Transverse pattern of microfilament bundles induced in epitheliocytes by cylindrical substrata

Plasma Membrane Blebbing Is Controlled by Subcellular Distribution of Vimentin Intermediate Filaments

The formation of specific cellular protrusions, plasma membrane blebs, underlies the amoeboid mode of cell motility, which is characteristic for free-living amoebae and leukocytes, and can also be adopted by stem and tumor cells to bypass unfavorable migration conditions and thus facilitate their long-distance migration. Not all cells are equally prone to bleb formation. We have previously shown that membrane blebbing can be experimentally induced in a subset of HT1080 fibrosarcoma cells, whereas other cells in the same culture under the same conditions retain non-blebbing mesenchymal morphology. Here we show that this heterogeneity is associated with the distribution of vimentin intermediate filaments (VIFs). Using different approaches to alter the VIF organization, we show that blebbing activity is biased toward cell edges lacking abundant VIFs, whereas the VIF-rich regions of the cell periphery exhibit low blebbing activity. This pattern is observed both in interphase fibroblasts, with and without experimentally induced blebbing, and during mitosis-associated blebbing. Moreover, the downregulation of vimentin expression or displacement of VIFs away from the cell periphery promotes blebbing even in cells resistant to bleb-inducing treatments. Thus, we reveal a new important function of VIFs in cell physiology that involves the regulation of non-apoptotic blebbing essential for amoeboid cell migration and mitosis.

Experimental and theoretical model for the origin of coiling of cellular protrusions around fibers

Protrusions at the leading-edge of a cell play an important role in sensing the extracellular cues during cellular spreading and motility. Recent studies provided indications that these protrusions wrap (coil) around the extracellular fibers. However, the physics of this coiling process, and the mechanisms that drive it, are not well understood. We present a combined theoretical and experimental study of the coiling of cellular protrusions on fibers of different geometry. Our theoretical model describes membrane protrusions that are produced by curved membrane proteins that recruit the protrusive forces of actin polymerization, and identifies the role of bending and adhesion energies in orienting the leading-edges of the protrusions along the azimuthal (coiling) direction. Our model predicts that the cell's leading-edge coils on fibers with circular cross-section (above some critical radius), but the coiling ceases for flattened fibers of highly elliptical cross-section. These predictions are verified by 3D visualization and quantitation of coiling on suspended fibers using Dual-View light-sheet microscopy (diSPIM). Overall, we provide a theoretical framework, supported by experiments, which explains the physical origin of the coiling phenomenon.

A minimal cell model for lamellipodia-based cellular dynamics and migration

One ubiquitous cellular structure for performing various tasks, such as spreading and migration over external surfaces, is the sheet-like protrusion called a lamellipodium, which propels the leading edge of the cell. Despite the detailed knowledge about the many components of this cellular structure, it is not yet fully understood how these components self-organize spatiotemporally to form lamellipodia. We review here recent theoretical works where we have demonstrated that membrane-bound protein complexes that have intrinsic curvature and recruit the protrusive forces of the cytoskeleton result in a simple, yet highly robust, organizing feedback mechanism that organizes the cytoskeleton and the membrane. This self-organization mechanism accounts for the formation of flat lamellipodia at the leading edge of cells spreading over adhesive substrates, allowing for the emergence of a polarized, motile 'minimal cell' model. The same mechanism describes how lamellipodia organize to drive robust engulfment of particles during phagocytosis and explains in simple physical terms the spreading and migration of cells over fibers and other curved surfaces. This Review highlights that despite the complexity of cellular composition, there might be simple general physical principles that are utilized by the cell to drive cellular shape dynamics.

Topographical curvature is sufficient to control epithelium elongation

How biophysical cues can control tissue morphogenesis is a central question in biology and for the development of efficient tissue engineering strategies. Recent data suggest that specific topographies such as grooves and ridges can trigger anisotropic tissue growth. However, the specific contribution of biologically relevant topographical features such as cell-scale curvature is still unclear. Here we engineer a series of grooves and ridges model topographies exhibiting specific curvature at the ridge/groove junctions and monitored the growth of epithelial colonies on these surfaces. We observe a striking proportionality between the maximum convex curvature of the ridges and the elongation of the epithelium. This is accompanied by the anisotropic distribution of F-actin and nuclei with partial exclusion of both in convex regions as well as the curvature-dependent reorientation of pluricellular protrusions and mitotic spindles. This demonstrates that curvature itself is sufficient to trigger and modulate the oriented growth of epithelia through the formation of convex “topographical barriers” and establishes curvature as a powerful tuning parameter for tissue engineering and biomimetic biomaterial design.

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.

Biological Tissues as Active Nematic Liquid Crystals

Live tissues can self-organize and be described as active materials composed of cells that generate active stresses through continuous injection of energy. In vitro reconstituted molecular networks, as well as single-cell cytoskeletons show that their filamentous structures can portray nematic liquid crystalline properties and can promote nonequilibrium processes induced by active processes at the microscale. The appearance of collective patterns, the formation of topological singularities, and spontaneous phase transition within the cell cytoskeleton are emergent properties that drive cellular functions. More integrated systems such as tissues have cells that can be seen as coarse-grained active nematic particles and their interaction can dictate many important tissue processes such as epithelial cell extrusion and migration as observed in vitro and in vivo. Here, a brief introduction to the concept of active nematics is provided, and the main focus is on the use of this framework in the systematic study of predominantly 2D tissue architectures and dynamics in vitro. In addition how the nematic state is important in tissue behavior, such as epithelial expansion, tissue homeostasis, and the atherosclerosis disease state, is discussed. Finally, how the nematic organization of cells can be controlled in vitro for tissue engineering purposes is briefly discussed.

Collective cell polarization and alignment on curved surfaces

Curvature as an important topological parameter of 3D extra-cellular matrix has drawn growing attention in recent years. But the underlying mechanism that curvature influences cell behaviors has remained unknown. In this study, we seeded cells on semi-cylindrical and hemispheric surfaces and tested cell alignment and polarization. We found that the surface curvature has profound effect on cell behaviors. With the decrease of diameter of the cylinder/sphere (i.e. increase of curvature), the cells would more preferentially align and polarize with large aspect ratio in the axial/peripheral direction. And the behaviors of the alignment and polarization were position-dependent. For example, at the end of the cylinder, the cells preferred to align circumferentially; while in the interior region, the cells preferred to align in the axial direction. We showed that the cell polarization and alignment were closely correlated with the in-plane stresses in cell layer. That is, the cell polarization and alignment were controlled by the maximum shear stress, which drove cells to align and polarize along the maximum principal stress. The curvature could influence the magnitude of the maximum shear stress and thus regulate cell behaviors. This study provided important insights into the mechanisms of surface curvature influencing cell behaviors in tissue morphogenesis. In addition, our theory of the stress dependent cellular polarity provides a generalized interpretation of the curvature and edge effects which might be extended to understand other steric effects in cell behaviors.

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.

Contraction and stress-dependent growth shape the forebrain of the early chicken embryo

During early vertebrate development, local constrictions, or sulci, form to divide the forebrain into the diencephalon, telencephalon, and optic vesicles. These partitions are maintained and exaggerated as the brain tube inflates, grows, and bends. Combining quantitative experiments on chick embryos with computational modeling, we investigated the biophysical mechanisms that drive these changes in brain shape. Chemical perturbations of contractility indicated that actomyosin contraction plays a major role in the creation of initial constrictions (Hamburger-Hamilton stages HH11-12), and fluorescent staining revealed that F-actin is circumferentially aligned at all constrictions. A finite element model based on these findings shows that the observed shape changes are consistent with circumferential contraction in these regions. To explain why sulci continue to deepen as the forebrain expands (HH12-20), we speculate that growth depends on wall stress. This idea was examined by including stress-dependent growth in a model with cerebrospinal fluid pressure and bending (cephalic flexure). The results given by the model agree with observed morphological changes that occur in the brain tube under normal and reduced eCSF pressure, quantitative measurements of relative sulcal depth versus time, and previously published patterns of cell proliferation. Taken together, our results support a biphasic mechanism for forebrain morphogenesis consisting of differential contractility (early) and stress-dependent growth (late).

Cell sheet mechanics: How geometrical constraints induce the detachment of cell sheets from concave surfaces

Despite of the progress made to engineer structured microtissues such as BioMEMS and 3D bioprinting, little control exists how microtissues transform as they mature, as the misbalance between cell-generated forces and the strength of cell-cell and cell-substrate contacts can result in unintended tissue deformations and ruptures. To develop a quantitative perspective on how cellular contractility, scaffold curvature and cell-substrate adhesion control such rupture processes, human aortic smooth muscle cells were grown on glass substrates with submillimeter semichannels. We quantified cell sheet detachment from 3D confocal image stacks as a function of channel curvature and cell sheet tension by adding different amounts of Blebbistatin and TGF-β to inhibit or enhance cell contractility, respectively. We found that both higher curvature and higher contractility increased the detachment probability. Variations of the adhesive strength of the protein coating on the substrate revealed that the rupture plane was localized along the substrate-extracellular matrix interface for non-covalently adsorbed adhesion proteins, while the collagen-integrin interface ruptured when collagen I was covalently crosslinked to the substrate. Finally, a simple mechanical model is introduced that quantitatively explains how the tuning of substrate curvature, cell sheet contractility and adhesive strength can be used as tunable parameters as summarized in a first semi-quantitative phase diagram. These parameters can thus be exploited to either inhibit or purposefully induce a collective detachment of sheet-like microtissues for the use in tissue engineering and regenerative therapies.

STATEMENT OF SIGNIFICANCE

Despite of the significant progress in 3D tissue fabrication technologies at the microscale, there is still no quantitative model that can predict if cells seeded on a 3D structure maintain the imposed geometry while they form a continuous microtissue. Especially, detachment or loss of shape control of growing tissue is a major concern when designing 3D-structured scaffolds. Utilizing semi-cylindrical channels and vascular smooth muscle cells, we characterized how geometrical and mechanical parameters such as curvature of the substrate, cellular contractility, or protein-substrate adhesion strength tune the catastrophic detachment of microtissue. Observed results were rationalized by a theoretical model. The phase diagram showing how unintended tissue detachment progresses would help in designing of mechanically-balanced 3D scaffolds in future tissue engineering applications.

Measuring cellular traction forces on non-planar substrates

Animal cells use traction forces to sense the mechanics and geometry of their environment. Measuring these traction forces requires a workflow combining cell experiments, image processing and force reconstruction based on elasticity theory. Such procedures have already been established mainly for planar substrates, in which case one can use the Green's function formalism. Here we introduce a workflow to measure traction forces of cardiac myofibroblasts on non-planar elastic substrates. Soft elastic substrates with a wave-like topology were micromoulded from polydimethylsiloxane and fluorescent marker beads were distributed homogeneously in the substrate. Using feature vector-based tracking of these marker beads, we first constructed a hexahedral mesh for the substrate. We then solved the direct elastic boundary volume problem on this mesh using the finite-element method. Using data simulations, we show that the traction forces can be reconstructed from the substrate deformations by solving the corresponding inverse problem with an L1-norm for the residue and an L2-norm for a zeroth-order Tikhonov regularization. Applying this procedure to the experimental data, we find that cardiac myofibroblast cells tend to align both their shapes and their forces with the long axis of the deformable wavy substrate.

Architecture and migration of an epithelium on a cylindrical wire

In a wide range of epithelial tissues such as kidney tubules or breast acini, cells organize into bidimensional monolayers experiencing an out-of-plane curvature. Cancer cells can also migrate collectively from epithelial tumors by wrapping around vessels or muscle fibers. However, in vitro experiments dealing with epithelia are mostly performed on flat substrates, neglecting this out-of-plane component. In this paper, we study the development and migration of epithelial tissues on glass wires of well-defined radii varying from less than 1 µm up to 85 µm. To uncouple the effect of out-of-plane curvature from the lateral confinement experienced by the cells in these geometries, we compare our results to experiments performed on narrow adhesive tracks. Because of lateral confinement, the velocity of collective migration increases for radii smaller than typically 20 µm. The monolayer dynamics is then controlled by front-edge protrusions. Conversely, high curvature is identified as the inducer of frequent cell detachments at the front edge, a phenotype reminiscent of the Epithelial-Mesenchymal Transition. High curvature also induces a circumferential alignment of the actin cytoskeleton, stabilized by multiple focal adhesions. This organization of the cytoskeleton is reminiscent of in vivo situations such as the development of the trachea of the Drosophila embryo. Finally, submicron radii halt the monolayer, which then reconfigures into hollow cysts.

Magnetophoretic assembly of flexible nanoparticles/lipid microfilaments

The directed assembly of colloidal particles into linear chains and clusters is of fundamental and practical importance. In this study we characterize and analyse the mechanism of the magnetic field driven assembly of lipid-coated iron oxide nanoparticles into flexible microfilaments. Recently we showed that nanocapillary lipid binding can form a new class of magnetic nanoparticle-lipid microfilaments with unprecedented flexibility and self-healing properties. In the presence of a uniform magnetic field, the magnetophoretic attraction of the particles combined with interparticle dipole–dipole attraction drives the microfilament assembly. The fluid like lipid layer on the particles leads to stickiness on the surface of the filaments and the magnetic field concentration overcomes the potential electrostatic repulsion in the water phase. The lipid capillary bridges formed between the particles facilitate their permanent binding and sustain the flexible microfilament structure. We demonstrate that this surface stickiness combined with the magnetic response of the filaments can be used further to twist, bend and bundle the microfilaments into unusual structures.

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.

Measuring anisotropic cell motility on curved substrates

Schwann cell motility was observed on laminin-coated quartz cylinders with different curvatures over an 18 hour period. A new analysis based on difference images helped to determine the minimal radius of curvature, 46 μm, which restricted motility along the cylinder axis. The migration speed, measured by calculating differences between successive images in the time series, ranged between 0.3 to 0.8 μm per minute and is similar to previously reported rates for Schwann cells. Difference images provide a rapid and simple method for the analysis of cell motility on large populations of cells.

Orthogonal (transverse) arrangements of actin in endothelia and fibroblasts

Though actin filaments running across the cell (transverse actin) have been occasionally reported for epithelial cells in groups and for cells growing on fibres, there has been no report heretofore of transverse actin in cells grown on planar substrata. This paper describes evidence in support of this possibility derived from actin staining, polarization microscopy and force measurements. The paper introduces two new methods for detecting the orientation and activity of contractile elements in cells. The orthogonal actin is most obvious in cells grown on groove ridge structures, but can be detected in cells grown on flat surfaces.

Combined microscale mechanical topography and chemical patterns on polymer cell culture substrates

This paper presents a technique to independently form mechanical topography and surface chemical patterns on polymer cell substrates, and studies the response of osteoblast cells to these surface patterns. The patterns were formed in two separate steps: hot embossing imprint lithography formed the mechanical topography and microcontact printing created the chemical pattern. The resulting substrate had surface features consisting of embossed grooves 4 microm deep and 8 microm wide spaced by 16 microm wide mesas and microcontact printed adhesive lanes 10 microm wide with spacings that ranged from 10 to 100 microm. When presented with either mechanical topography or chemical patterns alone, the cells significantly aligned to the pattern presented. When presented with mechanical topography overlaid with an orthogonal chemical pattern, the cells aligned to the mechanical topography. As the chemical pattern spacing was increased, osteoblasts remained aligned to the mechanical topography. Unlike traditional microfabrication approaches based on photolithography and wet chemistry, the patterning technique presented is compatible with a large number of biomaterials, could form patterns with features much smaller than 1 microm, and is highly scalable to large substrates.

Microfabrication of poly (glycerol-sebacate) for contact guidance applications

Controlling cell orientation and morphology through topographical patterning is a phenomenon that is applicable to a wide variety of medical applications such as implants and tissue engineering scaffolds. Previous work in this field, termed contact guidance, has demonstrated the application of this cellular response on a wide variety of material substrates such as silicon, quartz, glass, and poly(di-methyl siloxane) typically using ridge-groove geometries with sharp feature edges. One limitation of these studies in terms of biomedical applications is the choice of material. Therefore, demonstrating contact guidance and topography in a biodegradable material platform is a promising strategy for controlling cellular arrangements in tissue engineering scaffolds. This study investigates several strategies to advance contact guidance strategies and technology to more practical applications. Flexible biodegradable substrates with rounded features were fabricated by replica-molding poly(glycerol-sebacate) on sucrose-coated microfabricated silicon. Bovine aortic endothelial cells were cultured on substrates with microstructures between 2 and 5 microm in wavelength and with constant feature depth of 0.45 microm. Cells cultured on substrates with smaller pitches exhibited a substantially higher frequency of cell alignment and smaller circularity index. This work documents the first known use of using a flexible, biodegradable substrate with rounded features for use in contact guidance applications. The replica-molding technique described here is a general process that can be used to fabricate topographically patterned substrates with rounded features for many biomaterials. Furthermore, these results may lead to further elucidation of the mechanism of cell alignment and contact guidance on microfabricated substrates.

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.

Surface topography can interfere with epithelial tissue migration

Corneal epithelial tissue migration over the surface of a synthetic polymer can be inhibited by pores in the substrate. The effects of this substrate topography upon epithelial tissue migration were studied in vitro. Membranes of different porosities and structures were used to provide two series of surfaces having a graded increase in discontinuities: cellulose nitrate/acetate membranes with a tortuous network of pores, and track-etched polycarbonate membranes with columnar pores. Corneal epithelial tissue outgrowth was inhibited by increased pore size, and for both series of membranes, outgrowth was completely halted on membranes with mean diameter of the pores 0.9 microm at the pore densities measured. On the track-etched membranes with pores of <0.9 microm diameter, tissue outgrowth could be partially "rescued" by coating with fibronectin or collagen, but above this size, the inhibition predominated. The effect of porosity of the track-etched membranes upon the migration of dissociated epithelial cells was also examined. Although migration of these cells was reduced on membranes having pore sizes larger than 0.9 microm, it was not completely inhibited even on membranes of 2.3-microm pore diameter. Therefore, tissue movement of adult stratified epithelium may be inhibited by specific surface topographies, and in this assay system, epithelial tissue outgrowth was more affected than was the migration of dissociated epithelial cells.