Contact guidance induced organization of extracellular matrix

Aligned forces: Origins and mechanisms of cancer dissemination guided by extracellular matrix architecture

Organized extracellular matrix (ECM), in the form of aligned architectures, is a critical mediator of directed cancer cell migration by contact guidance, leading to metastasis in solid tumors. Current models suggest anisotropic force generation through the engagement of key adhesion and cytoskeletal complexes drives contact-guided migration. Likewise, disrupting the balance between cell-cell and cell-ECM forces, driven by ECM engagement for cells at the tumor-stromal interface, initiates and drives local invasion. Furthermore, processes such as traction forces exerted by cancer and stromal cells, spontaneous reorientation of matrix-producing fibroblasts, and direct binding of ECM modifying proteins lead to the emergence of collagen alignment in tumors. Thus, as we obtain a deeper understanding of the origins of ECM alignment and the mechanisms by which it is maintained to direct invasion, we are poised to use the new paradigm of stroma-targeted therapies to disrupt this vital axis of disease progression in solid tumors.

Is there a universal mechanism of cell alignment in response to substrate topography?

Cell alignment and elongation in the direction of anisotropic and aligned topographies are key manifestations of cellular contact guidance and are observed in many cell types. Whether this observation occurs through a universal mechanism remains to be established. In this Views article, we begin by presenting the most widely accepted model of topography-driven cell alignment which posits that anisotropic topographies impose lateral constraints on the growth of focal adhesions and actin stress fibers, thereby driving anisotropic force generation and cellular elongation and alignment. We then discuss particular scenarios where alternative or complementary mechanisms of cell alignment appear to be at play. These include the cases of specific cell types such as amoeboid-like cells and neurons as well as certain topography sizes. Finally, we review the role of the actin cytoskeleton in modulating topography-driven cell alignment and underscore the need for elucidating the role that other cytoskeletal elements play. We close by identifying key open questions the responses to which will significantly enhance our understanding of the role of cellular contact guidance in health and disease.

Printable Organic Electronic Materials for Precisely Positioned Cell Attachment

Over the past 3 decades, there has been a vast expansion of research in both tissue engineering and organic electronics. Although the two fields have interacted little, the materials and fabrication technologies which have accompanied the rise of organic electronics offer the potential for innovation and translation if appropriately adapted to pattern biological materials for tissue engineering. In this work, we use two organic electronic materials as adhesion points on a biocompatible poly(p-xylylene) surface. The organic electronic materials are precisely deposited via vacuum thermal evaporation and organic vapor jet printing, the proven, scalable processes used in the manufacture of organic electronic devices. The small molecular-weight organics prevent the subsequent growth of antifouling polyethylene glycol methacrylate polymer brushes that grow within the interstices between the molecular patches, rendering these background areas both protein and cell resistant. Last, fibronectin attaches to the molecular patches, allowing for the selective adhesion of fibroblasts. The process is simple, reproducible, and promotes a high yield of cell attachment to the targeted sites, demonstrating that biocompatible organic small-molecule materials can pattern cells at the microscale, utilizing techniques widely used in electronic device fabrication.

Cellular and Subcellular Contact Guidance on Microfabricated Substrates

Topography of the extracellular environment is now recognized as a major biophysical regulator of cell behavior and function. The study of the influence of patterned substrates on cells, named contact guidance, has greatly benefited from the development of micro and nano-fabrication techniques, allowing the emergence of increasingly diverse and elaborate engineered platforms. The purpose of this review is to provide a comprehensive view of the process of contact guidance from cellular to subcellular scales. We first classify and illustrate the large diversity of topographies reported in the literature by focusing on generic cellular responses to diverse topographical cues. Subsequently, and in a complementary fashion, we adopt the opposite approach and highlight cell type-specific responses to classically used topographies (arrays of pillars or grooves). Finally, we discuss recent advances on the key subcellular and molecular players involved in topographical sensing. Throughout the review, we focus particularly on neuronal cells, whose unique morphology and behavior have inspired a large body of studies in the field of topographical sensing and revealed fascinating cellular mechanisms. We conclude by using the current understanding of the cell-topography interactions at different scales as a springboard for identifying future challenges in the field of contact guidance.

The responses of human gingival fibroblasts to magnesium-doped titanium

The titanium (Ti) implant is widely used in implant dentistry; yet peri-implantitis has always been one of the most common and serious complications. Here, we demonstrated that magnesium-doping would be an effective way of enhancing the integration between implant surfaces and gingival tissues, which is critical to peri-implant health. The magnesium (2.76-6.35 at %) was immobilized onto the titanium substrate by a magnesium plasma immersion ion implantation (Mg-PIII) technique. Mg-PIII treatments did not alter surface topographies of the original titanium substrate but improved its hydrophilicity. The in vitro study including cell viability, adhesion, proliferation, migration, and real-time polymerase chain reaction assays disclosed improved adhesion, proliferation, migration, and extracellular matrix remodeling abilities of human gingival fibroblasts (HGFs) on the magnesium-doped titanium. The results of western blot suggested that the Mg-modified titanium induced the phosphorylation of AKT through the activation of PI3K. Our results revealed that magnesium-doping would potentially enhance soft tissue sealings by promoting cellular functions of HGFs in a dose-dependent manner, boding well for its applications on surfaces of implant necks in early peri-implant soft tissue integrations.

TRPV4-mediated calcium signaling in mesenchymal stem cells regulates aligned collagen matrix formation and vinculin tension

Microarchitectural cues drive aligned fibrillar collagen deposition in vivo and in biomaterial scaffolds, but the cell-signaling events that underlie this process are not well understood. Utilizing a multicellular patterning model system that allows for observation of intracellular signaling events during collagen matrix assembly, we investigated the role of calcium (Ca²⁺) signaling in human mesenchymal stem cells (MSCs) during this process. We observed spontaneous Ca²⁺ oscillations in MSCs during fibrillar collagen assembly, and hypothesized that the transient receptor potential vanilloid 4 (TRPV4) ion channel, a mechanosensitive Ca²⁺-permeable channel, may regulate this signaling. Inhibition of TRPV4 nearly abolished Ca²⁺ signaling at initial stages of collagen matrix assembly, while at later times had reduced but significant effects. Importantly, blocking TRPV4 activity dramatically reduced aligned collagen fibril assembly; conversely, activating TRPV4 accelerated aligned collagen formation. TRPV4-dependent Ca²⁺ oscillations were found to be independent of pattern shape or subpattern cell location, suggesting this signaling mechanism is necessary for aligned collagen formation but not sufficient in the absence of physical (microarchitectural) cues that force multicellular alignment. As cell-generated mechanical forces are known to be critical to the matrix assembly process, we examined the role of TRPV4-mediated Ca²⁺ signaling in force generated across the load-bearing focal adhesion protein vinculin within MSCs using an FRET-based tension sensor. Inhibiting TRPV4 decreased tensile force across vinculin, whereas TRPV4 activation caused a dynamic unloading and reloading of vinculin. Together, these findings suggest TRPV4 activity regulates forces at cell-matrix adhesions and is critical to aligned collagen matrix assembly by MSCs.

Spinal Cord Injury Scarring and Inflammation: Therapies Targeting Glial and Inflammatory Responses

Deficits in neuronal function are a hallmark of spinal cord injury (SCI) and therapeutic efforts are often focused on central nervous system (CNS) axon regeneration. However, secondary injury responses by astrocytes, microglia, pericytes, endothelial cells, Schwann cells, fibroblasts, meningeal cells, and other glia not only potentiate SCI damage but also facilitate endogenous repair. Due to their profound impact on the progression of SCI, glial cells and modification of the glial scar are focuses of SCI therapeutic research. Within and around the glial scar, cells deposit extracellular matrix (ECM) proteins that affect axon growth such as chondroitin sulfate proteoglycans (CSPGs), laminin, collagen, and fibronectin. This dense deposition of material, i.e., the fibrotic scar, is another barrier to endogenous repair and is a target of SCI therapies. Infiltrating neutrophils and monocytes are recruited to the injury site through glial chemokine and cytokine release and subsequent upregulation of chemotactic cellular adhesion molecules and selectins on endothelial cells. These peripheral immune cells, along with endogenous microglia, drive a robust inflammatory response to injury with heterogeneous reparative and pathological properties and are targeted for therapeutic modification. Here, we review the role of glial and inflammatory cells after SCI and the therapeutic strategies that aim to replace, dampen, or alter their activity to modulate SCI scarring and inflammation and improve injury outcomes.

Dynamics of 3D carcinoma cell invasion into aligned collagen

Carcinoma cells frequently expand and invade from a confined lesion, or multicellular clusters, into and through the stroma on the path to metastasis, often with an efficiency dictated by the architecture and composition of the microenvironment. Specifically, in desmoplastic carcinomas such as those of the breast, aligned collagen tracks provide contact guidance cues for directed cancer cell invasion. Yet, the evolving dynamics of this process of invasion remains poorly understood, in part due to difficulties in continuously capturing both spatial and temporal heterogeneity and progression to invasion in experimental systems. Therefore, to study the local invasion process from cell dense clusters into aligned collagen architectures found in solid tumors, we developed a novel engineered 3D invasion platform that integrates an aligned collagen matrix with a cell dense tumor-like plug. Using multiphoton microscopy and quantitative analysis of cell motility, we track the invasion of cancer cells from cell-dense bulk clusters into the pre-aligned 3D matrix, and define the temporal evolution of the advancing invasion fronts over several days. This enables us to identify and probe cell dynamics in key regions of interest: behind, at, and beyond the edge of the invading lesion at distinct time points. Analysis of single cell migration identifies significant spatial heterogeneity in migration behavior between cells in the highly cell-dense region behind the leading edge of the invasion front and cells at and beyond the leading edge. Moreover, temporal variations in motility and directionality are also observed between cells within the cell-dense tumor-like plug and the leading invasive edge as its boundary extends into the anisotropic collagen over time. Furthermore, experimental results combined with mathematical modeling demonstrate that in addition to contact guidance, physical crowding of cells is a key regulating factor orchestrating variability in single cell migration during invasion into anisotropic ECM. Thus, our novel platform enables us to capture spatio-temporal dynamics of cell behavior behind, at, and beyond the invasive front and reveals heterogeneous, local interactions that lead to the emergence and maintenance of the advancing front.

Surface micro- and nano-texturing of stainless steel by femtosecond laser for the control of cell migration

The precise control over the interaction between cells and the surface of materials plays a crucial role in optimizing the integration of implanted biomaterials. In this regard, material surface with controlled topographic features at the micro- and nano-scales has been proved to affect the overall cell behavior and therefore the final osseointegration of implants. Within this context, femtosecond (fs) laser micro/nano machining technology was used in this work to modify the surface structure of stainless steel aiming at controlling cell adhesion and migration. The experimental results show that cells tend to attach and preferentially align to the laser-induced nanopatterns oriented in a specific direction. Accordingly, the laser-based fabrication method here described constitutes a simple, clean, and scalable technique which allows a precise control of the surface nano-patterning process and, subsequently, enables the control of cell adhesion, migration, and polarization. Moreover, since our surface-patterning approach does not involve any chemical treatments and is performed in a single step process, it could in principle be applied to most metallic materials.

Electrospun Poly(γ-Benzyl-L-Glutamate) and Its Alkali-Treated Meshes: Their Water Wettability and Cell-Adhesion Potential

The aim of this study was (1) to fabricate non-woven meshes from a biodegradable polymer, poly(γ-benzylL-glutamate), by electrospinning and subsequent hydrolysis of the ester bond on the polymer side-chain in an aqueous solution of NaOH, creating surface carboxyl groups on the fibers, and (2) to determine the effect of hydrolysis time on water wettability and cellular behaviors, in order to perform a preliminary evaluation for use of this polymer as a wound dressing matrix. A non-woven mesh composed of fibers, with minimal formation of 'bead' structures, was produced by electrospinning from tetrahydrofuran solution under optimally controlled conditions. The surface wettability largely depended on the hydrolysis time: an increase in hydrolysis time significantly reduced the advancing water contact angle. Instantaneous spreading of water droplets occurred at long hydrolysis times. An increase in hydrolysis time decreased adhesion of endothelial cells, but increased cell spreading. Neither proliferation nor invasion into the mesh interior was observed. We conclude by discussing the use of partially hydrolyzed non-woven mesh as a promising burn dressing.

Winner of the Young Investigator Award of the Society for Biomaterials (USA) for 2016, 10th World Biomaterials Congress, May 17-22, 2016, Montreal QC, Canada: Aligned microribbon-like hydrogels for guiding three-dimensional smooth muscle tissue regeneration

Smooth muscle tissue is characterized by aligned structures, which is critical for its contractile functions. Smooth muscle injury is common and can be caused by various diseases and degenerative processes, and there remains a strong need to develop effective therapies for smooth muscle tissue regeneration with restored structures. To guide cell alignment, previously cells were cultured on 2D nano/microgrooved substrates, but such method is limited to fabricating 2D aligned cell sheets only. Alternatively, aligned electrospun nanofiber has been employed as 3D scaffold for cell alignment, but cells can only be seeded post fabrication, and nanoporosity of electrospun fiber meshes often leads to poor cell distribution. To overcome these limitations, we report aligned gelatin-based microribbons (µRBs) as macroporous hydrogels for guiding smooth muscle alignment in 3D. We developed aligned µRB-like hydrogels using wet spinning, which allows easy fabrication of tissue-scale (cm) macroporous matrices with alignment cues and supports direct cell encapsulation. The macroporosity within µRB-based hydrogels facilitated cell proliferation, new matrix deposition, and nutrient diffusion. In aligned µRB scaffold, smooth muscle cells showed high viability, rapid adhesion, and alignment following µRB direction. Aligned µRB scaffolds supported retention of smooth muscle contractile phenotype, and accelerated uniaxial deposition of new matrix (collagen I/IV) along the µRB. In contrast, cells encapsulated in conventional gelatin hydrogels remained round with matrix deposition limited to pericellular regions only. We envision such aligned µRB scaffold can be broadly applicable in growing other anisotropic tissues including tendon, nerves and blood vessel.

A Combined In Vitro Imaging and Multi-Scale Modeling System for Studying the Role of Cell Matrix Interactions in Cutaneous Wound Healing

Many cell types remodel the extracellular matrix of the tissues they inhabit in response to a wide range of environmental stimuli, including mechanical cues. Such is the case in dermal wound healing, where fibroblast migrate into and remodel the provisional fibrin matrix in a complex manner that depends in part on the local mechanical environment and the evolving multi-scale mechanical interactions of the system. In this study, we report on the development of an image-based multi-scale mechanical model that predicts the short-term (24 hours), structural reorganization of a fibrin gel by fibroblasts. These predictive models are based on an in vitro experimental system where clusters of fibroblasts (i.e., explants) were spatially arranged into a triangular geometry onto the surface of fibrin gels that were subjected to either Fixed or Free in-plane mechanical constraints. Experimentally, regional differences in short-term structural remodeling and cell migration were observed for the two gel boundary conditions. A pilot experiment indicated that these small differences in the short-term remodeling of the fibrin gel translate into substantial differences in long-term (4 weeks) remodeling, particularly in terms of collagen production. The multi-scale models were able to predict some regional differences in remodeling and qualitatively similar reorganization patterns for the two boundary conditions. However, other aspects of the model, such as the magnitudes and rates of deformation of gel, did not match the experiments. These discrepancies between model and experiment provide fertile ground for challenging model assumptions and devising new experiments to enhance our understanding of how this multi-scale system functions. These efforts will ultimately improve the predictions of the remodeling process, particularly as it relates to dermal wound healing and the reduction of patient scarring. Such models could be used to recommend patient-specific mechanical-based treatment dependent on parameters such as wound geometry, location, age, and health.

Nerve guidance conduits from aligned nanofibers: improvement of nerve regeneration through longitudinal nanogrooves on a fiber surface

A novel fibrous conduit consisting of well-aligned nanofibers with longitudinal nanogrooves on the fiber surface was prepared by electrospinning and was subjected to an in vivo nerve regeneration study on rats using a sciatic nerve injury model. For comparison, a fibrous conduit having a similar fiber alignment structure without surface groove and an autograft were also conducted in the same test. The electrophysiological, walking track, gastrocnemius muscle, triple-immunofluorescence, and immunohistological analyses indicated that grooved fibers effectively improved sciatic nerve regeneration. This is mainly attributed to the highly ordered secondary structure formed by surface grooves and an increase in the specific surface area. Fibrous conduits made of longitudinally aligned nanofibers with longitudinal nanogrooves on the fiber surface may offer a new nerve guidance conduit for peripheral nerve repair and regeneration.

Oriented matrix promotes directional tubulogenesis

Detailed control over the structural organization of scaffolds and engineered tissue constructs is a critical need in the quest to engineer functional tissues using biomaterials. This work presents a new approach to spatially direct endothelial tubulogenesis. Micropatterned fibronectin substrates were used to control lung fibroblast adhesion and growth and the subsequent deposition of fibroblast-derived matrix during culture. The fibroblast-derived matrix produced on the micropatterned substrates was tightly oriented by these patterns, with an average variation of only 8.5°. Further, regions of this oriented extracellular matrix provided directional control of developing endothelial tubes to within 10° of the original micropatterned substrate design. Endothelial cells seeded directly onto the micropatterned substrate did not form tubes. A metric for matrix anisotropy showed a relationship between the fibroblast-derived matrix and the endothelial tubes that were subsequently developed on the same micropatterns with a resulting aspect ratio over 1.5 for endothelial tubulogenesis. Micropatterns in "L" and "Y" shapes were used to direct endothelial tubes to turn and branch with the same level of precision. These data demonstrate that anisotropic fibroblast-derived matrices instruct the alignment and shape of endothelial tube networks, thereby introducing an approach that could be adapted for future design of microvascular implants featuring organ-specific natural matrix that patterns microvascular growth.

Effect of surface pore structure of nerve guide conduit on peripheral nerve regeneration

Polycaprolactone (PCL)/Pluronic F127 nerve guide conduits (NGCs) with different surface pore structures (nano-porous inner surface vs. micro-porous inner surface) but similar physical and chemical properties were fabricated by rolling the opposite side of asymmetrically porous PCL/F127 membranes. The effect of the pore structure on peripheral nerve regeneration through the NGCs was investigated using a sciatic nerve defect model of rats. The nerve fibers and tissues were shown to have regenerated along the longitudinal direction through the NGC with a nano-porous inner surface (Nanopore NGC), while they grew toward the porous wall of the NGC with a micro-porous inner surface (Micropore NGC) and, thus, their growth was restricted when compared with the Nanopore NGC, as investigated by immunohistochemical evaluations (by fluorescence microscopy with anti-neurofilament staining and Hoechst staining for growth pattern of nerve fibers), histological evaluations (by light microscopy with Meyer's modified trichrome staining and Toluidine blue staining and transmission electron microscopy for the regeneration of axon and myelin sheath), and FluoroGold retrograde tracing (for reconnection between proximal and distal stumps). The effect of nerve growth factor (NGF) immobilized on the pore surfaces of the NGCs on nerve regeneration was not so significant when compared with NGCs not containing immobilized NGF. The NGC system with different surface pore structures but the same chemical/physical properties seems to be a good tool that is used for elucidating the surface pore effect of NGCs on nerve regeneration.

Spatial and temporal evaluation of cell attachment to printed polycaprolactone microfibres

Surface topography plays a crucial role in influencing cellular responses and has therefore been utilized in the development of numerous implantable devices. Whilst numerous studies have either investigated cell attachment or migration post-attachment, few have looked at the early-stages of this process temporally. The aim of this study was to evaluate the use of time-lapse microscopy to study the behaviour of fibroblasts cultured with polycaprolactone microfibres and to assess spatially and temporally the cell-structure interaction over a 24h period. Ordered polymeric structures were printed (predetermined) onto glass substrates using an electrohydrodynamic direct write process to produce fine (3-5 μm wide) structures. Fibroblast attachment and migration were characterized as a function of distance perpendicular from structures (∼17.3, 34.6 and 51.9 μm). The use of time-lapse microscopy revealed a gradual decrease in cell attachment as the distance from the microfibres was increased. The technique also revealed that some cells were attaching and detaching from the microfibre multiple times. Our findings demonstrate that time-lapse microscopy is a useful technique for evaluating early-stage cell-biomaterial interaction that is capable of recording important events that might otherwise be overlooked.

Micrometer scale guidance of mesenchymal stem cells to form structurally oriented cartilage extracellular matrix

Tissue engineering is a possible method for long-term repair of cartilage lesions, but current tissue-engineered cartilage constructs have inferior mechanical properties compared to native cartilage. This problem may be due to the lack of an oriented structure in the constructs at the microscale that is present in the native tissue. In this study, we utilize contact guidance to develop constructs with microscale architecture for improved chondrogenesis and function. Stable channels of varying microscale dimensions were formed in collagen-based and polydimethylsiloxane membranes via a combination of microfabrication and soft-lithography. Human mesenchymal stem cells (MSCs) were selectively seeded in these channels. The chondrogenic potential of MSCs seeded in these channels was investigated by culturing them for 3 weeks under differentiating conditions, and then evaluating the subsequent synthesized tissue for mechanical function and by type II collagen immunohistochemistry. We demonstrate selective seeding of viable MSCs within the channels. MSC aligned and produced mature collagen fibrils along the length of the channel in smaller linear channels of widths 25-100 μm compared to larger linear channels of widths 500-1000 μm. Further, substrates with microchannels that led to cell alignment also led to superior mechanical properties compared to constructs with randomly seeded cells or selectively seeded cells in larger channels. The ultimate stress and modulus of elasticity of constructs with cells seeded in smaller channels increased by as much as fourfolds. We conclude that microscale guidance is useful to produce oriented cartilage structures with improved mechanical properties. These findings can be used to fabricate large clinically useful MSC-cartilage constructs with superior mechanical properties.

Cells cultured on microgrooves with or without surface coating: correlation between cell alignment, spreading and local membrane deformation

The behaviors of cells cultured on patterned substrates vary with the material stiffness, the geometry and the biochemical properties of the pattern. By using a reversed cell imprinting (RCI) technique, together with phase contrast microscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM), we have exploited reversed side cellular morphology on patterned microgrooves of different geometries with or without surface coating of adhesion molecules. We have shown a close correlation between the effect of contact guidance and penetration of cellular membrane. Without surface coating, roughly 80% of HeLa cells were aligned along the groove direction regardless of the groove spacing. When the microgrooves were coated with fibronectin, the area of cell spreading was increased but the percentage of aligned cells was significantly decreased. In both cases, the deformation of cell membrane at the cell-pattern interfaces could be measured. We found that the local penetration of the cellular membrane into the grooves was correlated to the cellular alignment for both HeLa and NIH 3T3 cells, and that such a correlation was cell-type dependent.

Guided gingival fibroblast attachment to titanium surfaces: an in vitro study

AIM

To assess the potential of gingival fibroblasts to attach in a predetermined linear orientation to a nano-topography of aligned fibres on titanium surfaces and determine the ability of such cells to deposit aligned collagen fibre matrix.

MATERIALS AND METHODS

smooth glass and rough titanium substrates were coated with polytetrafluoroethylene (PTFE) nano-fibres. Ammonia plasma treatment was used to modify the surface chemistry. Human gingival fibroblasts were cultured on substrates and orientation and collagen deposition was assessed.

RESULTS

Straight, unidirectional, parallel PTFE nano-fibres were deposited over the titanium features. By 7 days, the majority of cells were observed to orient to untreated fibres despite the presence of competing titanium surface features. On plasma-treated fibre-coated titanium substrates, cell orientation was mixed. On uncoated substrates, the majority of cells oriented to the titanium surface features. On fibre-coated glass substrates, cells oriented themselves with untreated and plasma-treated fibres and secreted collagen in the same direction after 1 week. On uncoated glass substrates, there was no preferred direction of collagen orientation.

CONCLUSION

Polytetrafluoroethylene nano-fibres induced cell and collagen orientation. Surface chemistry appeared only to affect cell behaviour at early time points. An implant surface that controls cell orientation may also influence the orientation of collagen, providing improved gingival support.

Regenerative scaffold electrodes for peripheral nerve interfacing

Advances in neural interfacing technology are required to enable natural, thought-driven control of a prosthetic limb. Here, we describe a regenerative electrode design in which a polymer-based thin-film electrode array is integrated within a thin-film sheet of aligned nanofibers, such that axons regenerating from a transected peripheral nerve are topographically guided across the electrode recording sites. Cultures of dorsal root ganglia were used to explore design parameters leading to cellular migration and neurite extension across the nanofiber/electrode array boundary. Regenerative scaffold electrodes (RSEs) were subsequently fabricated and implanted across rat tibial nerve gaps to evaluate device recording capabilities and influence on nerve regeneration. In 20 of these animals, regeneration was compared between a conventional nerve gap model and an amputation model. Characteristic shaping of regenerated nerve morphology around the embedded electrode array was observed in both groups, and regenerated axon profile counts were similar at the eight week end point. Implanted RSEs recorded evoked neural activity in all of these cases, and also in separate implantations lasting up to five months. These results demonstrate that nanofiber-based topographic cues within a regenerative electrode can influence nerve regeneration, to the potential benefit of a peripheral nerve interface suitable for limb amputees.

Nuclear and cellular alignment of primary corneal epithelial cells on topography

The basement membrane of the corneal epithelium presents biophysical cues in the form of topography and compliance that can modulate cytoskeletal dynamics, which, in turn, can result in altering cellular and nuclear morphology and alignment. In this study, the effect of topographic patterns of alternating ridges and grooves on nuclear and cellular shape and alignment was determined. Primary corneal epithelial cells were cultured on either planar or topographically patterned (400-4000 nm pitch) substrates. Alignment of individual cell body was correlated with respective nucleus for the analysis of orientation and elongation. A biphasic response in alignment was observed. Cell bodies preferentially aligned perpendicular to the 800 nm pitch; and with increasing pitch, cells increasingly aligned parallel to the substratum. Nuclear orientation largely followed this trend with the exception of those on 400 nm. On this biomimetic size scale, some nuclei oriented perpendicular to the topography while their cytoskeleton elements aligned parallel. Both nuclei and cell bodies were elongated on topography compared to those on flat surfaces. Our data demonstrate that nuclear orientation and shape are differentially altered by topographic features that are not mandated by alignment of the cell body. This novel finding suggests that nuanced differences in alignment of the nucleus versus the cell body exist and that these differences could have consequences on gene and protein regulation that ultimately regulate cell behaviors. A full understanding of these mechanisms could disclose novel pathways that would better inform evolving strategies in cell, stem cell, and tissue engineering as well as the design and fabrication of improved prosthetic devices.

Sacrificial nanofibrous composites provide instruction without impediment and enable functional tissue formation

The fibrous tissues prevalent throughout the body possess an ordered structure that underlies their refined and robust mechanical properties. Engineered replacements will require recapitulation of this exquisite architecture in three dimensions. Aligned nanofibrous scaffolds can dictate cell and matrix organization; however, their widespread application has been hindered by poor cell infiltration due to the tight packing of fibers during fabrication. Here, we develop and validate an enabling technology in which tunable composite nanofibrous scaffolds are produced to provide instruction without impediment. Composites were formed containing two distinct fiber fractions: slow-degrading poly(ε-caprolactone) and water-soluble, sacrificial poly(ethylene oxide), which can be selectively removed to increase pore size. Increasing the initial fraction of sacrificial poly(ethylene oxide) fibers enhanced cell infiltration and improved matrix distribution. Despite the removal of >50% of the initial fibers, the remaining scaffold provided sufficient instruction to align cells and direct the formation of a highly organized ECM across multiple length scales, which in turn led to pronounced increases in the tensile properties of the engineered constructs (nearly matching native tissue). This approach transforms what is an interesting surface phenomenon (cells on top of nanofibrous mats) into a method by which functional, 3D tissues (>1 mm thick) can be formed, both in vitro and in vivo. As such, this work represents a marked advance in the engineering of load-bearing fibrous tissues, and will find widespread applications in regenerative medicine.

Nanobiomaterials: a review of the existing science and technology, and new approaches

Nanotechnology has made great strides forward in the creation of new surfaces, new materials and new forms which also find application in the biomedical field. Traditional biomedical applications started benefiting from the use nanotechnology in an array of areas, such as biosensors, tissue engineering, controlled release systems, intelligent systems and nanocomposites used in implant design. In this manuscript a review of developments in these areas will be provided along with some applications from our laboratories.

Role of fibronectin in topographical guidance of neurite extension on electrospun fibers

Bridging of long peripheral nerve gaps remains a significant clinical challenge. Electrospun nanofibers have been used to direct and enhance neurite extension in vitro and in vivo. While it is well established that oriented fibers influence neurite outgrowth and Schwann cell migration, the mechanisms by which they influence these cells are still unclear. In this study, thin films consisting of aligned poly-acrylonitrile methylacrylate (PAN-MA) fibers or solvent casted smooth, PAN-MA films were fabricated to investigate the potential role of differential protein adsorption on topography-dependent neural cell responses. Aligned nanofiber films promoted enhanced adsorption of fibronectin compared to smooth films. Studies employing function-blocking antibodies against cell adhesion motifs suggest that fibronectin plays an important role in modulating Schwann cell migration and neurite outgrowth from dorsal root ganglion (DRG) cultures. Atomic Force Microscopy demonstrated that aligned PAN-MA fibers influenced fibronectin distribution, and promoted aligned fibronectin network formation compared to smooth PAN-MA films. In the presence of topographical cues, Schwann cell-generated fibronectin matrix was also organized in a topographically sensitive manner. Together these results suggest that fibronectin adsorption mediated the ability of topographical cues to influence Schwann cell migration and neurite outgrowth. These insights are significant to the development of rational approaches to scaffold designs to bridge long peripheral nerve gaps.

Synergic effects of crypt-like topography and ECM proteins on intestinal cell behavior in collagen based membranes

The basement membrane of small intestinal epithelium possesses complex topography at multiple scales ranging from the mesoscale to nanoscale. Specifically, intestinal crypt-villus units are comprised of hundred-micron-scale well-like invaginations and finger-like projections; intestinal cell phenotype is related to location on this crypt-villus unit. A biomimetic intestinal cell culture system composed of type I collagen based permeable cell culture membranes incorporating both micron-scale intestinal crypt-like topography and nanometer scale topography was fabricated. Membranes were pre-incubated with either laminin (Ln) or fibronectin (Fn), inoculated with intestinal epithelial Caco-2 cells and cultured for 1-21 days to study the relative significance of influence of crypt-like topography and biomimetic substrate chemistry on cell phenotype. Crypt-like topography inhibited Caco-2 differentiation during early culture, as evidenced by slower cell spreading and lower brush border enzyme activity. For example, alanine aminopeptidase activity was lower on Ln-coated patterned collagen ( approximately 3.4+/-0.24mU/mg) compared to flat collagen (10.84+/-0.55mU/mg) at day 7. Caco-2 cultured on Fn-coated collagen started to spread earlier (1 day vs 3 days) and formed longer protrusions than on Ln-coated collagen. Pre-coating of Ln enhanced cell differentiation, as the maximum activity of a cell differentiation marker (alkaline phosphatase) was 2-3 times higher than on Fn-coated collagen, and maintained differentiated phenotype in long term (up to 21 days) culture. In general, compared to substrate topography, coating with ECM protein had more prominent and longer effect on cell behavior. Crypt-like topography affected Caco-2 spreading and differentiation during early culture, however the effect diminished as culture progressed. This information will benefit intestinal tissue engineering scaffold design, and modification of in vitro intestinal cell models.

Radially aligned, electrospun nanofibers as dural substitutes for wound closure and tissue regeneration applications

This paper reports the fabrication of scaffolds consisting of radially aligned poly(ε-caprolactone) nanofibers by utilizing a collector composed of a central point electrode and a peripheral ring electrode. This novel class of scaffolds was able to present nanoscale topographic cues to cultured cells, directing and enhancing their migration from the periphery to the center. We also established that such scaffolds could induce faster cellular migration and population than nonwoven mats consisting of random nanofibers. Dural fibroblast cells cultured on these two types of scaffolds were found to express type I collagen, the main extracellular matrix component in dural mater. The type I collagen exhibited a high degree of organization on the scaffolds of radially aligned fibers and a haphazard distribution on the scaffolds of random fibers. Taken together, the scaffolds based on radially aligned, electrospun nanofibers show great potential as artificial dural substitutes and may be particularly useful as biomedical patches or grafts to induce wound closure and/or tissue regeneration.

Development of a micromanipulator-based loading device for mechanoregulation study of human mesenchymal stem cells in three-dimensional collagen constructs

Mechanical signal is important for regulating cellular activities, including proliferation, metabolism, matrix production, and orientation. Bioreactors with loading functions can be used to precondition cells in three-dimensional (3D) constructs so as to study the cellular responses to mechanical stimulation. However, full-scale bioreactor is not always an affordable option considering the high cost of equipment and the liter-sized medium with serum and growth factor supplements. In this study, a custom-built loading system was developed by coupling a conventional camera-equipped inverted research microscope with two micromanipulators. The system was programmed to deliver either cyclic compressive loading with different frequencies or static compressive loading for 1 week to investigate the cellular responses of human mesenchymal stem cells (hMSCs) entrapped in a 3D construct consists of reconstituted collagen fibers. Cellular properties, including their alignment, cytoskeleton, and cell metabolism, and properties of matrix molecules, such as collagen fiber alignment and glycosaminoglycan deposition, were evaluated. Using a MatLab-based image analysis program, reorientation of the entrapped cells from a random distribution to a preferred alignment along the loading direction in constructs with both static and cyclic compression has been demonstrated, but no such alignment was found in the free-floating controls. Fluorescent staining on filamentous actin cytoskeleton also confirmed the finding. Nevertheless, the collagen fiber meshwork entrapping the hMSCs remained randomly distributed, and no change in cellular metabolism and glycosaminoglycans production was noted. The current study provides a simple and affordable option toward setting up a mechanoregulation facility based on existing laboratory equipment and sheds new insights on the effect of mechanical loading on the alignment of hMSCs in 3D collagen constructs.

The ability of corneal epithelial cells to recognize high aspect ratio nanostructures

The basement membrane of the human corneal epithelium comprises topographic features including fibers, pores, and elevations with feature dimensions on the order of 20-400 nm. Understanding the impact of sub-micron and nanotopography on corneal cell behavior will contribute to our understanding of biomechanical cues and will assist in the design of improved synthetic corneal implants. We utilized well defined ridge and groove wave-like nanostructures (wave ordered structures, WOS) of 60-140 pitches (30-70 nm ridge widths) and 200 nm depths to assess human corneal epithelial cell (HCEC) contact guidance and to establish HCEC contact acuity defined as the lower limit in feature dimensions at which cells respond to biomimetic topographic cues. Results using the WOS substrates demonstrate that HCEC contact acuity is in the range of 60 nm pitch for cells in a serum-free basal medium (EpiLife) and in the range of 90 nm pitch for cells in epithelial medium. To further investigate the influence of HCEC contact acuity in the presence of larger topographic cues, we fabricated 70 nm pitch WOS-overlaid parallel to the top of the ridges of 800-4000 nm pitch. HCEC cultured in epithelial medium demonstrate a significant increase in the percent of cells aligning to 4000 nm pitch topography with WOS-overlay compared to controls (both flat and 70 nm WOS alone) and 4000 nm pitch topography alone. These results highlight the significance of the lower range of basement membrane scale topographic cues on cell response and allow for improved prosthetic design.

Interfacial flow processing of collagen

A new method for creating substrates made out of ordered collagen fibers, on which cells in culture can align, is proposed. The substrates can be used for research in cell culture, and this research presents a significant advance in the technology to coat implants in order to improve cell adhesion. In the procedure presented here, a molecular solution of collagen is spread at the interface of a saline solution and air to induce fiber formation, compressed at a high speed to induce orientation and deposited on solid substrates via Langmuir-Blodgett transfer. Several interfacial techniques are employed to investigate the behavior of collagen, which is shown to be dependent on the salt concentration of the subphase as well as the temperature. After Langmuir-Blodgett transfer, primary human fibroblasts and adipose-derived stem cells are cultured on the collagen substrates. Both types of cells respond favorably to the collagen orientation and align with the deposited fibers. The technique presented here provides a simple method to produce well-controlled, oriented collagen substrates that can be used in tissue culture research or scaffolding applications without the use of additives and/or bioincompatible materials.

Mechanosensitivity of fibroblast cell shape and movement to anisotropic substratum topography gradients

In this report, we describe using ultraviolet (UV)-assisted capillary force lithography (CFL) to create a model substratum of anisotropic micro- and nanotopographic pattern arrays with variable local density for the analysis of cell-substratum interactions. A single cell adhesion substratum with the constant ridge width (1 microm), and depth (400 nm) and variable groove widths (1-9.1 microm) allowed us to characterize the dependence of cellular responses, including cell shape, orientation, and migration, on the anisotropy and local density of the variable micro- and nanotopographic pattern. We found that fibroblasts adhering to the denser pattern areas aligned and elongated more strongly along the direction of ridges, vs. those on the sparser areas, exhibiting a biphasic dependence of the migration speed on the pattern density. In addition, cells responded to local variations in topography by altering morphology and migrating along the direction of grooves biased by the direction of pattern orientation (short term) and pattern density (long term), suggesting that single cells can sense the topography gradient. Molecular dynamic live cell imaging and immunocytochemical analysis of focal adhesions and actin cytoskeleton suggest that variable substratum topography can result in distinct types of cytoskeleton reorganization. We also demonstrate that fibroblasts cultured as monolayers on the same substratum retain most of the properties displayed by single cells. This result, in addition to demonstrating a more sophisticated method to study aspects of wound healing processes, strongly suggests that even in the presence of adhesive cell-cell interactions, the cues provided by the underlying substratum topography continue to exercise substantial influence on cell behavior. The described experimental platform might not only further our understanding of biomechanical regulation of cell-matrix interactions, but also contribute to bioengineering of devices with the optimally structured design of cell-material interface.

The use of nanoscale topography to modulate the dynamics of adhesion formation in primary osteoblasts and ERK/MAPK signalling in STRO-1+ enriched skeletal stem cells

The physiochemical characteristics of a material with in vivo applications are critical for the clinical success of the implant and regulate both cellular adhesion and differentiated cellular function. Topographical modification of an orthopaedic implant may be a viable method to guide tissue integration and has been shown in vitro to dramatically influence osteogenesis, inhibit bone resorption and regulate integrin mediated cell adhesion. Integrins function as force dependant mechanotransducers, acting via the actin cytoskeleton to translate tension applied at the tissue level to changes in cellular function via intricate signalling pathways. In particular the ERK/MAPK signalling cascade is a known regulator of osteospecific differentiation and function. Here we investigate the effects of nanoscale pits and grooves on focal adhesion formation in human osteoblasts (HOBs) and the ERK/MAPK signalling pathway in mesenchymal populations. Nanopit arrays disrupted adhesion formation and cellular spreading in HOBs and impaired osteospecific differentiation in skeletal stem cells. HOBs cultured on 10 microm wide groove/ridge arrays formed significantly less focal adhesions than cells cultured on planar substrates and displayed negligible differentiation along the osteospecific lineage, undergoing up-regulations in the expression of adipospecific genes. Conversely, osteospecific function was correlated to increased integrin mediated adhesion formation and cellular spreading as noted in HOBS cultured on 100 microm wide groove arrays. Here osteospecific differentiation and function was linked to focal adhesion growth and FAK mediated activation of the ERK/MAPK signalling pathway in mesenchymal populations.

Thin-film enhanced nerve guidance channels for peripheral nerve repair

It has been demonstrated that nerve guidance channels containing stacked thin-films of aligned poly(acrylonitrile-co-methylacrylate) fibers support peripheral nerve regeneration across critical sized nerve gaps, without the aid of exogenous cells or proteins. Here, we explore the ability of tubular channels minimally supplemented with aligned nanofiber-based thin-films to promote endogenous nerve repair. We describe a technique for fabricating guidance channels in which individual thin-films are fixed into place within the lumen of a polysulfone tube. Because each thin-film is <10 microm thick, this technique allows fine control over the positioning of aligned scaffolding substrate. We evaluated nerve regeneration through a 1-film guidance channel--containing a single continuous thin-film of aligned fibers--in comparison to a 3-film channel that provided two additional thin-film tracks. Thirty rats were implanted with one of the two channel types, and regeneration across a 14 mm tibial nerve gap was evaluated after 6 weeks and 13 weeks, using a range of morphological and functional measures. Both the 1-film and the 3-film channels supported regeneration across the nerve gap resulting in functional muscular reinnervation. Each channel type characteristically influenced the morphology of the regeneration cable. Interestingly, the 1-film channels supported enhanced regeneration compared to the 3-film channels in terms of regenerated axon profile counts and measures of nerve conduction velocity. These results suggest that minimal levels of appropriately positioned topographical cues significantly enhance guidance channel function by modulating endogenous repair mechanisms, resulting in effective bridging of critically sized peripheral nerve gaps.

Attachment of human primary osteoblast cells to modified polyethylene surfaces

Ultra-high-molecular-weight polyethylene (UHMWPE) has a long history of use in medical devices, primarily for articulating surfaces due to its inherent low surface energy which limits tissue integration. To widen the applications of UHMWPE, the surface energy can be increased. The increase in surface energy would improve the adsorption of proteins and attachment of cells to allow tissue integration, thereby allowing UHMWPE to potentially be used for a wider range of implants. The attachment and function of human primary osteoblast-like (HOB) cells to surfaces of UHMWPE with various levels of incorporated surface oxygen have been investigated. The surface modification of the UHMWPE was produced by exposure to a UV/ozone treatment. The resulting surface chemistry was studied using X-ray photoelectron spectroscopy (XPS), and the topography and surface structure were probed by atomic force microscopy (AFM) and scanning electron microscopy (SEM), which showed an increase in surface oxygen from 11 to 26 atom % with no significant change to the surface topography. The absolute root mean square roughness of both untreated and UV/ozone-treated surfaces was within 350-450 nm, and the water contact angles decreased with increasing oxygen incorporation, i.e., showing an increase in surface hydrophilicity. Cell attachment and functionality were assessed over a 21 day period for each cell-surface combination studied; these were performed using SEM and the alamarBlue assay to study cell attachment and proliferation and energy-dispersive X-ray (EDX) analysis to confirm extracellular mineral deposits, and total protein assay to examine the intra- and extracellular protein expressed by the cells. HOB cells cultured for 21 days on the modified UHMWPE surfaces with 19 and 26 atom % oxygen incorporated showed significantly higher cell densities compared to cells cultured on tissue culture polystyrene (TCPS) from day 3 onward. This indicated that the cells attached and proliferated more readily on the UV/ozone-treated UHMWPE surfaces than on untreated UHMWPE and TCPS surfaces. Contact guidance of the cells was observed on the UHMWPE surfaces by both SEM and AFM. Scanning electron micrographs showed that the cells were confluent on the modified UHMWPE surfaces by day 10, which led to visible layering of the cells by day 21, an indicator of nodule formation. In vitro mineralization of the extracellular matrix expressed by the HOB cells on the modified UHMWPE surfaces was confirmed by SEM and EDX analysis; spherulite structures were observed near cell protrusions by day 21. EDX analysis confirmed the spherulites to contain calcium and phosphorus, the major constituents in calcium phosphate apatite, the mineral phase of bone. Overall cell attachment, functionality, and mineralization were found to be enhanced on the UV/ozone-modified UHMWPE surfaces, demonstrating the importance of optimizing the surface chemistry for primary HOB cells.

Positive changes in bone marrow-derived cells in response to culture on an aligned bioscaffold

Previous studies have shown that cultured cells align with the local topography of their substrate following a concept called "contact guidance." Additionally, if the topography is highly aligned, the cells produce newly synthesized matrix that is also aligned. The objective of this study was to elucidate the positive effect of cell seeding on an elongated porcine small intestinal submucosa (SIS), which has been shown to improve ligament and tendon healing, by measuring the cellular response as a result of the changes in alignment. Because elongation is known to align the fibers of SIS through recruitment along the direction of elongation, we hypothesized that rabbit bone marrow-derived cells (BMDCs) seeded on SIS with improved fiber alignment would increase the expression and production of collagen following the concept of contact guidance. Using the small-angle light-scattering technique, it was found that a 15% elongation together with BMDC seeding on SIS (elongated, seeded group) improved its alignment of collagen fibers up to 16 times more than no elongation and no BMDC seeding (non-elongated, non-seeded group). Furthermore, BMDCs were also aligned along the direction of elongation and showed 200% greater collagen type I gene expression in the elongated, seeded group than in Petri dish controls. More importantly, the production of collagen was also 24% greater. The results of this study demonstrate that alignment of a bioscaffold can result in positive changes in cellular response, making the bioscaffold more attractive for functional tissue engineering to potentially enhance healing of ligaments and tendons.

Creation of highly aligned electrospun poly-L-lactic acid fibers for nerve regeneration applications

Aligned, electrospun polymer fibers have shown considerable promise in directing regenerating axons in vitro and in vivo. However, in several studies, final electrospinning parameters are presented for producing aligned fiber scaffolds, and alignment where minimal fiber crossing occurs is not achieved. Highly aligned species are necessary for neural tissue engineering applications to ensure that axonal extension occurs through a regenerating environment efficiently. Axonal outgrowth on fibers that deviate from the natural axis of growth may delay axonal extension from one end of a scaffold to the other. Therefore, producing aligned fiber scaffolds with little fiber crossing is essential. In this study, the contributions of four electrospinning parameters (collection disk rotation speed, needle size, needle tip shape and syringe pump flow rate) were investigated thoroughly with the goal of finding parameters to obtain highly aligned electrospun fibers made from poly-L-lactic acid (PLLA). Using an 8 wt% PLLA solution in chloroform, a collection disk rotation speed of 1000 revolutions per minute (rpm), a 22 gauge, sharp-tip needle and a syringe pump rate of 2 ml h(-1) produced highly aligned fiber (1.2-1.6 microm in diameter) scaffolds verified using a fast Fourier transform and a fiber alignment quantification technique. Additionally, the application of an insulating sheath around the needle tip improved the rate of fiber deposition (electrospinning efficiency). Optimized scaffolds were then evaluated in vitro using embryonic stage nine (E9) chick dorsal root ganglia (DRGs) and rat Schwann cells (SCs). To demonstrate the importance of creating highly aligned scaffolds to direct neurite outgrowth, scaffolds were created that contained crossing fibers. Neurites on these scaffolds were directed down the axis of the aligned fibers, but neurites also grew along the crossed fibers. At times, these crossed fibers even stopped further axonal extension. Highly aligned PLLA fibers generated under optimized electrospinning conditions guided neurite and SC growth along the aligned fibers. Schwann cells demonstrated the bipolar phenotype seen along the fibers. Using a novel technique to determine fiber density, an increase in fiber density correlated to an increase in the number of neurites, but average neurite length was not statistically different between the two different fiber densities. Together, this work presents methods by which to produce highly aligned fiber scaffolds efficiently and techniques for assessing neurite outgrowth on different fiber scaffolds, while suggesting that crossing fibers may be detrimental in fostering efficient, directed axonal outgrowth.

Brain cortex regeneration affected by scaffold architectures

Object

The aim of this study was to compare designed scaffolds with a random-pored sponge scaffold to determine what role scaffold architecture plays in a cortical injury model.

Methods

Cylindrical scaffolds (3 × 3 mm) were made of a poly-(ε-caprolactone) polymer with 2 different molds from a 3D printer and had either: 1) unidirectional channels and microgrooves oriented longitudinally within the cylinder or 2) orthogonally intersecting channels and axial microgrooves within the cylinder. Additional randomized porosity was imparted using a salt-leaching method. A control scaffold without channels or microgrooves but containing random pores was also made. Scaffolds were implanted for 1, 4, and 8 weeks in a cylindrical defect created 3 mm posterior to the bregma in rat cortex. Control animals had tissue removed but received no implant. Brains were coronally cryosectioned and sections were stained. Antibodies for nestin, glial fibrillary acidic protein (GFAP), and TUJ1 were used to identify neural progenitors, activated astrocytes, and neuronal axons. Tissue ingrowth (H & E), astrocytic infiltration (GFAP), parenchymal inflammation (GFAP), and defect width (H & E) were quantified from images.

Results

Defect widths grew and parenchymal inflammation decreased over time with no statistical difference between groups. Total tissue ingrowth and astrocytic infiltration increased over time and was greatest in the orthogonal design group. Specific cell ingrowth, which was aligned with microgrooves interiorly in the orthogonal group and exteriorly in the longitudinal channel group, was qualitatively assessed from nestin and TUJ1 labeling.

Conclusions

Scaffold architecture can benefit brain tissue regeneration by integrating the following design principles: 1) large (100s of micrometers) pores or channels oriented toward the parenchyma for increased astrocytic infiltration; 2) microgrooves oriented in the desired direction of cellular migration and neuronal alignment; and 3) fully interconnecting channels for cellular migration and tissue integration.

Evaluation of neurite outgrowth anisotropy using a novel application of circular analysis

Precise axon growth is required for making proper connections in development and after injury. One method of studying axon guidance and growth is through in vitro outgrowth assays that present controlled microenvironments. In this study, we applied circular statistical methods to evaluate directional neurite response. Visualization of data on a circular scale allows more accurate representation of the data, as neurite angles are inherently expressed on a circle. Here, the direction of neurite outgrowth from dorsal root ganglia derived neurons on different substrate types was quantitatively measured. Further, simulations of datasets with known circular parameters reflecting expected neurite angle distributions from different substrate types were also generated. Circular statistical methods were utilized and compared to linear statistical models widely used in the neuroscience literature. For small samples, Rao's spacing test showed the smallest occurrence of Type I errors (false positives) when tested against simulated uniform distributions. V-test and Rayleigh's test showed highest statistical power when tested against a unimodal distribution with known and unknown mean direction, respectively. For bimodal samples, Watson's U(2)-test showed the highest statistical power. Overall, circular statistical uniformity tests showed higher statistical power than linear non-parametric tests, particularly for small samples (n=5). Circular analysis methods represent a useful tool for evaluation of directionality of neurite outgrowth with applications including: (1) assessment of neurite outgrowth potential; (2) determination of isotropy of cellular responses to single and multiple cues and (3) determination of the relative strengths of cues present in a complex environment.

Remodeling of engineered tissue anisotropy in response to altered loading conditions

Structural and mechanical anisotropy are critical to the function of many engineered tissues. This study examined the ability of anisotropic tissue constructs to overcome contact guidance cues and remodel in response to altered mechanical loading conditions. Square tissues engineered from dermal fibroblasts and type-I collagen were uniaxially loaded to induce cell and matrix alignment. After an initial time, t*, of 5-72 h, loading was switched from the x-axis to the y-axis. Cell alignment was examined throughout the experiment until a steady state was reached. Before t*, cells spontaneously aligned in the x-direction. After t*, the strength of alignment transiently decreased then increased, and mean cell orientation transitioned from the x- to the y-direction following an exponential time course with a time constant that increased with t*. Collagen fiber orientation exhibited similar trends that could not be explained by passive kinematics alone. Structural realignment resulted in concomitant changes in biaxial tissue mechanical properties. The findings suggest that even highly aligned engineered tissue constructs retain the capacity to remodel in response to altered mechanical stimuli. This may have important functional consequences when an anisotropic engineered tissue designed in vitro is surgically implanted into a mechanically complex graft site.