topographical features along the scaffold surface. This presence of an initial fibrillar network and the high-aspect ratio vessel were both required to generate alignment. The diameter of fabricated scaffolds was found to vary significantly with both the collagen concentration of the hydrogel suspension and the diameter of conduits used for fabrication. Additionally, the size of individual aligned topographical features was significantly dependent on the conduit diameter and the freezing temperature. When cultured on aligned collagen scaffolds, both rat dermal fibroblasts and axons emerging from chick dorsal root ganglia explants demonstrated elongated, aligned morphology and growth on the aligned topographical features. Overall, this method presents a simple means for generating aligned collagen scaffolds that can be applied to a wide variety of tissue types, particularly those where such alignment is critical to native function.

Evaluation of polycaprolactone scaffold with basic fibroblast growth factor and fibroblasts in an athymic rat model for anterior cruciate ligament reconstruction

Anterior cruciate ligament (ACL) rupture is a common ligamentous injury often necessitating surgery. Current surgical treatment options include ligament reconstruction with autograft or allograft, which have their inherent limitations. Thus, there is interest in a tissue-engineered substitute for use in ACL regeneration. However, there have been relatively few in vivo studies to date. In this study, an athymic rat model of ACL reconstruction was used to evaluate electrospun polycaprolactone (PCL) grafts, with and without the addition of basic fibroblast growth factor (bFGF) and human foreskin fibroblasts. We examined the regenerative potential of tissue-engineered ACL grafts using histology, immunohistochemistry, and mechanical testing up to 16 weeks postoperatively. Histology showed infiltration of the grafts with cells, and immunohistochemistry demonstrated aligned collagen deposition with minimal inflammatory reaction. Mechanical testing of the grafts demonstrated significantly higher mechanical properties than immediately postimplantation. Acellular grafts loaded with bFGF achieved 58.8% of the stiffness and 40.7% of the peak load of healthy native ACL. Grafts without bFGF achieved 31.3% of the stiffness and 28.2% of the peak load of healthy native ACL. In this in vivo rodent model study for ACL reconstruction, the histological and mechanical evaluation demonstrated excellent healing and regenerative potential of our electrospun PCL ligament graft.

Nanofibrous scaffold with incorporated protein gradient for directing neurite outgrowth

Concentration gradient of diffusible bioactive chemicals assumes many important roles in regulating cellular behavior. Among the many factors influencing functional recovery after nerve injury, such as topographical and biochemical signals, concentration gradients of neurotrophic factors provide chemotactic cues for neurite outgrowth and targeted renervation. In this study, a concentration gradient of nerve growth factor (NGF, 0-250 μg/ml) was incorporated throughout the thickness of poly(ε-caprolactone)-poly(ethylene glycol) coaxial electrospun nanofibrous scaffolds (∼700 μm thick with ∼800 nm average fiber diameter). The existence of the protein gradient upon protein release was demonstrated using a customized under-agarose-PC12 neurite outgrowth assay. When exposed to scaffolds endowed with NGF concentration gradient (NGF-CG), a significant difference in the percentage of cells bearing neurite outgrowth was observed (7.1 ± 1.9% vs. 0.8 ± 0.3% for cells exposed to high vs. low concentration surface, respectively; p < 0.05). In contrast, no significant difference was observed when cells were exposed to scaffolds that encapsulated a fixed concentration of NGF. Direct culture of PC12 cells on the substrates demonstrated the cytocompatibility and the effect of diffusible NGF gradient on neurite outgrowth. A significant difference in the percentage of cells with neurite extensions was observed when PC12 cells were seeded on NGF-CG scaffolds (21.2 ± 3.6% vs. 10.4 ± 1.3% on high vs. low concentration surface, respectively; p < 0.05). Furthermore, Z-stack confocal microscopy tracking of neurite extensions revealed the chemotatic guidance effect of NGF concentration gradient. Directed and enhanced neurite penetration into the scaffolds towards increasing NGF concentration was observed. In vitro release study indicated that the encapsulated NGF was released in a sustained manner for at least 30 days (80.4 ± 3.6% released). Taken together, this study demonstrates the feasibility of incorporating concentration gradient of diffusible bioactive chemicals in nanofibrous scaffolds via the coaxial electrospinning technique.

In vitro and in vivo evaluation of heparin mediated growth factor release from tissue-engineered constructs for anterior cruciate ligament reconstruction

Anterior cruciate ligament (ACL) rupture is a common injury often necessitating surgical treatment with graft reconstruction. Due to limitations associated with current graft options, there is interest in a tissue-engineered substitute for use in ACL regeneration. While they represent an important step in translation to clinical practice, relatively few in vivo studies have been performed to evaluate tissue-engineered ACL grafts. In the present study, we immobilized heparin onto electrospun polycaprolactone scaffolds as a means of incorporating basic fibroblast growth factor (bFGF) onto the scaffold. In vitro, we demonstrated that human foreskin fibroblasts (HFFs) cultured on bFGF-coated scaffolds had significantly greater cell proliferation. In vivo, we implanted electrospun polycaprolactone grafts with and without bFGF into athymic rat knees. We analyzed the regenerated ACL using histological methods up to 16 weeks post-implantation. Hematoxylin and eosin staining demonstrated infiltration of the grafts with cells, and picrosirius red staining demonstrated aligned collagen fibers. At 16 weeks postop, mechanical testing of the grafts demonstrated that the grafts had approximately 30% the maximum load to failure of the native ACL. However, there were no significant differences observed between the graft groups with or without heparin-immobilized bFGF. While this study demonstrates the potential of a regenerative medicine approach to treatment of ACL rupture, it also demonstrates that in vitro results do not always predict what will occur in vivo.

Inducing chemotactic and haptotactic cues in microfluidic devices for three-dimensional in vitro assays

Microfluidic devices allow for the production of physiologically relevant cellular microenvironments by including biomimetic hydrogels and generating controlled chemical gradients. During transport, the biomolecules interact in distinct ways with the fibrillar networks: as purely diffusive factors in the soluble fluid or bound to the matrix proteins. These two main mechanisms may regulate distinct cell responses in order to guide their directional migration: caused by the substrate-bound chemoattractant gradient (haptotaxis) or by the gradient established within the soluble fluid (chemotaxis). In this work 3D diffusion experiments, in combination with ELISA assays, are performed using microfluidic platforms in order to quantify the distribution of PDGF-BB and TGF-β1 across collagen and fibrin gels. Furthermore, to gain a deeper understanding of the fundamental processes, the experiments are reproduced by computer simulations based on a reaction-diffusion transport model. This model yields an accurate prediction of the experimental results, confirming that diffusion and binding phenomena are established within the microdevice.

Defining an optimal stromal derived factor-1 presentation for effective recruitment of mesenchymal stem cells in 3D

In "situ" tissue engineering is a promising approach in regenerative medicine, envisaging to potentiate the physiological tissue repair processes by recruiting the host's own cellular progenitors at the lesion site by means of bioactive materials. Despite numerous works focused the attention in characterizing novel chemoattractant molecules, only few studied the optimal way to present signal in the microenvironment, in order to recruit cells more effectively. In this work, we have analyzed the effects of gradients of stromal derived factor-1 (SDF-1) on the migratory behavior of human mesenchymal stem cells (MSCs). We have characterized the expression of the chemokine-associated receptor, CXCR4, using cytofluorimetric and real-time PCR analyses. Gradients of SDF-1 were created in 3D collagen gels in a chemotaxis chamber. Migration parameters were evaluated using different chemoattractant concentrations. Our results show that cell motion is strongly affected by the spatio-temporal features of SDF-1 gradients. In particular, we demonstrated that the presence of SDF-1 not only influences cell motility but alters the cell state in terms of SDF-1 receptor expression and productions, thus modifying the way cells perceive the signal itself. Our observations highlight the importance of a correct stimulation of MSCs by means of SDF-1 in order to implement on effective cell recruitment. Our results could be useful for the creation of a "cell instructive material" that is capable to communicate with the cells and control and direct tissue regeneration. Biotechnol. Bioeng. 2014;111: 2303-2316. © 2014 Wiley Periodicals, Inc.

Controlled release and gradient formation of human glial-cell derived neurotrophic factor from heparinated poly(ethylene glycol) microsphere-based scaffolds

Introduction of spatial patterning of proteins, while retaining activity and releasability, is critical for the field of regenerative medicine. Reversible binding to heparin, which many biological molecules exhibit, is one potential pathway to achieve this goal. We have covalently bound heparin to poly(ethylene glycol) (PEG) microspheres to create useful spatial patterns of glial-cell derived human neurotrophic factor (GDNF) in scaffolds to promote peripheral nerve regeneration. Labeled GDNF was incubated with heparinated microspheres that were subsequently centrifuged into cylindrical scaffolds in distinct layers containing different concentrations of GDNF. The GDNF was then allowed to diffuse out of the scaffold, and release was tracked via fluorescent scanning confocal microscopy. The measured release profile was compared to predicted Fickian models. Solutions of reaction-diffusion equations suggested the concentrations of GDNF in each discrete layer that would result in a nearly linear concentration gradient over much of the length of the scaffold. The agreement between the predicted and measured GDNF concentration gradients was high. Multilayer scaffolds with different amounts of heparin and GDNF and different crosslinking densities allow the design of a wide variety of gradients and release kinetics. Additionally, fabrication is much simpler and more robust than typical gradient-forming systems due to the low viscosity of the microsphere solutions compared to gelating solutions, which can easily result in premature gelation or the trapping of air bubbles with a nerve guidance conduit. The microsphere-based method provides a framework for producing specific growth factor gradients in conduits designed to enhance nerve regeneration.

Directed migration in neural tissue engineering

Directed cell migration is particularly important in neural tissue engineering, where the goal is to direct neurons and support cells across injured nerve gaps. Investigation of the gradients present in the body during development provides an approach to guiding cells in peripheral and central nervous system tissue, but many different types of gradients and patterns can accomplish directed migration. The focus of this review is to describe current research paradigms in neural tissue gradients and review their effectiveness for directed migration. The review explores directed migration achieved through the use of chemical, adhesive, mechanical, topographical, and electrical types of gradients. Few studies investigate combined gradients, though it is known that a combination of therapies is necessary for reconnection of neuronal circuitry. To date, there has been no systematic review of gradient approaches to neural tissue engineering. By looking at effectiveness of various scaffold cue presentation and methods to combine these strategies, the potential for nerve repair is increased.

A novel chemotaxis assay in 3-D collagen gels by time-lapse microscopy

The directional cell response to chemical gradients, referred to as chemotaxis, plays an important role in physiological and pathological processes including development, immune response and tumor cell invasion. Despite such implications, chemotaxis remains a challenging process to study under physiologically-relevant conditions in-vitro, mainly due to difficulties in generating a well characterized and sustained gradient in substrata mimicking the in-vivo environment while allowing dynamic cell imaging. Here, we describe a novel chemotaxis assay in 3D collagen gels, based on a reusable direct-viewing chamber in which a chemoattractant gradient is generated by diffusion through a porous membrane. The diffusion process has been analysed by monitoring the concentration of FITC-labelled dextran through epifluorescence microscopy and by comparing experimental data with theoretical and numerical predictions based on Fick's law. Cell migration towards chemoattractant gradients has been followed by time-lapse microscopy and quantified by cell tracking based on image analysis techniques. The results are expressed in terms of chemotactic index (I) and average cell velocity. The assay has been tested by comparing the migration of human neutrophils in isotropic conditions and in the presence of an Interleukin-8 (IL-8) gradient. In the absence of IL-8 stimulation, 80% of the cells showed a velocity ranging from 0 to 1 µm/min. However, in the presence of an IL-8 gradient, 60% of the cells showed an increase in velocity reaching values between 2 and 7 µm/min. Furthermore, after IL-8 addition, I increased from 0 to 0.25 and 0.25 to 0.5, respectively, for the two donors examined. These data indicate a pronounced directional migration of neutrophils towards the IL-8 gradient in 3D collagen matrix. The chemotaxis assay described here can be adapted to other cell types and may serve as a physiologically relevant method to study the directed locomotion of cells in a 3D environment in response to different chemoattractants.

Gene-activated and cell-migration guiding PEG matrices based on three dimensional patterning of RGD peptides and DNA complexes

Essential to the design of genetic bioreactors used in the human body is a consideration of how the properties of biomaterials can combine to envelope, spatially guide, reprogramme by gene transfer, and then release cells. In order to approach this goal, poly(ethylene glycol) (PEG) matrices with modulated structural features and defined spatial patterns of bioactive signals have been designed and produced. In particular, within such PEG matrices, both an adhesive RGD peptide gradient, to directionally attract NIH3T3 cells, and a designed spatial distribution of immobilized poly(ethylenimine) (PEI)/DNA complexes, to obtain a localized transfection, have been realized. These bioactive biomaterials have been designed bearing in mind that cells following an RGD gradient migrate through the matrix, in which they find the bound DNA and become transfected. Both cell migration and transfection have been monitored by fluorescence microscopy. Results show that this system is able to envelope cells, spatially guide them towards the immobilized gene complexes and locally transfect them. Therefore, the system, acting as a genetic bioreactor potentially useful for the regulation of biology at a distance, could be used to directly control cell trafficking and activation in the human body, and has many potential biomedical applications.

Mechanical regulation of fibroblast migration and collagen remodelling in healing myocardial infarcts

Effective management of healing and remodelling after myocardial infarction is an important problem in modern cardiology practice. We have recently shown that the level of infarct anisotropy is a critical determinant of heart function following a large anterior infarction, which suggests that therapeutic gains may be realized by controlling infarct anisotropy. However, factors regulating infarct anisotropy are not well understood. Mechanical, structural and chemical guidance cues have all been shown to regulate alignment of fibroblasts and collagen in vitro, and prior studies have proposed that each of these cues could regulate anisotropy of infarct scar tissue, but understanding of fibroblast behaviour in the complex environment of a healing infarct is lacking. We developed an agent-based model of infarct healing that accounted for the combined influence of these cues on fibroblast alignment, collagen deposition and collagen remodelling. We pooled published experimental data from several sources in order to determine parameter values, then used the model to test the importance of each cue for predicting collagen alignment measurements from a set of recent cryoinfarction experiments. We found that although chemokine gradients and pre-existing matrix structures had important effects on collagen organization, a response of fibroblasts to mechanical cues was critical for correctly predicting collagen alignment in infarct scar. Many proposed therapies for myocardial infarction, such as injection of cells or polymers, alter the mechanics of the infarct region. Our modelling results suggest that such therapies could change the anisotropy of the healing infarct, which could have important functional consequences. This model is therefore a potentially important tool for predicting how such interventions change healing outcomes.

Automated and adaptable quantification of cellular alignment from microscopic images for tissue engineering applications

Cellular alignment plays a critical role in functional, physical, and biological characteristics of many tissue types, such as muscle, tendon, nerve, and cornea. Current efforts toward regeneration of these tissues include replicating the cellular microenvironment by developing biomaterials that facilitate cellular alignment. To assess the functional effectiveness of the engineered microenvironments, one essential criterion is quantification of cellular alignment. Therefore, there is a need for rapid, accurate, and adaptable methodologies to quantify cellular alignment for tissue engineering applications. To address this need, we developed an automated method, binarization-based extraction of alignment score (BEAS), to determine cell orientation distribution in a wide variety of microscopic images. This method combines a sequenced application of median and band-pass filters, locally adaptive thresholding approaches and image processing techniques. Cellular alignment score is obtained by applying a robust scoring algorithm to the orientation distribution. We validated the BEAS method by comparing the results with the existing approaches reported in literature (i.e., manual, radial fast Fourier transform-radial sum, and gradient based approaches). Validation results indicated that the BEAS method resulted in statistically comparable alignment scores with the manual method (coefficient of determination R(2)=0.92). Therefore, the BEAS method introduced in this study could enable accurate, convenient, and adaptable evaluation of engineered tissue constructs and biomaterials in terms of cellular alignment and organization.

Gradients with depth in electrospun fibrous scaffolds for directed cell behavior

A major obstacle in creating viable tissue-engineered constructs using electrospinning is the lack of complete cellularization and vascularization due to the limited porosity in these densely packed fibrous scaffolds. One potential approach to circumvent this issue is the use of various gradients of chemical and biophysical cues to drive the infiltration of cells into these structures. Toward this goal, this study focused on creating durotactic (mechanical) and haptotactic (adhesive) gradients through the thickness of electrospun hyaluronic acid (HA) scaffolds using a unique, yet simple, modification of common electrospinning protocols. Specifically, both mechanical (via cross-linking: ranging from 27-100% modified methacrylated HA, MeHA) and adhesive (via inclusion of the adhesive peptide RGD: 0-3 mM RGD) gradients were each fabricated by mixing two solutions (one ramping up, one ramping down) prior to electrospinning and fiber collection. Gradient formation was verified by fluorescence microscopy, FTIR, atomic force microscopy, and cellular morphology assessment of scaffolds at different points of collection (i.e., with scaffold thickness). To test further the functionality of gradient scaffolds, chick aortic arch explants were cultured on adhesive gradient scaffolds for 7 days, and low RGD-high RGD gradient scaffolds showed significantly greater cell infiltration compared with high RGD-low RGD gradients and uniform high RGD or uniform low RGD control scaffolds. In addition to enhanced infiltration, this approach could be used to fabricate graded tissue structures, such as those that occur at interfaces.

Gradient biomaterials for soft-to-hard interface tissue engineering

Interface tissue engineering (ITE) is a rapidly developing field that aims to fabricate biological tissue alternates with the goal of repairing or regenerating the functions of diseased or damaged zones at the interface of different tissue types (also called "interface tissues"). Notable examples of the interface tissues in the human body include ligament-to-bone, tendon-to-bone and cartilage-to-bone. Engineering interface tissues is a complex process, which requires a combination of specialized biomaterials with spatially organized material composition, cell types and signaling molecules. Therefore, the use of conventional biomaterials (monophasic or composites) for ITE has certain limitations to help stimulate the tissue integration or recreating the structural organization at the junction of different tissue types. The advancement of micro- and nanotechnologies enable us to develop systems with gradients in biomaterials properties that encourage the differentiation of multiple cell phenotypes and subsequent tissue development. In this review we discuss recent developments in the fabrication of gradient biomaterials for controlling cellular behavior such as migration, differentiation and heterotypic interactions. Moreover, we give an overview of potential uses of gradient biomaterials in engineering interface tissues such as soft tissues (e.g. cartilage) to hard tissues (e.g. bone), with illustrated experimental examples. We also address fundamentals of interface tissue organization, various gradient biomaterials used in ITE, micro- and nanotechnologies employed for the fabrication of those gradients, and certain challenges that must be met in order for ITE to reach its full potential.

Combinatorial guidance by CCR7 ligands for T lymphocytes migration in co-existing chemokine fields

Chemokines mediate the trafficking and positioning of lymphocytes in lymphoid tissues that is crucial for immune surveillance and immune responses. In particular, a CCR7 ligand, CCL21, plays important roles in recruiting T cells to secondary lymphoid tissues (SLT). Furthermore, CCL21 together with another CCR7 ligand, CCL19, direct the navigation and compartmentation of T cells within SLT. However, the distinct roles of these two chemokines for regulating cell trafficking and positioning are not clear. In this study, we explore the effect of co-existing CCL19 and CCL21 concentration fields on guiding T cell migration. Using microfluidic devices that can configure single and superimposed chemokine fields we show that under physiological gradient conditions, human peripheral blood T cells chemotax to CCL21 but not CCL19. Furthermore, T cells migrate away from the CCL19 gradient in a uniform background of CCL21. This repulsive migratory response is predicted by mathematical modeling based on the competition of CCL19 and CCL21 for CCR7 signaling and the differential ability of the two chemokines for desensitizing CCR7. These results suggest a new combinatorial guiding mechanism by CCL19 and CCL21 for the migration and trafficking of CCR7 expressing leukocytes.

Varying assay geometry to emulate connective tissue planes in an in vitro model of acupuncture needling

During traditional acupuncture, fine needles are inserted subcutaneously and rotated, which causes loose fascial tissue to wind around the needle. This coupling is stronger at acupuncture points, which tend to fall above intermuscular fascial planes, than control points, which lay above skeletal muscle. These different anatomical constraints may affect the mechanical coupling. Fascia at acupuncture points is bounded on two sides by skeletal muscle, but at control points is essentially unbounded. These differences were approximated in simple in vitro models. To emulate the narrower boundary within the intermuscular plane, type I collagen was cast in circular gels of different radii. To model the channel-like nature of these planes, collagen was cast in elliptical gels with major and minor axes matching the large and small circular gels, respectively, and in planar gels constrained on two sides. Acupuncture needles were inserted into the gels and rotated via a computer-controlled motor while capturing the evolution of fiber alignment under cross-polarization. Small circular gels aligned faster, but failed earlier than large circular gels. Rotation in elliptical and planar gels generated more alignment-per-revolution than circular gels. Planar gels were particularly resistant to failure. Fiber alignment in circular gels was isotropic, but was stronger in the direction of the minor axis in elliptical and planar gels. In fibroblast-populated gels, cells followed the alignment of the collagen fibers, and also became denser in regions of stronger alignment. These results suggest that the anatomy at acupuncture points provides unique boundaries that accentuate the mechanical response to needle manipulation.

The formation of protein concentration gradients mediated by density differences of poly(ethylene glycol) microspheres

A critical element in the formation of scaffolds for tissue engineering is the introduction of concentration gradients of bioactive molecules. We explored the use of poly(ethylene glycol) (PEG) microspheres fabricated via a thermally induced phase separation to facilitate the creation of gradients in scaffolds. PEG microspheres were produced with different densities (buoyancies) and centrifuged to develop microsphere gradients. We previously found that the time to gelation following phase separation controlled the size of microspheres in the de-swollen state, while crosslink density affected swelling following buffer exchange into PBS. The principle factors used here to control microsphere densities were the temperature at which the PEG solutions were reacted following phase separation in aqueous sodium sulfate solutions and the length of the incubation period above the 'cloud point'. Using different temperatures and incubation times, microspheres were formed that self-assembled into gradients upon centrifugation. The gradients were produced with sharp interfaces or gradual transitions, with up to 5 tiers of different microsphere types. For proof-of-concept, concentration gradients of covalently immobilized proteins were also assembled. PEG microspheres containing heparin were also fabricated. PEG-heparin microspheres were incubated with fluorescently labeled protamine and used to form gradient scaffolds. The ability to form gradients in microspheres may prove to be useful to achieve better control over the kinetics of protein release from scaffolds or to generate gradients of immobilized growth factors.

Covalently immobilized RGD gradient on PEG hydrogel scaffold influences cell migration parameters

Understanding the influence of a controlled spatial distribution of biological cues on cell activities can be useful to design "cell instructive" materials, able to control and guide the formation of engineered tissues in vivo and in vitro. To this purpose, biochemical and mechanical properties of the resulting biomaterial must be carefully designed and controlled. In this work, the effect of covalently immobilized RGD peptide gradients on poly(ethylene glycol) diacrylate hydrogels on cell behaviour was studied. We set up a mechanical device generating gradients based on a fluidic chamber. Cell response to RGD gradients with different slope (0.7, 1 and 2 mM cm(-1)) was qualitatively and quantitatively assessed by evaluating cell adhesion and, in particular, cell migration, compared to cells seeded on hydrogels with uniform distribution of RGD peptides. To evaluate the influence of RGD gradient and to exclude any concentration effect on cell response, all analyses were carried out in a specific region of the gradients which displayed the same average concentration of RGD (1.5 mM). Results suggest that cells recognize the RGD gradient and adhere onto it assuming a stretched shape. Moreover, cells tend to migrate in the direction of the gradient, as their speed is higher than that of cells migrating on hydrogels with a uniform distribution of RGD and increases by increasing RGD gradient steepness. This increment is due to an augmentation of bias speed component of the mean squared speed, that is, the drift of the cell population migrating on the anisotropic surface provided by the RGD gradient.

Osteochondral interface tissue engineering using macroscopic gradients of bioactive signals

Continuous gradients exist at osteochondral interfaces, which may be engineered by applying spatially patterned gradients of biological cues. In the present study, a protein-loaded microsphere-based scaffold fabrication strategy was applied to achieve spatially and temporally controlled delivery of bioactive signals in three-dimensional (3D) tissue engineering scaffolds. Bone morphogenetic protein-2 and transforming growth factor-beta(1)-loaded poly(D,L-lactic-co-glycolic acid) microspheres were utilized with a gradient scaffold fabrication technology to produce microsphere-based scaffolds containing opposing gradients of these signals. Constructs were then seeded with human bone marrow stromal cells (hBMSCs) or human umbilical cord mesenchymal stromal cells (hUCMSCs), and osteochondral tissue regeneration was assessed in gradient scaffolds and compared to multiple control groups. Following a 6-week cell culture, the gradient scaffolds produced regionalized extracellular matrix, and outperformed the blank control scaffolds in cell number, glycosaminoglycan production, collagen content, alkaline phosphatase activity, and in some instances, gene expression of major osteogenic and chondrogenic markers. These results suggest that engineered signal gradients may be beneficial for osteochondral tissue engineering.

Stochastic modelling of biased cell migration and collagen matrix modification

Matrix dynamics plays a crucial role in several physiological and pathological processes. In this paper we develop a model framework, which describes the temporal fibre network evolution depending on the influence of migrating fibroblasts. The cells are regarded as discrete objects in the plane, whose velocities are determined by a generalised Langevin equation. For its solution we verify existence and uniqueness. The courses of the trajectories are affected by two external impulses, chemotaxis and contact guidance, respectively. The extracellular matrix is described by a continuous vector field which contains both information on density and orientation of the fibrous material. Modelling dynamic interaction between the discrete and the continuum variables is an essential point of this paper. In particular, the smoothing of the fluctuating paths plays a key role. Besides a detailed description of the formulated equations, we also supply the condensed pseudo code of the algorithm. We investigate several examples and present results both from artificial and real data.

Regulating fibrinolysis to engineer skeletal muscle from the C2C12 cell line

Muscles engineered from transformed cells would be a powerful model for the study of muscle physiology by allowing long-term in vitro studies of muscle adaptation. However, previously described methods either take >5 weeks to produce a tissue or use collagen as a scaffold, which decreases the specific force of the muscle, making it hard to measure the function of the constructs. The aim of this study was to rapidly engineer muscle using the C2C12 cell line in fibrin, which has a stiffness similar to muscle tissue, allowing accurate functional testing. Both the protease inhibitor aprotinin and the natural cross-linker genipin increased the length of time that muscle could be cultured, with genipin increasing the time in culture to 10 weeks. The function of the tissues was significantly affected by the batch of serum (64-78%) or thrombin (41%), the differentiation medium (78%), and the seeding protocol (38%), but was unaffected by initial cell number. Strikingly, different C2C12 clones produced up to a 3.6-fold variation in force production. Under optimal conditions, the tissues form in 10.4+/-0.3 days and remain fully functional for 5 weeks over which time they continue to mature. The optimized model described here provides rapid, reliable, and functional tissues that will be useful in the study of skeletal muscle physiology.

Cell Guidance by 3D-Gradients in Hydrogel Matrices: Importance for Biomedical Applications

Concentration gradients of soluble and matrix-bound guidance cues in the extracellular matrix direct cell growth in native tissues and are of great interest for design of biomedical scaffolds and on implant surfaces. The focus of this review is to demonstrate the importance of gradient guidance for cells as it would be desirable to direct cell growth onto/into biomedical devices. Many studies have been described that illustrate the production and characterization of surface gradients, but three dimensional (3D)-gradients that direct cellular behavior are not well investigated. Hydrogels are considered as synthetic replacements for native extracellular matrices as they share key functions such as 2D- or 3D-solid support, fibrous structure, gas- and nutrition permeability and allow storage and release of biologically active molecules. Therefore this review focuses on current studies that try to implement soluble or covalently-attached gradients of growth factors, cytokines or adhesion sequences into 3D-hydrogel matrices in order to control cell growth, orientation and migration towards a target. Such gradient architectures are especially desirable for wound healing purposes, where defined cell populations need to be recruited from the blood stream and out of the adjacent tissue, in critical bone defects, for vascular implants or neuronal guidance structures where defined cell populations should be guided by appropriate signals to reach their proper positions or target tissues in order to accomplish functional repair.

Microsphere-based seamless scaffolds containing macroscopic gradients of encapsulated factors for tissue engineering

Spatial and temporal control of bioactive signals in three-dimensional (3D) tissue engineering scaffolds is greatly desired. Coupled together, these attributes may mimic and maintain complex signal patterns, such as those observed during axonal regeneration or neovascularization. Seamless polymer constructs may provide a route to achieve spatial control of signal distribution. In this study, a novel microparticle-based scaffold fabrication technique is introduced as a method to create 3D scaffolds with spatial control over model dyes using uniform poly(D,L-lactide-co-glycolide) microspheres. Uniform microspheres were produced using the Precision Particle Fabrication technique. Scaffolds were assembled by flowing microsphere suspensions into a cylindrical glass mold, and then microspheres were physically attached to form a continuous scaffold using ethanol treatment. An ethanol soak of 1 h was found to be optimum for improved mechanical characteristics. Morphological and physical characterization of the scaffolds revealed that microsphere matrices were porous (41.1 +/- 2.1%) and well connected, and their compressive stiffness ranged from 142 to 306 kPa. Culturing chondrocytes on the scaffolds revealed the compatibility of these substrates with cell attachment and viability. In addition, bilayered, multilayered, and gradient scaffolds were fabricated, exhibiting excellent spatial control and resolution. Such novel scaffolds can serve as sustained delivery devices of heterogeneous signals in a continuous and seamless manner, and may be particularly useful in future interfacial tissue engineering investigations.

Positively and negatively modulating cell adhesion to type I collagen via peptide grafting

The biophysical interactions between cells and type I collagen are controlled by the level of cell adhesion, which is dictated primarily by the density of ligands on collagen and the density of integrin receptors on cells. The native adhesivity of collagen was modulated by covalently grafting glycine-arginine-glycine-aspartic acid-serine (GRGDS), which includes the bioactive RGD sequence, or glycine-arginine-aspartic acid-glycine-serine (GRDGS), which includes the scrambled RDG sequence, to collagen with the hetero-bifunctional coupling agent 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide. The peptide-grafted collagen self-assembled into a fibrillar gel with negligible changes in gel structure and rheology. Rat dermal fibroblasts (RDFs) and human smooth muscle cells demonstrated increased levels of adhesion on gels prepared from RGD-grafted collagen, and decreased levels of adhesion on RDG-grafted collagen. Both cell types demonstrated an increased ability to compact free-floating RGD-grafted collagen gels, and an impaired ability to compact RDG-grafted gels. RDF migration on and within collagen was increased with RDG-grafted collagen and decreased with RGD-grafted collagen, and dose-response experiments indicated a biphasic response of RDF migration to adhesion. Smooth muscle cells demonstrated similar, though not statistically significant, trends. The ability to both positively and negatively modulate cell adhesion to collagen increases the versatility of this natural biomaterial for regenerative therapies.

Fluorescence-based assays for in vitro analysis of cell adhesion and migration

Cell adhesion and cell migration are two primary cellular phenomena for which in vitro approaches may be exploited to effectively dissect the individual events and underlying molecular mechanisms. The use of assays dedicated to the analysis of cell adhesion and migration in vitro also afford an efficient way of conducting larger basic and applied research screenings on the factors affecting these processes and are potentially exploitable in the context of routine diagnostic, prognostic, and predictive tests in the biological and medical fields. Therefore, there is a longstanding continuum in the interest in devising more rationale such assays and major contributions in this direction have been provided by the advent of procedures based on fluorescence cell tagging, the design of instruments capable of detecting fluorescent signals with high sensitivity, and informatic tools allowing sophisticated elaboration of data generated through these instruments. In this report, we describe three representative fluorescence-based model assays for the qualitative and quantitative assessment of cell adhesion and cell locomotion in static and dynamic conditions. The assays are easily performed, accurate and reproducible, and can be automated for high-to-medium throughput screenings of cell behavior in vitro. Performance of the assays involves the use of certain dedicated disposable accessories, which are commercially available, and a few instruments that, due to their versatility, can be regarded as constituents of a more generic laboratory setup.

Quantitative studies of neuronal chemotaxis in 3D

During development a variety of cell types are guided by molecular concentration gradients to form tissues and organ systems. In the nervous system, the migration and neuronal pathfinding that occurs during development is organized and driven by "guidance cues." Some of these cues are substrate bound or nondiffusible, while many are diffusible and form gradients within the developing embryo to guide neurons and neurites to their appropriate destination. There have been many approaches used to discover and characterize the multitude of guidance cues, their cognate receptors, and how these cues and receptors are regulated to achieve the highly detailed connections found in the nervous system. Here we present a method for creating precisely controlled gradients of molecular factors within a three-dimensional culture environment. The method is based on a non contact mediated delivery of biomolecules to the surface of a collagen gel. The factors are printed in a pattern on the top of a gel containing the tissue or cell type of interest embedded in the gel. The formation of the gradient is dependent upon the diffusion of the printed molecule in the gel. The concentration of the factor within the gel becomes independent of depth rapidly, and the gradient becomes smooth on a similar time scale. The gradients formed can remain relatively stable for a day or more. Moreover, the steepness and molar concentration of tropic or trophic factors within the gradient can be controlled.

Neutrophil migration assay from a drop of blood

Neutrophil directional migration in response to chemical gradients, also known as chemotaxis, is one of the key phenomena in the immune responses against bacterial infection. To better study neutrophils chemotaxis, several in vitro assays have been developed that replicate chemotactic gradients around neutrophils isolated from whole blood. One drawback for most of these assays is the lengthy processing of blood required for neutrophils isolation, which can alter the responsiveness of neutrophils compared to the in vivo conditions. To address this limitation, we have designed a microfluidic chip for chemotaxis studies which can use neutrophils isolated on the chip, directly from whole blood. We have tested three different cell adhesion molecules as substrates for neutrophil isolation (P-selectin, E-selectin and fibronectin) and found average capture efficiencies of 20-40 neutrophils/mm2 at optimized concentrations. Subsequent analysis of neutrophil migration in chemoattractant gradients of N-formyl-methyl-leucyl-phenylalanine (fMLP) or Interleukin-8 (IL-8) shows higher average velocities over E-selectin as compared to the P-selectin. Our microfluidic assay uses just a drop of whole blood (<10 microL) for neutrophil isolation and provides a robust platform to perform chemotaxis assays in the competing environment of different chemokines.

Strategies and applications for incorporating physical and chemical signal gradients in tissue engineering

From embryonic development to wound repair, concentration gradients of bioactive signaling molecules guide tissue formation and regeneration. Moreover, gradients in cellular and extracellular architecture as well as in mechanical properties are readily apparent in native tissues. Perhaps tissue engineers can take a cue from nature in attempting to regenerate tissues by incorporating gradients into engineering design strategies. Indeed, gradient-based approaches are an emerging trend in tissue engineering, standing in contrast to traditional approaches of homogeneous delivery of cells and/or growth factors using isotropic scaffolds. Gradients in tissue engineering lie at the intersection of three major paradigms in the field-biomimetic, interfacial, and functional tissue engineering-by combining physical (via biomaterial design) and chemical (with growth/differentiation factors and cell adhesion molecules) signal delivery to achieve a continuous transition in both structure and function. This review consolidates several key methodologies to generate gradients, some of which have never been employed in a tissue engineering application, and discusses strategies for incorporating these methods into tissue engineering and implant design. A key finding of this review was that two-dimensional physicochemical gradient substrates, which serve as excellent high-throughput screening tools for optimizing desired biomaterial properties, can be enhanced in the future by transitioning from two dimensions to three dimensions, which would enable studies of cell-protein-biomaterial interactions in a more native tissue-like environment. In addition, biomimetic tissue regeneration via combined delivery of graded physical and chemical signals appears to be a promising strategy for the regeneration of heterogeneous tissues and tissue interfaces. In the future, in vivo applications will shed more light on the performance of gradient-based mechanical integrity and signal delivery strategies compared to traditional tissue engineering approaches.

Fabrication of combinatorial polymer scaffold libraries

We have designed a novel combinatorial research platform to help accelerate tissue engineering research. Combinatorial methods combine many samples into a single specimen to enable accelerated experimentation and discovery. The platform for fabricating combinatorial polymer scaffold libraries can be used to rapidly identify scaffold formulations that maximize tissue formation. Many approaches for screening cell-biomaterial interactions utilize a two-dimensional format such as a film or surface to present test substrates to cells. However, cells in vivo exist in a three-dimensional milieu of extracellular matrix and cells in vitro behave more naturally when cultured in a three-dimensional environment than when cultured on a two-dimensional surface. Thus, we have designed a method for fabricating combinatorial biomaterial libraries where the materials are presented to cells in the form of three-dimensional, porous, salt-leached, polymer scaffolds. Many scaffold variations and compositions can be screened in a single experiment so that optimal scaffold formulations for tissue formation can be rapidly identified. In summary, we have developed a platform technology for fabricating combinatorial polymer scaffold libraries that can be used to screen cell response to materials in a three-dimensional, scaffold format.

Generation of stable concentration gradients in 2D and 3D environments using a microfluidic ladder chamber

We have developed a simple microfluidic device for generating stable concentration gradients in 2D and 3D environments. The device, termed the Ladder Chamber, uses a two-compartment diffusion system to generate steady state gradients across flow-free channels that connect the source and sink channels. To demonstrate the utility of the Ladder Chamber for cell migration, neutrophil chemotaxis was successfully observed in soluble chemoattractant (IL-8) gradient. The Ladder Chamber's simple design and experimental implementation make it an attractive approach for investigating cell migration and other biological experiments in well-defined gradients in 2D surfaces as well as in 3D gels.

Covalent immobilization of RGDS on hydrogel surfaces to direct cell alignment and migration

This study extends the capability for directing cell behavior using PEG-based hydrogels in tissue-engineering applications to include control over the spatial distribution of the adhesive peptide, RGDS. A continuous linear gradient was formed by simultaneously using a gradient maker to combine precursor solutions and using photopolymerization to lock the RGDS gradient in place. Hydrogels containing entrapped gradients of bovine serum albumin (BSA) were characterized using Coomassie brilliant blue stain, which indicated that BSA concentration increases along the hydrogel's length and that the steepness of the gradient's slope can be varied by changing the relative BSA concentrations in the precursor solutions. Human dermal fibroblasts responded to covalently immobilized RGDS gradients by changing their morphology to align in the direction of increasing RGDS concentration. After 24 h, approximately 46% of fibroblasts were aligned with the RGDS-gradient axis. This proportion of cells further increased to approximately 53% (p < 0.05) and approximately 58% after 48 and 96 h, respectively. Also, fibroblasts migrated differentially depending on the concentration of RGDS. Fibroblasts migrated approximately 48% further going up the concentration gradient (0 to 6 micromol/ml PEG-RGDS) than going down the concentration gradient. Migration up the concentration gradient was also approximately 33% greater than migration on control surfaces with a constant concentration of RGDS (2 micromol/ml), while migration down the gradient was reduced approximately 12% relative to the control surface. In addition, directed migration was further enhanced by increasing the RGDS gradient's slope. This hydrogel system is expected to be useful for directing cell migration to enhance the formation of engineered tissues.

Generating controlled molecular gradients in 3D gels

A new method for producing molecular gradients of arbitrary shape in thin three dimensional gels is described. Patterns are produced on the surface of the gel by printing with a micropump that dispenses small droplets of solution at controlled rates. The molecules in the solution rapidly diffuse into the gel and create a smooth concentration profile that is independent of depth. The pattern is relatively stable for long times, and its evolution can be accurately described by finite element modeling of the diffusion equation. As a demonstration of the method, direct measurements of protein gradients are performed by quantitative fluorescence microscopy. A complementary technique for measuring diffusion coefficients is also presented. This rapid, flexible, contactless approach to gradient generation is ideally suited for cell culture experiments to investigate the role of gradients of diffusible substances in processes such as chemotaxis, morphogenesis, and pattern formation, as well as for high-throughput screening of system responses to a wide range of chemical concentrations.

Molecularly engineered PEG hydrogels: a novel model system for proteolytically mediated cell migration

Model systems mimicking the extracellular matrix (ECM) have greatly helped in quantifying cell migration in three dimensions and elucidated the molecular determinants of cellular motility in morphogenesis, regeneration, and disease progression. Here we tested the suitability of proteolytically degradable synthetic poly(ethylene glycol) (PEG)-based hydrogels as an ECM model system for cell migration research and compared this designer matrix with the two well-established ECM mimetics fibrin and collagen. Three-dimensional migration of dermal fibroblasts was quantified by time-lapse microscopy and automated single-cell tracking. A broadband matrix metalloproteinase (MMP) inhibitor and tumor necrosis factor-alpha, a potent MMP-inducer in fibroblasts, were used to alter MMP regulation. We demonstrate a high sensitivity of migration in synthetic networks to both MMP modulators: inhibition led to an almost complete suppression of migration in PEG hydrogels, whereas MMP upregulation increased the fraction of migrating cells significantly. Conversely, migration in collagen and fibrin proved to be less sensitive to the above MMP modulators, as their fibrillar architecture allowed for MMP-independent migration through preexisting pores. The possibility of molecularly recapitulating key functions of the natural extracellular microenvironment and the improved protease sensitivity makes PEG hydrogels an interesting model system that allows correlation between protease activity and cell migration.

Neutrophil Migration in Opposing Chemoattractant Gradients Using Microfluidic Chemotaxis Devices

Neutrophils migrating in tissue respond to complex overlapping signals generated by a variety of chemotactic factors (CFs). Previous studies suggested a hierarchy between bacteria-derived CFs and host-derived CFs but could not differentiate neutrophil response to potentially equal host-derived CFs (IL-8 and LTB4). This paper reports neutrophil migration in conflicting gradients of IL-8 and LTB4 using a microfluidic chemotaxis device that can generate stable and well-defined gradients. We quantitatively characterized the movement of cells from time-lapse images. Neutrophils migrate more efficiently toward single IL-8 gradients than single LTB4 gradients as measured by the effective chemotactic index (ECI). In opposing gradients of IL-8 and LTB4, neutrophils show obvious chemotaxis toward a distant gradient, consistent with previous reports. When an opposing gradient of LTB4 is present, neutrophils show less effective chemotaxis toward IL-8 than when they are in a gradient of IL-8 alone. In contrast, the chemotactic response of neutrophils to LTB4 is not reduced in opposing gradients as compared to that in a single LTB4 gradient. These results indicate that the presence of one host-derived CF modifies the response of neutrophils to a second CF suggesting a subtle hierarchy between them.

Cellular alignment by grafted adhesion peptide surface density gradients

The extracellular matrix and extracellular matrix-associated proteins play a major role in growth and differentiation of tissues and organs. To date, few methods have been developed that allow researchers to examine the affect of surface density gradients of adhesion molecules in a controlled manner. Fibroblasts cultured on surfaces with a surface density gradient of RGD peptide aligned parallel to the gradient while fibroblasts on constant density RGD surfaces spread but did not align as has been shown in numerous earlier studies. Not only did fibroblasts align on the gradient surfaces, but they also showed significantly greater elongation than on constant density peptide surfaces or on control surfaces. This type of method is easy to replicate and can be used by laboratories interested in investigating alignment of various cell types without mechanical force or other stimulation, and without cell-cell interaction or for investigation of affects of surface density gradients of molecules on cellular biochemistry and biophysics. This method also has potential applications for developing scaffolds for tissue engineering applications where cellular alignment is necessary.

A new chemotaxis assay shows the extreme sensitivity of axons to molecular gradients

Axonal chemotaxis is believed to be important in wiring up the developing and regenerating nervous system, but little is known about how axons actually respond to molecular gradients. We report a new quantitative assay that allows the long-term response of axons to gradients of known and controllable shape to be examined in a three-dimensional gel. Using this assay, we show that axons may be nature's most-sensitive gradient detectors, but this sensitivity exists only within a narrow range of ligand concentrations. This assay should also be applicable to other biological processes that are controlled by molecular gradients, such as cell migration and morphogenesis.

Fabrication of Patterned Multicomponent Protein Gradients and Gradient Arrays Using Microfluidic Depletion

We demonstrate that depletion effects in the fluids used to fill a poly(dimethylsiloxane) microfluidic device can be used in conjunction with its design rules to generate patterned protein gradients. The linear portions of these structures can be designed to present gradients of bound protein coverage-varying from near-saturation to effectively zero-over distances ranging from a few hundred micrometers to more than 1 cm by design. Such patterns can be developed in a simple, single-channel form as well as in a multichannel gradient array of more complex design. The patterning protocols also support the use of multiple protein sources, and we demonstrate an assembly process mediated by a protein that inhibits adsorption to generate a gradient array in pixel form. We describe examples of multiple protein gradient patterns along with simple immunoassays to illustrate the scope of the methodology, the activity of the patterned proteins, and their recognition in gradient form on a surface. These gradients should prove useful to studies in biosensor and bioassay development and as substrates for cell culture to study growth and motility.

Temporal variations in cell migration and traction during fibroblast-mediated gel compaction

Current models used in our laboratory to assess the migration and traction of a population of cells within biopolymer gels are extended to investigate temporal changes in these parameters during compaction of mechanically constrained gels. The random cell migration coefficient, micro (t) is calculated using a windowing technique by regressing the mean-squared displacement of cells tracked at high magnification in three dimensions with a generalized least squares algorithm for a subset of experimental time intervals, and then shifting the window interval-by-interval until all time points are analyzed. The cell traction parameter, tau(0)(t), is determined by optimizing the solution of our anisotropic biphasic theory to tissue equivalent compaction. The windowing technique captured simulated sinusoidal and step changes in cell migration superposed on a persistent random walk in simulated cell movement. The optimization software captured simulated time dependence of compaction on cell spreading. Employment of these techniques on experimental data using rat dermal fibroblasts (RDFs) and human foreskin fibroblasts (HFFs) demonstrated that these cells exhibit different migration-traction relationships. Rat dermal fibroblast migration was negatively correlated to traction, suggesting migration was not the driving force for compaction with these cells, whereas human foreskin fibroblast migration was positively correlated to traction.

Effect of platelet releasate on bone cell migration and recruitment in vitro

The use of platelets and platelet products has become increasingly popular clinically as a means of accelerating endosseous wound healing. It is likely that growth factors released by activated platelets at the site of injury play a role in bone regeneration by stimulating the migration and proliferation of bone cells. In this study, a novel in vitro assay was developed to study the effects of platelet releasate (PR) collected from activated platelet concentrate on rat bone marrow-derived cells. Cultures of primary rat bone marrow cells were overlaid with a fibrin matrix, and the number of cells migrating within the three-dimensional matrix and the leading front of migration were quantified. The addition of PR to the top of the fibrin gels at different time points caused a 25% increase in the leading front of migration and a 3.5-fold increase in the number of migrating cells. Platelet releasate was also shown to have a mitogenic effect on bone cells in proliferation studies. Comparison between migration and proliferation data indicated that PR stimulates the initial recruitment of bone marrow cells to migration. This assay further allowed the determination that rat bone marrow cells are capable of exerting contractile forces on fibrin matrices and that matrix contraction is directly related to the migratory activity of cells. The results provide a potential mechanism to explain why biologically active platelet-derived factors enhance endosseous wound healing.

GRGDSP peptide-bound silicone membranes withstand mechanical flexing in vitro and display enhanced fibroblast adhesion

Mechanobiological studies of cardiac tissue require devices that allow forces to be exerted on cells in vitro. Silicone elastomer is often used in these devices because it is flexible and transparent, permitting optical imaging of the cells. However, native untreated silicone is hydrophobic and is unsuitable for cell culture. Peptides covalently bound to silicone surfaces are examined here for the enhancement of cellular adhesion during in vitro dynamic flexing. A procedure is described for the chemical modification of medical grade silicone membranes with covalently bound GRGDSP peptides. The conditions for mechanical studies of cardiac cell cultures are then duplicated and it is demonstrated that the peptide layers survive 48 h of mechanical flexing in vitro. Specifically, mechanical flexing in vitro of the 30 pmol/cm2 peptide-modified silicone membranes has no significant effect on the amount of peptides that remains bound to the surface. Cardiac fibroblasts display enhanced adhesion to these peptide-bound silicone membranes for at least 24 h of growth, compared with native silicone or tissue culture polystyrene. The effects of serum versus serum-free media on fibroblast growth are also examined.