Morphogenetic response of cultured normal and transformed fibroblasts, and epitheliocytes, to a cylindrical substratum surface. Possible role for the actin filament bundle pattern

Emergent collective organization of bone cells in complex curvature fields

Individual cells and multicellular systems respond to cell-scale curvatures in their environments, guiding migration, orientation, and tissue formation. However, it remains largely unclear how cells collectively explore and pattern complex landscapes with curvature gradients across the Euclidean and non-Euclidean spectra. Here, we show that mathematically designed substrates with controlled curvature variations induce multicellular spatiotemporal organization of preosteoblasts. We quantify curvature-induced patterning and find that cells generally prefer regions with at least one negative principal curvature. However, we also show that the developing tissue can eventually cover unfavorably curved territories, can bridge large portions of the substrates, and is often characterized by collectively aligned stress fibers. We demonstrate that this is partly regulated by cellular contractility and extracellular matrix development, underscoring the mechanical nature of curvature guidance. Our findings offer a geometric perspective on cell-environment interactions that could be harnessed in tissue engineering and regenerative medicine applications.

Substrate curvature as a cue to guide spatiotemporal cell and tissue organization

Recent evidence clearly shows that cells respond to various physical cues in their environments, guiding many cellular processes and tissue morphogenesis, pathology, and repair. One aspect that is gaining significant traction is the role of local geometry as an extracellular cue. Elucidating how geometry affects cell and tissue behavior is, indeed, crucial to design artificial scaffolds and understand tissue growth and remodeling. Perhaps the most fundamental descriptor of local geometry is surface curvature, and a growing body of evidence confirms that surface curvature affects the spatiotemporal organization of cells and tissues. While well-defined in differential geometry, curvature remains somewhat ambiguously treated in biological studies. Here, we provide a more formal curvature framework, based on the notions of mean and Gaussian curvature, and summarize the available evidence on curvature guidance at the cell and tissue levels. We discuss the involved mechanisms, highlighting the interplay between tensile forces and substrate curvature that forms the foundation of curvature guidance. Moreover, we show that relatively simple computational models, based on some application of curvature flow, are able to capture experimental tissue growth remarkably well. Since curvature guidance principles could be leveraged for tissue regeneration, the implications for geometrical scaffold design are also discussed. Finally, perspectives on future research opportunities are provided.

Cell-Cell Adhesion and Cortical Actin Bending Govern Cell Elongation on Negatively Curved Substrates

Physiologically, cells experience and respond to a variety of mechanical stimuli such as rigidity and topography of the extracellular matrix. However, little is known about the effects of substrate curvature on cell behavior. We developed a novel, to our knowledge, method to fabricate cell culture substrates with semicylindrical grooves of negative curvatures (radius of curvature, R_c = 20-100 μm). We found that negative substrate curvatures induced elongation of mesenchymal and epithelial cells along the cylinder axis. As R_c decreases, mesenchymal National Institutes of Health 3T3 fibroblasts increasingly elongate along the long axis of the grooves, whereas elongation of epithelial Madin-Darby Canine Kidney (MDCK) cells is biphasic with maximal cell elongation when R_c = 40 μm. Addition of blebbistatin to MDCK cells to reduce cortical actin rigidity resulted in a decrease in cell elongation across all curvatures while preserving the biphasic trend. However, addition of calyculin A or ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid, to increase cortical rigidity or reduce intercellular adhesion, respectively, resulted in a monotonic increase in MDCK cell elongation with decreasing R_c. Using an energy minimization model, we showed that cell elongation in epithelial cell sheet is governed by the competition between two energies as R_c decreases: curvature-dependent intercellular adhesion that prevents elongation; and intracellular cortical actin bending that enhances elongation. Therefore, our results of cellular elongation induced by negatively curved substrates offer insights into how tubule elongation or growth of tubular structures such as kidney tubules can be controlled by the substrate curvature in vivo.

Gaussian Curvature Directs Stress Fiber Orientation and Cell Migration

We show that substrates with nonzero Gaussian curvature influence the organization of stress fibers and direct the migration of cells. To study the role of Gaussian curvature, we developed a sphere-with-skirt surface in which a positive Gaussian curvature spherical cap is seamlessly surrounded by a negative Gaussian curvature draping skirt, both with principal radii similar to cell-length scales. We find significant reconfiguration of two subpopulations of stress fibers when fibroblasts are exposed to these curvatures. Apical stress fibers in cells on skirts align in the radial direction and avoid bending by forming chords across the concave gap, whereas basal stress fibers bend along the convex direction. Cell migration is also strongly influenced by the Gaussian curvature. Real-time imaging shows that cells migrating on skirts repolarize to establish a leading edge in the azimuthal direction. Thereafter, they migrate in that direction. This behavior is notably different from migration on planar surfaces, in which cells typically migrate in the same direction as the apical stress fiber orientation. Thus, this platform reveals that nonzero Gaussian curvature not only affects the positioning of cells and alignment of stress fiber subpopulations but also directs migration in a manner fundamentally distinct from that of migration on planar surfaces.

Overlooked? Underestimated? Effects of Substrate Curvature on Cell Behavior

In biological systems, form and function are inherently correlated. Despite this strong interdependence, the biological effect of curvature has been largely overlooked or underestimated, and consequently it has rarely been considered in the design of new cell-material interfaces. This review summarizes current understanding of the interplay between the curvature of a cell substrate and the related morphological and functional cellular response. In this context, we also discuss what is currently known about how, in the process of such a response, cells recognize curvature and accordingly reshape their membrane. Beyond this, we highlight state-of-the-art microtechnologies for engineering curved biomaterials at cell-scale, and describe aspects that impair or improve readouts of the pure effect of curvature on cells.

Role of the Nucleus as a Sensor of Cell Environment Topography

The proper integration of biophysical cues from the cell vicinity is crucial for cells to maintain homeostasis, cooperate with other cells within the tissues, and properly fulfill their biological function. It is therefore crucial to fully understand how cells integrate these extracellular signals for tissue engineering and regenerative medicine. Topography has emerged as a prominent component of the cellular microenvironment that has pleiotropic effects on cell behavior. This progress report focuses on the recent advances in the understanding of the topography sensing mechanism with a special emphasis on the role of the nucleus. Here, recent techniques developed for monitoring the nuclear mechanics are reviewed and the impact of various topographies and their consequences on nuclear organization, gene regulation, and stem cell fate is summarized. The role of the cell nucleus as a sensor of cell-scale topography is further discussed.

Curvature and Rho activation differentially control the alignment of cells and stress fibers

In vivo, cells respond to a host of physical cues ranging from substrate stiffness to the organization of micro- and nanoscale fibrous networks. We show that macroscale substrates with radii of curvature from tens to hundreds of micrometers influence cell alignment. In a model system of fibroblasts, isolated cells aligned strongly in the axial direction on cylinders with radii similar to the cell length and more weakly on cylinders of much larger radius. Isolated vascular smooth muscle cells did not align as effectively as fibroblasts. However, both cell types aligned robustly in weak curvature fields when in confluent monolayers. We identified two distinct populations of stress fibers in both cell types: long, apical stress fibers that aligned axially and short, basal stress fibers that aligned circumferentially. Circumferential alignment of the basal stress fibers is in apparent disagreement with a long-standing hypothesis that energetic penalties for bending enforce axial alignment on cylinders. To explore this phenomenon, we manipulated stress fibers by activating Rho, a small guanosine triphosphatase that regulates stress fiber assembly. In response, apical stress fibers disassembled, whereas basal stress fibers thickened and aligned more strongly in the circumferential direction. By activating Rho in confluent monolayers of vascular smooth muscle cells, we recapitulated the circumferential alignment pattern of F-actin within these cells that is observed in cylindrical vessels in vivo. In agreement with recent theory, these results suggest that stress fiber bending penalties are overcome when stress fiber contractility is enhanced and motivate deeper study of the mechanics of these distinct stress fiber populations.

Fiber-assisted molding (FAM) of surfaces with tunable curvature to guide cell alignment and complex tissue architecture

A simple and robust method termed "fiber-assisted molding (FAM)" is presented to create biomimetic three-dimensional surfaces with controllable curvature and helical twist. The alignment of muscle fibrils and the assembly of helically patterned extracellular matrix by cells demonstrate the potential of this method for tissue engineering and other materials science applications.

Static axial stretching enhances the mechanical properties and cellular responses of fibrin microthreads

Fibrin microthreads are a platform technology that can be used for a variety of applications, and therefore the mechanical requirements of these microthreads differ for each tissue or device application. To develop biopolymer microthreads with tunable mechanical properties, we analyzed fibrin microthread processing conditions to strengthen the scaffold materials without the use of exogenous crosslinking agents. Fibrin microthreads were extruded, dried, rehydrated and static axially stretched 0-200% of their original lengths; then the mechanical and structural properties of the microthreads were assessed. Stretching significantly increased the tensile strength of microthreads 3-fold, yielding scaffolds with tensile strengths and stiffnesses that equaled or exceeded values reported previously for carbodiimide crosslinked threads without affecting intrinsic material properties such as strain hardening or Poisson's ratio. Interestingly, these stretching conditions did not affect the rate of proteolytic degradation of the threads. The swelling ratios of stretched microthreads decreased, and scanning electron micrographs showed increases in grooved topography with increased stretch, suggesting that stretching may increase the fibrillar alignment of fibrin fibrils. The average cell alignment with respect to the longitudinal axis of the microthreads increased 2-fold with increased stretch, further supporting the hypothesis that stretching microthreads increases the alignment of fibrin fibrils on the surfaces of the scaffolds. Together, these data suggest that stretching fibrin microthreads generates stronger materials without affecting their proteolytic stability, making stretched microthreads ideal for implantable scaffolds that require short degradation times and large initial loading properties. Further modifications to stretched microthreads, such as carbodiimide crosslinking, could generate microthreads to direct cell orientation and align tissue deposition, with additional resistance to degradation for use as a long-term scaffold for tissue regeneration.

Brain microvascular endothelial cells resist elongation due to curvature and shear stress

The highly specialized endothelial cells in brain capillaries are a key component of the blood-brain barrier, forming a network of tight junctions that almost completely block paracellular transport. In contrast to vascular endothelial cells in other organs, we show that brain microvascular endothelial cells resist elongation in response to curvature and shear stress. Since the tight junction network is defined by endothelial cell morphology, these results suggest that there may be an evolutionary advantage to resisting elongation by minimizing the total length of cell-cell junctions per unit length of vessel.

Salivary gland cell differentiation and organization on micropatterned PLGA nanofiber craters

There is a need for an artificial salivary gland as a long-term remedy for patients suffering from salivary hypofunction, a leading cause of chronic xerostomia (dry mouth). Current salivary gland tissue engineering approaches are limited in that they either lack sufficient physical cues and surface area needed to facilitate epithelial cell differentiation, or they fail to provide a mechanism for assembling an interconnected branched network of cells. We have developed highly-ordered arrays of curved hemispherical "craters" in polydimethylsiloxane (PDMS) using wafer-level integrated circuit (IC) fabrication processes, and lined them with electrospun poly-lactic-co-glycolic acid (PLGA) nanofibers, designed to mimic the three-dimensional (3-D) in vivo architecture of the basement membrane surrounding spherical acini of salivary gland epithelial cells. These micropatterned scaffolds provide a method for engineering increased surface area and were additionally investigated for their ability to promote cell polarization. Two immortalized salivary gland cell lines (SIMS, ductal and Par-C10, acinar) were cultured on fibrous crater arrays of various radii and compared with those grown on flat PLGA nanofiber substrates, and in 3-D Matrigel. It was found that by increasing crater curvature, the average height of the cell monolayer of SIMS cells and to a lesser extent, Par-C10 cells, increased to a maximum similar to that seen in cells grown in 3-D Matrigel. Increasing curvature resulted in higher expression levels of tight junction protein occludin in both cell lines, but did not induce a change in expression of adherens junction protein E-cadherin. Additionally, increasing curvature promoted polarity of both cell lines, as a greater apical localization of occludin was seen in cells on substrates of higher curvature. Lastly, substrate curvature increased expression of the water channel protein aquaporin-5 (Aqp-5) in Par-C10 cells, suggesting that curved nanofiber substrates are more suitable for promoting differentiation of salivary gland cells.

Passive control of cell locomotion using micropatterns: the effect of micropattern geometry on the migratory behavior of adherent cells

Directed cell migration is critical to a variety of biological and physiological processes. Although simple topographical patterns such as parallel grooves and three-dimensional post arrays have been studied to guide cell migration, the effect of the dimensions and shape of micropatterns, which respectively represent the amount and gradient of physical spatial cues, on cell migration has not yet been fully explored. This motivates a quantitative characterization of cell migration in response to micropatterns having different widths and divergence angles. The changes in the migratory (and even locational) behavior of adherent cells, when the cells are exposed to physical spatial cues imposed by the micropatterns, are explored here using a microfabricated biological platform, nicknamed the "Rome platform". The Rome platform, made of a biocompatible, ultraviolet (UV) curable polymer (ORMOCOMP), consists of 3 μm thick micropatterns with different widths of 3 to 75 μm and different divergence angles of 0.5 to 5.0°. The migration paths through which NIH 3T3 fibroblasts move on the micropatterns are analyzed with a persistent random walk model, thus quantifying the effect of the divergence angle of micropatterns on cell migratory characteristics such as cell migration speed, directional persistence time, and random motility coefficient. The effect of the width of micropatterns on cell migratory characteristics is also extensively investigated. Cell migration direction is manipulated by creating the gradient of physical spatial cues (that is, divergence angle of micropatterns), while cell migration speed is controlled by modulating the amount of them (namely, width of micropatterns). In short, the amount and gradient of physical spatial cues imposed by changing the width and divergence angle of micropatterns make it possible to control the rate and direction of cell migration in a passive way. These results offer a potential for reducing the healing time of open wounds with a smart wound dressing engraved with micropatterns (or microscaffolds).

Restoration of skeletal muscle defects with adult human cells delivered on fibrin microthreads

Large-scale musculoskeletal wounds, such as those seen in trauma injuries, present poor functional healing prognoses. In severe trauma, when the native tissue architecture is destroyed or lost, the regenerative capacity of skeletal muscle is diminished by scar formation. Here we demonstrate that a scaffold system composed of fibrin microthreads can provide an efficient delivery system for cell-based therapies and improve regeneration of a large defect in the tibialis anterior of the mouse. Cell-loaded fibrin microthread bundles implanted into a skeletal muscle resection reduced the overall fibroplasia-associated deposition of collagen in the wound bed and promoted in-growth of new muscle tissue. When fibrin microthreads were seeded with adult human cells, implanted cells contributed to the nascent host tissue architecture by forming skeletal muscle fibers, connective tissue, and PAX7-positive cells. Stable engraftment was observed at 10 weeks postimplant and was accompanied by reduced levels of collagen deposition. Taken together, these data support the design and development of a platform for microthread-based delivery of autologous cells that, when coupled to an in vitro cellular reprogramming process, has the potential to improve healing outcomes in large skeletal muscle wounds.

Fibrin microthreads support mesenchymal stem cell growth while maintaining differentiation potential

We developed a method to produce discrete fibrin microthreads, which can be seeded with human mesenchymal stem cells (hMSCs) and used as a suture to enhance the efficiency and localization of cell delivery. To assess the efficacy of fibrin microthreads to support hMSC attachment, proliferation, and survival, microthreads (100 μm diameter per microthread) were bundled together, seeded with 50,000 hMSCs for 2 h, and cultured for 5 days. Cell density on microthread bundles increased over time in culture to a maximum average density of 731 ± 101 cells/mm(2) after 5 days. A LIVE/DEAD assay confirmed that the cells were viable, and Ki-67 staining verified hMSC proliferation. In addition, functional differentiation assays demonstrated that hMSCs cultured on microthreads retained their ability to differentiate into adipocytes and osteocytes. The results of this study demonstrate that fibrin microthreads support hMSC viability and proliferation, while maintaining their multipotency. We anticipate that these cell-seeded fibrin microthreads will serve as a platform technology to improve localized delivery and engraftment of viable cells to damaged tissue.

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.

Enhanced proliferation and migration of fibroblasts on the surface of fibroblast growth factor-2-loaded fibrin microthreads

Fibrin microthreads are discrete biopolymer fibers, 50-100 μm in diameter, produced from the natural extracellular matrix protein of the provisional matrix that promotes tissue regeneration in the in vivo wound healing environment. The goals of this study were to investigate the feasibility of creating fibrin microthreads containing fibroblast growth factor-2 (FGF-2), and to study the potential of a fibrin matrix to bind signaling proteins known to promote wound healing and regulate cell function in localized cellular microenvironments on scaffold surfaces. FGF-2 was loaded into fibrin microthreads in concentrations ranging from 0 to 200 ng/mL, to investigate the effect of the material on fibroblast attachment, proliferation, cellular outgrowth, and alignment. Although FGF-2-loaded microthreads did not affect fibroblast attachment, they significantly increased cellular outgrowth and proliferation relative to unloaded microthreads. The most pronounced effects were observed at day 7 of cell culture. Further, all of the fibrin microthreads promoted the alignment of fibroblasts and their cytoskeletal components along the long axis of threads, independent of the FGF-2 concentration. Ultimately, we anticipate that these fibrin microthreads will be a promising biopolymer material to promote the regeneration of injured tissues because of their mechanical stability and their matrix signaling capabilities, particularly when loaded with matrix-bound growth factors such as FGF-2.

A Novel Approach for Automated Analysis of Cell Attachment and Spreading Based on Backscattered Electron Imaging by Scanning Electron Microscopy

The development of new materials for biological application requires in vitro testing of cell/surface interactions. Cell adhesion and spreading are difficult to quantify as most materials are non-transparent and transmission microscopy cannot be used. Contrast in reflection microscopy is rather poor. We propose an alternative method for the automated screening of cell attachment and spreading using backscattered electron imaging of scanning electron microscopy. The enhanced cell contrast permits study of cell/material interactions by little differences between cells and material.

Self-standing aligned fiber scaffold fabrication by two photon photopolymerization

Development of materials and fabrication techniques lead the growth of three-dimensional cell culture matrices in biomedical engineering. In this work, we present a method for fabricating self-standing fiber scaffolds by two-photon polymerization induced by a femtosecond laser. The aligned fibers are 330 microm long with a diameter of 6-9 microm. Depending on the pitch of the aligned fibers, various cell morphologies are distinguished via three-dimensional images. Furthermore, the morphologies of fibroblast cells (NIH-3T3) and epithelial cells (MDCK) on the fiber scaffolds are studied to show the effect of high curvature (3-4.5 microm radii) on cell morphology. NIH-3T3 cells that contain straight pattern of actin microfilament bundles are extended and partly wrap single fibers or tend to reside between fibers. On the other hand, MDCK cells that contain circular pattern of actin microfilament bundles cover the fiber peripheral surface exhibiting high aspect ratio elongation. These results indicate that cell morphology on fiber scaffolds is influenced by the pattern of actin microfilament bundles.

Response of mesenchymal stem cells to the biomechanical environment of the endothelium on a flexible tubular silicone substrate

Understanding the response of mesenchymal stem cells (MSCs) to forces in the vasculature is very important in the field of cardiovascular intervention for a number of reasons. These include the development of MSC seeded tissue engineered vascular grafts, targeted or systemic delivery of MSCs in the dynamic environment of the coronary artery and understanding the potential pathological calcifying role of mechanically conditioned multipotent cells already present in the vessel wall. In vivo, cells present in the coronary artery are exposed to the primary biomechanical forces of shear stress, radial stress and hoop stress. To date, many studies have examined the effect of these stresses in isolation, thereby not presenting the complete picture. Therefore, the main aim of this study is to examine the combined role of these stresses on MSC behaviour. To this end, a bioreactor was configured to expose MSCs seeded on flexible silicone substrates to physiological forces - namely, a pulsatile pressure between 40 and 120mmHg (5.33-1.6x10(4)Pa), radial distention of 5% and a shear stress of 10dyn/cm(2) (1Pa) at frequency of 1Hz for up to 24h. Thereafter, the 'pseudovessel' was assessed for changes in morphology, orientation and expression of endothelial and smooth muscle cell (SMC) specific markers. Hematoxylin and eosin (H&E) staining revealed that MSCs exhibit a similar mechanosensitive response to that of endothelial cells (ECs); they reorientate parallel with direction of flow and have adapted their morphology to be similar to that of ECs. However, gene expression results show the cells exhibit greater levels of SMC-associated markers alpha-smooth muscle actin and calponin (p<0.05).

Discrete crosslinked fibrin microthread scaffolds for tissue regeneration

In this study, we report on the development of discrete fibrin microthreads as well as novel scaffolds composed of arrays of fibrin threads. These scaffolds exhibit mechanical properties that are significantly greater than fibrin gels and cellular responses suggesting that the materials are conducive to the development of organized, aligned tissues. Fibrin microthreads were produced by coextruding solutions of 70 mg/mL fibrinogen and 6 U/mL thrombin through small diameter polyethylene tubing. Uncrosslinked fibrin microthreads averaged 55-65 microm in hydrated diameter and achieved ultimate tensile strengths approaching 4.5 MPa. The strengths and stiffnesses of the microthreads were approximately twofold greater when the materials were treated with exposure to ultraviolet (UV) light. Although UV crosslinking attenuated fibroblast proliferation, uncrosslinked fibrin microthreads supported fibroblast attachment, proliferation, and alignment, suggesting that they represent a viable biomaterial for the aligned regeneration of tissues. Because of the physiologic roles of fibrin matrices in the early phase of wound healing, we anticipate that these fibrin-based microthreads will direct the spatially and temporally complex processes of cell-mediated tissue ingrowth and regeneration.

Probing cell shape regulation with patterned substratum: requirement of myosin II-mediated contractility

Adherent cells cultured on flat, homogeneous surfaces typically maintain an intact cell body with a polygonal or fan shape, despite active migration and strong mechanical interactions with the substratum. We hypothesized that, in addition to the constraint of the surface membrane, an active mechanism may be involved in maintaining the shape and integrity of the cell body particularly where cells encounter complex topographic patterns of guidance cues. To detect if there is a mechanism that constrains cell shape, we plated NIH 3T3 fibroblasts on ring-patterned substrata 8-17.5 microns in width and 53-133 microns in outer diameter. Untreated cells have a limited angular span, encompassing an average of 108 degrees around the ring, even though these cells were able to cover a much larger surface when plated on flat surfaces of the same material. Treatment of 3T3 cells with a myosin II inhibitor, blebbistatin, induced a striking increase in the bending ability, causing cells to cover more than 60% of the ring. Inhibition of the Rho-dependent kinase with Y-27632 caused a similar but smaller increase in the angular span. Our results suggest that cell shape is controlled not only by the passive constraint of the surface membrane but also by an active mechanism driven by myosin II-mediated contractility under the regulation of Rho-dependent kinase. The inward surface tension-like forces allow the cell to maintain its integrity while navigating through complex physiological environments.

Utilizing acid pretreatment and electrospinning to improve biocompatibility of poly(glycolic acid) for tissue engineering

Poly(glycolic acid) (PGA) has a long history as a bioresorbable polymer. Its biocompatibility is widely accepted, yet PGA is often rejected as a soft-tissue scaffold because of fibrous encapsulation. The goal of this study was to improve the soft-tissue biocompatibility of PGA by producing scaffolds composed of small-diameter fibers through electrospinning and subjecting these scaffolds to a concentrated hydrochloric acid (HCL) pretreatment. The theory is that small-diameter fibers will elicit a reduced immune response and HCl treatment will improve cellular interactions. Scaffolds were characterized in terms of fiber diameter and pore area via image-analysis software. Biocompatibility was assessed through a WST-1 cell-proliferation assay (in vitro) with the use of rat cardiac fibroblasts and rat intramuscular implantations (in vivo). Fibers produced ranged in diameter from 0.22 to 0.88 microm with pore areas from 1.84 to 13.22 microm(2). The untreated scaffold composed of 0.88-microm fibers was encapsulated in vivo and supported the lowest rates of cell proliferation. On the contrary, the acid pretreated scaffold with 0.22-microm fibers was incorporated into the surrounding tissue and exhibited proliferation rates that exceeded the control populations on tissue-culture plastic. In conclusion, this study has shown the ability to improve the biocompatibility of PGA through acid pretreatment of scaffolds comprised of submicron fiber diameters.

Production of tissue-engineered three-dimensional human bronchial models

We have reported morphological and functional features of cells isolated from human bronchial biopsies. Both epithelial and fibroblastic cells were isolated from the same biopsies using collagenase. A few models have been established to study normal bronchial response to various agents and to understand the mechanisms responsible for some disorders, such as asthma. We produced three-dimensional bronchial equivalents in culture, using human epithelial and fibroblastic cells. We previously showed that peripheral anchorage can prevent the dramatic collagen contraction in gels seeded with fibroblasts when properly adapted to the size and type of cultured tissues. Our bilayered bronchial constructs were anchored and cultured under submerged conditions and at the air-liquid interface. Three culture media were compared. Serum-free medium supplemented with retinoic acid (5 x 10(-8) M) was found to be the best for maintenance of bronchial cell properties in the reconstructed bronchial tissue. Immunohistological and ultrastructural analyses showed that these equivalents present good structural organization, allowing ciliogenesis to occur in culture. Moreover, human bronchial goblet cells could differentiate and secrete mucus with culture time. Laminin, a major constituent of the basement membrane and basal cells, was also detected at the mesenchymoepithelial interface. Such models will be useful for studying human bronchial properties in vitro.

Nanoscale features influence epithelial cell morphology and cytokine production

Available, easy and fast fabrication methods of nanostructured surfaces, and the knowledge that cells in vivo interacts with nanometer-sized structures/objects, led us to study the impact of nanotopography on cell morphology and cytokine production. Uroepithelial cells were seeded on three different substrate types: two with defined nanometer topographies and a flat control, all three having identical surface chemistry. The nanostructured substrates contained hemispherical pillars or step edges, the latter in the form of parallel grooves and ridges. Qualitative and quantitative analysis of cell morphology and cytokine production were studied. Both quantities were significantly different between cells cultured on hemispherically structured surfaces compared to flat control surfaces. Cells cultured on hemispherically structured surfaces showed a decrease in IL-6 and IL-8 production and were less spread, less round and more stellate (larger dispersion). Only cell morphology differed between cells cultured on grooved surfaces and flat control surfaces. These findings suggest that epithelial cell morphology and cytokine production are dependent on the underlying nanotopography.

Influence of systematically varied nanoscale topography on the morphology of epithelial cells

With the knowledge that cells can react to lithographically manufactured nanometer-sized surface objects, our interest concerned whether cells would respond to surface structures of systematically increasing size. Our approach to answer this question was to fabricate surfaces with the same surface chemistry and similar surface roughness but increasing size of structural features. To fabricate large areas of patterned surfaces, required for cell culture studies, we used colloidal lithography utilizing colloidal particles as a template for surface nanostructuring. The fabricated surfaces contained hemispherical nanopillars with diameters ranging from 60 to 170 nm. Changes in cell morphology of a pancreatic epithelial cell line (AR4-2J) were studied by evaluating cell area and cell shape. The latter was studied by applying the cell shape classification method using three shape descriptors. The pancreatic cells responded in a systematic way to the surface nanostructures. The cells spread more and became more nonround when cultured on surfaces with increasing size of the topographic features. Index Terms-Biological cells, image analysis, nanotechnology, shape measurement, surfaces.

Adhesion, alignment, and migration of cultured Schwann cells on ultrathin fibronectin fibres

Individual fibres of fibronectin (Fn-fibres), an extracellular matrix cell adhesion glycoprotein, were produced from a purified solution of fibronectin. These fibres range from 0.5-7 microm in width and have been engineered to produce mats (Fn-mats) by using a unidirectional shear force to orientate the fibres. Fn-fibres have been shown to promote alignment by contact guidance of human dermal fibroblasts, neurites, macrophages, and epitenon fibroblasts. Fn-mats have been used to orientate and enhance the regeneration of peripheral nerve components. We investigated cell spreading, orientation, formation of focal contacts, and the speed of cell movement on individual Fn-fibres, glass-covered with poly-L-lysine and poly-L-lysine/laminin/Fn. Fibronectin fibres significantly promoted cell spreading and the speed of cell migration with alignment of focal contacts and F-actin filaments to the axis of the fibres. The study reveals the potential of Fn-fibres to guide and direct cellular behaviour by contact guidance. The increase in migration and other behaviour exhibited by Schwann cells on Fn-fibres justifies the use of Fn-mats for peripheral nerve repair and is clinically important in that atrophy of the target organ, which is the most common failure of nerve repair, may be minimised.

Locomotory behaviour of epitheliocytes and fibroblasts on metallic grids

Behaviour of epitheliocytes and fibroblasts on special discontinuous substrata (metallic grids with square openings of 45x45 microm2) was examined in order to compare the ability of these cells to spread in two mutually perpendicular directions and to stretch over the void spaces. Two cell types with typical fibroblastic morphology, the AGO 1523 line of human foreskin fibroblasts and secondary cultures of mouse embryo fibroblasts, and three cell types with typical epithelial morphology, primary mouse hepatocytes, the IAR-2 line of rat liver cells and the MDCK line of canine kidney epithelial cells (clone 20) were used. We also examined the epitheliocytes (MDCK cells, clone 20) transformed to fibroblast-like morphology by treatment with hepatocyte growth factor/scatter factor (HGF/SF). Time-lapse video microscopy, scanning electron microscopy and immunofluorescence microscopy were used to examine cell reorganizations at various stages of spreading. It was found that early stages of spreading of fibroblasts and epitheliocytes were similar: the cell spread along two bars, perpendicular to each other (bar and crossbar), with the formation of a small triangular lamellar cytoplasm stretched over the opening. Later central parts of the bodies of the fibroblasts retracted from the bars so that the cells remained attached only by their polar lamellae. Successive expansions and partial retractions of these lamellae led to elongation of the cell body crossing several openings of the grid. Epitheliocytes, in contrast to fibroblasts, at the late stages of spreading did not retract their bodies and did not contract polar lamellae. As a result, their central lamellae stretched progressively over the openings. As a result of the treatment of MDCK epitheliocytes with HGF/SF the behaviour of the cells on the grids became similar to that of fibroblasts. It is suggested that these distinct spreading patterns of epitheliocytes and fibroblasts are due to the type-specific differences in the actin-myosin cortex. Experiments with microtubule-specific drugs, colcemid and taxol, indicate that the organization of this cortex is under microtubular control.

Topographical control of cells

We review the literature on the reaction of cells to their surrounding topography. The topography may be that of surrounding cells, intercellular materials or biomaterials. The reactions include cell orientation, rates of movement, and activations of the cells. We concentrate on those papers where quantitative measurements of the reactions have been made and largely ignore those on subjective impressions. A wide range of topographies are considered but special attention is given to results on groove-ridge topographies. The question of whether the cells are reacting to the topography directly or to patterned substratum chemistry formed on the topography is discussed. The review ends with a summary of the types of prosthesis where advantage has been taken of the ability to fabricate topography.

Adhesion, orientation, and movement of cells cultured on ultrathin fibronectin fibers

This study examined the behavior of rat tendon fibroblasts, baby hamster kidney fibroblasts, macrophage-like P388D1 cells, and neurons from rat dorsal root ganglia, cultured on fibronectin strands 0.2-5 micrograms in diameter. We investigated cell spreading, orientation, formation of focal contacts, the speed of cell movement, and the speed of neurite outgrowth in cells cultured on fibronectin strands, glass covered with fibronectin, and plain, nontreated glass. Fibronectin strands significantly promoted cell spreading and caused a marked alignment of all kinds of cells to the direction of the fiber. The fibers caused the alignment of actin filaments in fibroblasts and focal contacts in fibroblasts and macrophages and increased polymerization of F-actin in cells. Fibronectin fibers also increased the speed and persistence of cell movement and the rate of neurite outgrowth. Macrophages grown on fibronectin fibers produced numerous actin-rich microspikes and adopted a polarized, migratory phenotype. These findings indicate that fibronectin strands, resembling natural components of the extracellular matrix, are more effective in activating various types of cells then two-dimensional, fibronectin-covered substrata. The results also confirm the suitability of the three-dimensionally oriented fibronectin form for use in clinical practice.

After embryonic day 17, distribution of cells on surface of primary muscle fibres in mouse is non-random

During the formation of skeletal muscle, secondary fibres form, by cell fusion, on the surface of primary fibres. Three-dimensional reconstructions of primary fibres with the secondary fibres and cells on their surfaces were produced from spaced serial transmission electron micrographs. Reconstructions were made of fibres from embryonic day (E) 17, E19, and E21 of the Extensor Digitorum muscle of Balb/c mice. Cell distribution was analysed in two ways. Firstly, nearest neighbour analysis was used to see if cells were randomly arranged or clustered. Secondly, the association of cells and secondary fibres was tested by measuring the distances between cells and each secondary fibre. Cells were found to be randomly distributed on the surface of primaries at E17 but significantly clustered, and associated with smaller secondary fibres at E19 and E21. Cells were not associated with the ends of secondary fibres. Cells associated with secondary fibres lay in the groove formed by contact between adjacent primary and secondary fibres. This apparent response to the topography of the surface on which the cells are lying has previously been mimicked in vitro by growing cells on grooved surfaces. It is likely that cells associated with secondaries will fuse laterally with these fibres and their response to topography is part of the process of bringing them into correct alignment with the fibre before fusion.

After embryonic day 17, distribution of cells on surface of primary muscle fibres in mouse is non-random

During the formation of skeletal muscle, secondary fibres form, by cell fusion, on the surface of primary fibres. Three-dimensional reconstructions of primary fibres with the secondary fibres and cells on their surfaces were produced from spaced serial transmission electron micrographs. Reconstructions were made of fibres from embryonic day (E) 17, E19, and E21 of the Extensor Digitorum muscle of Balb/c mice. Cell distribution was analysed in two ways. Firstly, nearest neighbour analysis was used to see if cells were randomly arranged or clustered. Secondly, the association of cells and secondary fibres was tested by measuring the distances between cells and each secondary fibre. Cells were found to be randomly distributed on the surface of primaries at E17 but significantly clustered, and associated with smaller secondary fibres at E19 and E21. Cells were not associated with the ends of secondary fibres. Cells associated with secondary fibres lay in the groove formed by contact between adjacent primary and secondary fibres. This apparent response to the topography of the surface on which the cells are lying has previously been mimicked in vitro by growing cells on grooved surfaces. It is likely that cells associated with secondaries will fuse laterally with these fibres and their response to topography is part of the process of bringing them into correct alignment with the fibre before fusion.

Development of fibroblast-seeded ligament analogs for ACL reconstruction

We fabricated "ligament analogs" in vitro by seeding high-strength resorbable collagen fiber scaffolds with intraarticular (anterior cruciate ligament, ACL) or extraarticular (patellar tendon, PT) rabbit fibroblasts. Fibroblasts attached, proliferated, and secreted new collagen on the ligament analogs in vitro. Fibroblast function depended on the tissue culture substrate (ligament analog vs. tissue culture plate) and the origin of the fibroblasts (ACL vs. PT) PT fibroblasts proliferated more rapidly than ACL fibroblasts when cultured on ligament analogs. Collagen synthesis by ACL and PT fibroblasts was approximately tenfold greater on ligament analogs than on tissue culture plates. The composition, structure, and geometry of the collagen fiber scaffolds may promote collagen synthesis within ligament analogs in vitro. Ligament analogs roughly approximate the structure and strength of native ligament tissue. Ongoing in vivo studies suggest that autogenous fibroblast-seeded ligament analogs remain viable after implantation into the knee joint. With further development, ligament analogs may be useful as implants for ACL reconstruction surgery.

Transverse pattern of microfilament bundles induced in epitheliocytes by cylindrical substrata

Cylindrical culture substrata are known to induced longitudinal orientation of polarized fibroblasts and corresponding alignment of actin microfilament bundles in these cells. We studied microfilament bundle distribution in two cell types, fibroblasts and epitheliocytes, spread on two kinds of anisotropic substrata, quartz glass cylinders with a diameter 32 microns and narrow (25-40 microns wide) flat glass adhesive strips with non-adhesive borders. Rat embryo and human diploid fibroblasts, as expected, formed predominantly longitudinally aligned bundles on both substrata. In contrast, transverse bundles on cylinders and randomly oriented bundles on flat strips were formed in IAR-2 and MDCK epithelial cells. We interpret these data as showing that the epitheliocyte attempts to override the guiding influence of anisotropic substrata. The microfilament bundle pattern on cylinders depends on the integrity of the microtubules. Colcemid-induced microtubule depolymerization caused formation of longitudinal as well as transverse bundles both in fibroblasts and epitheliocytes, thus diminishing the differences in microfilament bundle patterns in two cell types. These results show that microtubules control the cell-type-specific distribution of microfilament bundles both in polarized fibroblasts and in discoid epitheliocytes. However, the results of this control are opposite: microtubules enhance cell polarization in fibroblasts, but prevent it in epithelial cells.

Role of the cytoskeleton in the reaction of fibroblasts to multiple grooved substrata

The role of the cytoskeleton and cell attachments in the alignment of baby hamster kidney fibroblasts to ridge and groove substratum topography was investigated using confocal scanning microscopy. This was carried out with normal cells and cells treated with the cytoskeleton modifiers cytochalasin D, colcemid, and taxol. Actin was localised with fluorescent phalloidin. Tubulin, vinculin, and intracellular adhesion molecule-1 were visualised by indirect immunofluorescence. The spreading, elongation, and orientation of the cells after 24 h of culture in these conditions were measured on grooves of 5, 10, and 25 microns width and 0.5, 1, 2, and 5 microns depth. We have also observed events over the first 30 min of cell attachment. Five minutes after cell attachment, F-actin condensations were seen close to the intersection of groove wall and ridge top, that is, at a topographic discontinuity. The condensations were often at right angles to the groove edge and showed a periodicity of 0.6 microns. Vinculin arrangement at the early stages of cell spreading was similar to that of actin. Organisation of the microtubule system followed later, becoming obvious at about 30 min after cell plating. The Curtis and Clark theory (that cells react to topography primarily at lines of discontinuity in the substratum by actin nucleation) is supported by these results. The use of cytoskeletal poisons did not entirely abolish cell reaction to grooves. Colcemid increased cell spreading and reduced cell orientation and elongation. Cytochalasin D reduced cell spreading, orientation, and elongation. Taxol reduced cell elongation but did not affect cell spreading and orientation. We conclude that the aggregation of actin along groove/ridge boundaries is a primary driving event in determining fibroblast orientation on microgrooved substrata.