Fibrillogenesis in dense collagen solutions: a physicochemical study

Type I collagen D-spacing in fibril bundles of dermis, tendon, and bone: bridging between nano- and micro-level tissue hierarchy

Fibrillar collagens in connective tissues are organized into complex and diverse hierarchical networks. In dermis, bone, and tendon, one common phenomenon at the micrometer scale is the organization of fibrils into bundles. Previously, we have reported that collagen fibrils in these tissues exhibit a 10 nm width distribution of D-spacing values. This study expands the observation to a higher hierarchical level by examining fibril D-spacing distribution in relation to the bundle organization. We used atomic force microscopy imaging and two-dimensional fast Fourier transform analysis to investigate dermis, tendon, and bone tissues. We found that, in each tissue type, collagen fibril D-spacings within a single bundle were nearly identical and frequently differ by less than 1 nm. The full 10 nm range in D-spacing values arises from different values found in different bundles. The similarity in D-spacing was observed to persist for up to 40 μm in bundle length and width. A nested mixed model analysis of variance examining 107 bundles and 1710 fibrils from dermis, tendon, and bone indicated that fibril D-spacing differences arise primarily at the bundle level (∼76%), independent of species or tissue types.

Controlling the nano-bio interface to build collagen-silica self-assembled networks

Bio-hybrid networks are designed based on the self-assembly of surface-engineered collagen-silica nanoparticles. Collagen triple helices can be confined on the surface of sulfonate-modified silica particles in a controlled manner. This gives rise to hybrid building blocks with well-defined diameters and surface potentials. Taking advantage of the self-assembling properties of collagen, collagen-silica networks are further built-up in solution. The structural and specific recognition properties of the collagen fibrils are well-preserved within the hybrid assembly. A combination of calorimetry, dynamic light scattering, zetametry and microscopy studies indicates that network formation occurs via a surface-mediated mechanism where pre-organization of the protein chains on the particle surface favors the fibrillogenesis process. These results enlighten the importance of the nano-bio interface on the formation and properties of self-assembled bionanocomposites.

Occupy tissue: the movement in cancer metastasis

The critical role of migration and invasion in cancer metastasis warrants new therapeutic approaches targeting the machinery regulating cell migration and invasion. While 2-dimensional (2D) models have helped identify a range of adhesion molecules, cytoskeletal components and regulators that are potentially important for cell migration, the use of models that better mimic the 3-dimensional (3D) environment has yielded new insights into the physiology of cell movement. For example, studying cells in 3D models has revealed that invading cancer cells may switch between heterogeneous invasion modes and thus evade pharmacological inhibition of invasion. Here we summarize published data in which the role of cell adhesion molecules in 2D vs. 3D migration have been directly compared and discuss mechanisms that regulate migration speed and persistence in 2D and 3D. Finally we discuss limits of 3D culture models to recapitulate the in vivo situation.

Thick collagen-based 3D matrices including growth factors to induce neurite outgrowth

Designing synthetic microenvironments for cellular investigations is a very active area of research at the crossroads of cell biology and materials science. The present work describes the design and functionalization of a three-dimensional (3D) culture support dedicated to the study of neurite outgrowth from neural cells. It is based on a dense self-assembled collagen matrix stabilized by 100-nm-wide interconnected native fibrils without chemical crosslinking. The matrices were made suitable for cell manipulation and direct observation in confocal microscopy by anchoring them to traditional glass supports with a calibrated thickness of ∼50μm. The matrix composition can be readily adapted to specific neural cell types, notably by incorporating appropriate neurotrophic growth factors. Both PC-12 and SH-SY5Y lines respond to growth factors (nerve growth factor and brain-derived neurotrophic factor, respectively) impregnated and slowly released from the support. Significant neurite outgrowth is reported for a large proportion of cells, up to 66% for PC12 and 49% for SH-SY5Y. It is also shown that both growth factors can be chemically conjugated (EDC/NHS) throughout the matrix and yield similar proportions of cells with longer neurites (61% and 52%, respectively). Finally, neurite outgrowth was observed over several tens of microns within the 3D matrix, with both diffusing and immobilized growth factors.

Molecular crowding of collagen: a pathway to produce highly-organized collagenous structures

Collagen in vertebrate animals is often arranged in alternating lamellae or in bundles of aligned fibrils which are designed to withstand in vivo mechanical loads. The formation of these organized structures is thought to result from a complex, large-area integration of individual cell motion and locally-controlled synthesis of fibrillar arrays via cell-surface fibripositors (direct matrix printing). The difficulty of reproducing such a process in vitro has prevented tissue engineers from constructing clinically useful load-bearing connective tissue directly from collagen. However, we and others have taken the view that long-range organizational information is potentially encoded into the structure of the collagen molecule itself, allowing the control of fibril organization to extend far from cell (or bounding) surfaces. We here demonstrate a simple, fast, cell-free method capable of producing highly-organized, anistropic collagen fibrillar lamellae de novo which persist over relatively long-distances (tens to hundreds of microns). Our approach to nanoscale organizational control takes advantage of the intrinsic physiochemical properties of collagen molecules by inducing collagen association through molecular crowding and geometric confinement. To mimic biological tissues which comprise planar, aligned collagen lamellae (e.g. cornea, lamellar bone or annulus fibrosus), type I collagen was confined to a thin, planar geometry, concentrated through molecular crowding and polymerized. The resulting fibrillar lamellae show a striking resemblance to native load-bearing lamellae in that the fibrils are small, generally aligned in the plane of the confining space and change direction en masse throughout the thickness of the construct. The process of organizational control is consistent with embryonic development where the bounded planar cell sheets produced by fibroblasts suggest a similar confinement/concentration strategy. Such a simple approach to nanoscale organizational control of structure not only makes de novo tissue engineering a possibility, but also suggests a clearer pathway to organization for fibroblasts than direct matrix printing.

The predominant role of collagen in the nucleation, growth, structure and orientation of bone apatite

The involvement of collagen in bone biomineralization is commonly admitted, yet its role remains unclear. Here we show that type I collagen in vitro can initiate and orientate the growth of carbonated apatite mineral in the absence of any other vertebrate extracellular matrix molecules of calcifying tissues. We also show that the collagen matrix influences the structural characteristics on the atomic scale, and controls the size and the three-dimensional distribution of apatite at larger length scales. These results call into question recent consensus in the literature on the need for Ca-rich non-collagenous proteins for collagen mineralization to occur in vivo. Our model is based on a collagen/apatite self-assembly process that combines the ability to mimic the in vivo extracellular fluid with three major features inherent to living bone tissue, that is, high fibrillar density, monodispersed fibrils and long-range hierarchical organization.

Imaging and 3D morphological analysis of collagen fibrils

The recent booming of multiphoton imaging of collagen fibrils by means of second harmonic generation microscopy generates the need for the development and automation of quantitative methods for image analysis. Standard approaches sequentially analyse two-dimensional (2D) slices to gain knowledge on the spatial arrangement and dimension of the fibrils, whereas the reconstructed three-dimensional (3D) image yields better information about these characteristics. In this work, a 3D analysis method is proposed for second harmonic generation images of collagen fibrils, based on a recently developed 3D fibre quantification method. This analysis uses operators from mathematical morphology. The fibril structure is scanned with a directional distance transform. Inertia moments of the directional distances yield the main fibre orientation, corresponding to the main inertia axis. The collaboration of directional distances and fibre orientation delivers a geometrical estimate of the fibre radius. The results include local maps as well as global distribution of orientation and radius of the fibrils over the 3D image. They also bring a segmentation of the image into foreground and background, as well as a classification of the foreground pixels into the preferred orientations. This accurate determination of the spatial arrangement of the fibrils within a 3D data set will be most relevant in biomedical applications. It brings the possibility to monitor remodelling of collagen tissues upon a variety of injuries and to guide tissues engineering because biomimetic 3D organizations and density are requested for better integration of implants.

In situ three-dimensional monitoring of collagen fibrillogenesis using SHG microscopy

We implemented in situ time-lapse Second Harmonic Generation (SHG) microscopy to monitor the three-dimensional (3D) self-assembly of collagen in solution. As a proof of concept, we tuned the kinetics of fibril formation by varying the pH and measured the subsequent exponential increase of fibril volume density in SHG images. We obtained significantly different time constants at pH = 6.5 ± 0.3 and at pH = 7.5 ± 0.3. Moreover, we showed that we could focus on the growth of a single isolated collagen fibril because SHG microscopy is sensitive to well-organized fibrils with diameter below the optical resolution. This work illustrates the potential of SHG microscopy for the rational design and characterization of collagen-based biomaterials.

Development of a nonlinear fiber-optic spectrometer for human lung tissue exploration

Several major lung pathologies are characterized by early modifications of the extracellular matrix (ECM) fibrillar collagen and elastin network. We report here the development of a nonlinear fiber-optic spectrometer, compatible with an endoscopic use, primarily intended for the recording of second-harmonic generation (SHG) signal of collagen and two-photon excited fluorescence (2PEF) of both collagen and elastin. Fiber dispersion is accurately compensated by the use of a specific grism-pair stretcher, allowing laser pulse temporal width around 70 fs and excitation wavelength tunability from 790 to 900 nm. This spectrometer was used to investigate the excitation wavelength dependence (from 800 to 870 nm) of SHG and 2PEF spectra originating from ex vivo human lung tissue samples. The results were compared with spectral responses of collagen gel and elastin powder reference samples and also with data obtained using standard nonlinear microspectroscopy. The excitation-wavelength-tunable nonlinear fiber-optic spectrometer presented in this study allows performing nonlinear spectroscopy of human lung tissue ECM through the elastin 2PEF and the collagen SHG signals. This work opens the way to tunable excitation nonlinear endomicroscopy based on both distal scanning of a single optical fiber and proximal scanning of a fiber-optic bundle.

Biophysical control of invasive tumor cell behavior by extracellular matrix microarchitecture

Fibrillar collagen gels, which are used extensively in vitro to study tumor-microenvironment interactions, are composed of a cell-instructive network of interconnected fibers and pores whose organization is sensitive to polymerization conditions such as bulk concentration, pH, and temperature. Using confocal reflectance microscopy and image autocorrelation analysis to quantitatively assess gel microarchitecture, we show that additional polymerization parameters including culture media formulation and gel thickness significantly affect the dimensions and organization of fibers and pores in collagen gels. These findings enabled the development of a three-dimensional culture system in which cell-scale gel microarchitecture was decoupled from bulk gel collagen concentration. Interestingly, morphology and migration characteristics of embedded MDA-MB-231 cells were sensitive to gel microarchitecture independently of collagen gel concentration. Cells adopted a polarized, motile phenotype in gels with larger fibers and pores and a rounded or stellate, less motile phenotype in gels with small fibers and pores regardless of bulk gel density. Conversely, cell proliferation was sensitive to gel concentration but not microarchitecture. These results indicate that cell-scale gel microarchitecture may trump bulk-scale gel density in controlling specific cell behaviors, underscoring the biophysical role of gel microarchitecture in influencing cell behavior.

Tailoring material properties of a nanofibrous extracellular matrix derived hydrogel

In the native tissue, the interaction between cells and the extracellular matrix (ECM) is essential for cell migration, proliferation, differentiation, mechanical stability, and signaling. It has been shown that decellularized ECMs can be processed into injectable formulations, thereby allowing for minimally invasive delivery. Upon injection and increase in temperature, these materials self-assemble into porous gels forming a complex network of fibers with nanoscale structure. In this study we aimed to examine and tailor the material properties of a self-assembling ECM hydrogel derived from porcine myocardial tissue, which was developed as a tissue specific injectable scaffold for cardiac tissue engineering. The impact of gelation parameters on ECM hydrogels has not previously been explored. We examined how modulating pH, temperature, ionic strength, and concentration affected the nanoscale architecture, mechanical properties, and gelation kinetics. These material characteristics were assessed using scanning electron microscopy, rheometry, and spectrophotometry, respectively. Since the main component of the myocardial matrix is collagen, many similarities between the ECM hydrogel and collagen gels were observed in terms of the nanofibrous structure and modulation of properties by altering ionic strength. However, variation from collagen gels was noted for the gelation temperature along with varied times and rates of gelation. These discrepancies when compared to collagen are likely due to the presence of other ECM components in the decellularized ECM based hydrogel. These results demonstrate how the material properties of ECM hydrogels could be tailored for future in vitro and in vivo applications.

Epitaxially guided assembly of collagen layers on mica surfaces

Ordered assembly of collagen molecules on flat substrates has potential for various applications and serves as a model system for studying the assembly process. While previous studies demonstrated self-assembly of collagen on muscovite mica into highly ordered layers, the mechanism by which different conditions affect the resulting morphology remains to be elucidated. Using atomic force microscopy, we follow the assembly of collagen on muscovite mica at a concentration lower than the critical fibrillogenesis concentration in bulk. Initially, individual collagen molecules adsorb to mica and subsequently nucleate into fibrils possessing the 67 nm D-periodic bands. Emergence of fibrils aligned in parallel despite large interfibril distances agrees with an alignment mechanism guided by the underlying mica. The epitaxial growth was further confirmed by the formation of novel triangular networks of collagen fibrils on phlogopite mica, whose surface lattice is known to have a hexagonal symmetry, whereas the more widely used muscovite does not. Comparing collagen assembly on the two types of mica at different potassium concentrations revealed that potassium binds to the negatively charged mica surface and neutralizes it, thereby reducing the binding affinity of collagen and enhancing surface diffusion. These results suggest that collagen assembly on mica follows the surface adsorption, diffusion, nucleation, and growth pathway, where the growth direction is determined at the nucleation step. Comparison with other molecules that assemble similarly on mica supports generality of the proposed assembly mechanism, the knowledge of which will be useful for controlling the resulting surface morphologies.

Effects of phosphate-buffered saline concentration and incubation time on the mechanical and structural properties of electrochemically aligned collagen threads

A key step during the synthesis of collagen constructs is the incubation of monomeric collagen in phosphate buffer saline (PBS) to promote fibrillogenesis in the collagen network. Optimal PBS-treatment conditions for monomeric collagen solutions to induce gelation are well established in the literature. Recently, a report in the literature (Cheng et al 2008 Biomaterials 29 3278-88) showed a novel method to fabricate highly oriented electrochemically aligned collagen (ELAC) threads which have orders of magnitude greater packing density than collagen gels. The optimal PBS-treatment conditions for induction of D-banding pattern in such a dense and anisotropic collagen network are unknown. This study aimed to optimize PBS treatment of ELAC threads by investigating the effect of phosphate ion concentration (0.5×, 1×, 5× and 10×) and incubation time (3, 12 and 96 h) on the mechanical strength and ultrastructural organization by monotonic mechanical testing, small angle x-ray scattering and transmission electron microscopy (TEM). ELAC threads incubated in water (no PBS) served as the control. ELAC threads incubated in 1× PBS showed significantly higher extensibility compared to those in 0.5× or 10× PBS along with the presence of D-banded patterns with a periodicity of 63.83 nm. Incubation of ELAC threads in 1× PBS for 96 h resulted in significantly higher ultimate stress compared to 3 or 12 h. However, these threads lacked the D-banding pattern. TEM observations showed no significant differences in the microfibril diameter distribution of ELAC threads treated with or without PBS. This indicates that microfibrils lacked D-banding following electrochemical alignment and the subsequent PBS-treatment-induced D-banding by reorganization within microfibrils. It was concluded that incubation of aligned collagen in 1× PBS for 12 h results in mechanically competent, D-banded ELAC threads which can be used for the regeneration of load bearing tissues such as tendons and ligaments.

Preparation of mineralized nanofibers: collagen fibrils containing calcium phosphate

We report a straightforward, bottom-up, scalable process for preparing mineralized nanofibers. Our procedure is based on flowing feed solution, containing both inorganic cations and polymeric molecules, through a nanoporous membrane into a receiver solution with anions, which leads to the formation of mineralized nanofibers at the exit of the pores. With this strategy, we were able to achieve size control of the nanofiber diameters. We illustrate this approach by producing collagen fibrils with calcium phosphate incorporated inside the fibrils. This structure, which resembles the basic constituent of bones, assembles itself without the addition of noncollagenous proteins or their polymeric substitutes. Rheological experiments demonstrated that the stiffness of gels derived from these fibrils is enhanced by mineralization. Growth experiments of human adipose derived stem cells on these gels showed the compatibility of the fibrils in a tissue-regeneration context.

Collagen hydrogel characterization: multi-scale and multi-modality approach

The complex supramolecular architecture of collagen biopolymer plays an important role in tissue development and integrity. Developing methods to report on collagen structures assembled in vitro would accelerate the pace of utilizing it in biomedical applications. Employing imaging techniques and turbidity measurements, we mapped the light scattering properties of 3D collagen hydrogels formed at initial concentrations of 1 mg ml-1 to about 5 mg ml-1 and several incubation temperatures. The transmission electron microscopy (TEM) images show that collagen scattering features consist of both native-like fibrils and filamentous structures that do not have the characteristic fibrillar striation observed in this protein. Spindle-shaped fibrils appear at the concentrations of 1, 2, 2.5 and 4 mg ml-1 and the spiral-shaped fibrils are formed at the concentrations of 2 and 2.5 mg ml-1. The multiphoton microscopy (MPM) images reveal that in the 3D collagen hydrogels a unified relationship between second harmonic generation (SHG) signal directionality and fibril morphology and/or sizes is not likely. The MPM images, however, showed important micro-structural details. These details lead us to conclude that the dependence of SHG signals on the number of interfaces created upon assembly of 3D collagen hydrogels can account for the strength of the detected backscattered signals.

Collagen supramolecular and suprafibrillar organizations on osteoblasts long-term behavior: benefits for bone healing materials

This study compares the behavior of osteoblastic cells seeded on three structurally distinct collagen-based materials. Adhesion and long-term behavior were evaluated in vitro in regard to collagen scaffolds forming loose or dense fibrillar networks or exempt of fibrils. In this purpose collagen solutions at concentrations of 5 and 40 mg/mL were processed by freeze-drying or by sol/gel fibrillogenesis to form either sponges or hydrogels. Macroscopic and microscopic images of sponges showed a light material exhibiting large pores surrounded by dense collagen walls made of thin unstriated microfibrils of 20 nm in diameter. In comparison collagen hydrogels are more homogeneous materials, at 5 mg/mL the material consists of a regular network of cross-striated collagen fibrils of 100 nm in diameter. At 40 mg/mL the material appears stiffer, the ultrastructure exhibits cross-striated collagen fibrils packed in large bundles of 300-800 nm of width. Human osteoblastic cells seeded on top of the 5 mg/mL matrices exhibit a squared shaped osteoblast-like morphology over 28 days of culture and express both alkaline phosphatase and osteocalcin. Osteoblastic cells seeded on top of sponges or of 40 mg/mL matrices exhibit both flat and elongated resting-osteoblast morphology. Osteoblastic cells have mineralized the three collagen-based materials after 28 days of culture but collagen sponges spontaneously mineralized in absence of cells. These results highlight, in an in vitro cell culture approach, the benefit of fibrils and of dense fibrillar networks close to in vivo-like tissues, as positive criteria for new bone tissue repair materials.

Effects of matrix composition, microstructure, and viscoelasticity on the behaviors of vocal fold fibroblasts cultured in three-dimensional hydrogel networks

Vocal fold diseases and disorders are difficult to treat surgically or therapeutically. Tissue engineering offers an alternative strategy for the restoration of functional vocal folds. As a first step toward vocal fold tissue engineering, we investigated the responses of primary vocal fold fibroblasts (PVFFs) to two types of collagen and hyaluronic acid (HA)-based hydrogels that are compositionally similar, but structurally variable and mechanically different. Type A hydrogels were composed of mature collagen fibers reinforced by oxidized HA, whereas type B hydrogels contained immature collagen fibrils interpenetrated in an amorphous, covalently cross-linked HA matrix. PVFFs encapsulated in either matrix adopted a fibroblastic morphology and expressed genes related to important extracellular matrix proteins. DNA analysis indicated a linear growth profile for cells encapsulated in type B gels from day 0 to 21, in contrast to an initial dormant, nonproliferative period from day 0 to 3 experienced by cells in type A gels. At the end of the culture, similar DNA content was detected in both types of constructs. A reduction in collagen content was observed for both types of constructs after 28 days of culture, with type A constructs generally retaining higher amounts of collagen than type B constructs. The HA content in the constructs decreased steadily throughout the culture, with type A constructs consistently exhibiting less HA than type B constructs. Using the torsional wave analysis, we found that the elastic moduli for type A constructs decreased sharply during the first week of culture, followed by 2 weeks of matrix stabilization without significant changes in matrix stiffness. Conversely, the elastic modulus for type B constructs increased moderately over time. It is postulated that PVFFs residing in gels alter the matrix organization, chemical compositions, and viscoelasticity through cell-mediated remodeling processes.

Nonlinear optical imaging of lyotropic cholesteric liquid crystals

We use nonlinear optical microscopy combining Second Harmonic Generation (SHG) microscopy and Two-Photon Excited Fluorescence (2PEF) signals to characterize collagen lyotropic liquid crystals. We show that SHG signals provide highly contrasted images of the three-dimensional texture of cholesteric patterns with submicrometer lateral resolution. Moreover, simultaneous recording of the 2PEF signal enables in situ quantitative mapping of the molecular concentration and its correlation with the observed textures. We apply this technique to the characterization of biomimetic textures obtained in concentrated collagen liquid solutions. We successfully image biologically relevant organizations that are similar to the collagen organization found as a stabilized state in compact bones.

Collagen-based cell migration models in vitro and in vivo

Fibrillar collagen is the most abundant extracellular matrix (ECM) constituent which maintains the structure of most interstitial tissues and organs, including skin, gut, and breast. Density and spatial alignments of the three-dimensional (3D) collagen architecture define mechanical tissue properties, i.e. stiffness and porosity, which guide or oppose cell migration and positioning in different contexts, such as morphogenesis, regeneration, immune response, and cancer progression. To reproduce interstitial cell movement in vitro with high in vivo fidelity, 3D collagen lattices are being reconstituted from extracted collagen monomers, resulting in the re-assembly of a fibrillar meshwork of defined porosity and stiffness. With a focus on tumor invasion studies, we here evaluate different in vitro collagen-based cell invasion models, employing either pepsinized or non-pepsinized collagen extracts, and compare their structure to connective tissue in vivo, including mouse dermis and mammary gland, chick chorioallantoic membrane (CAM), and human dermis. Using confocal reflection and two-photon-excited second harmonic generation (SHG) microscopy, we here show that, depending on the collagen source, in vitro models yield homogeneous fibrillar texture with a quite narrow range of pore size variation, whereas all in vivo scaffolds comprise a range from low- to high-density fibrillar networks and heterogeneous pore sizes within the same tissue. Future in-depth comparison of structure and physical properties between 3D ECM-based models in vitro and in vivo are mandatory to better understand the mechanisms and limits of interstitial cell movements in distinct tissue environments.

Correlating mechanical properties with aggregation processes in electrochemically fabricated collagen membranes

We show that mechanical stiffness is a useful metric for characterizing complex collagen assemblies, providing insight about aggregation products and pathways in collagen-based materials. This study focuses on mechanically robust collagenous membranes produced by an electrochemical synthesis process. Changing the duration of the applied electric field, or adjusting the electrolyte composition (by adding Ca(2+), K(+), or Na(+) or by changing pH), produces membranes with a range of Young's moduli as determined from force-displacement measurements with an atomic force microscope. The structural organization, characterized by UV-visible spectroscopy, Raman spectroscopy, optical microscopy, and atomic force microscopy, correlates with the mechanical stiffness. These data provide insights into the relative importance of different aggregation pathways enabled by our multiparameter electrochemically induced collagen assembly process.

Fully automated, quantitative, noninvasive assessment of collagen fiber content and organization in thick collagen gels

Collagen is the most prominent protein of human tissues. Its content and organization define to a large extent the mechanical properties of tissue as well as its function. Methods that have been used traditionally to visualize and analyze collagen are invasive, provide only qualitative or indirect information, and have limited use in studies that aim to understand the dynamic nature of collagen remodeling and its interactions with the surrounding cells and other matrix components. Second harmonic generation (SHG) imaging emerged as a promising noninvasive modality for providing high-resolution images of collagen fibers within thick specimens, such as tissues. In this article, we present a fully automated procedure to acquire quantitative information on the content, orientation, and organization of collagen fibers. We use this procedure to monitor the dynamic remodeling of collagen gels in the absence or presence of fibroblasts over periods of 12 or 14 days. We find that an adaptive thresholding and stretching approach provides great insight to the content of collagen fibers within SHG images without the need for user input. An additional feature-erosion and feature-dilation step is useful for preserving structure and noise removal in images with low signal. To quantitatively assess the orientation of collagen fibers, we extract the orientation index (OI), a parameter based on the power distribution of the spatial-frequency-averaged, two-dimensional Fourier transform of the SHG images. To measure the local organization of the collagen fibers, we access the Hough transform of small tiles of the image and compute the entropy distribution, which represents the probability of finding the direction of fibers along a dominant direction. Using these methods we observed that the presence and number of fibroblasts within the collagen gel significantly affects the remodeling of the collagen matrix. In the absence of fibroblasts, gels contract, especially during the first few days, in a manner that allows the fibers to remain mostly disoriented, as indicated by small OI values. Subtle changes in the local organization of fibers may be taking place as the corresponding entropy values of these gels show a small decrease. The presence of fibroblasts affects the collagen matrix in a manner that is highly dependent on their number. A low density of fibroblasts enhances the rate of initial gel contraction, but ultimately leads to degradation of collagen fibers, which start to organize in localized clumps. This degradation and reorganization is seen within the first days of incubation with fibroblasts at a high density and is followed by de novo collagen fiber deposition by the fibroblasts. These collagen fibers are more highly oriented and organized than the fibers of the original collagen gel. These initial studies demonstrate that SHG imaging in combination with automated image analysis approaches offer a noninvasive and easily implementable method for characterizing important features of the content and organization of collagen in tissuelike specimens. Therefore, these studies could offer important insights for improving tissue engineering and disease diagnostic efforts.

pH-induced contrast in viscoelasticity imaging of biopolymers

Understanding contrast mechanisms and identifying discriminating features is at the heart of diagnostic imaging development. This paper focuses on how pH influences the viscoelastic properties of biopolymers to better understand the effects of extracellular pH on breast tumour elasticity imaging. Extracellular pH is known to decrease as much as 1 pH unit in breast tumours, thus creating a dangerous environment that increases cellular mutation rates and therapeutic resistance. We used a gelatin hydrogel phantom to isolate the effects of pH on a polymer network with similarities to the extracellular matrix in breast stroma. Using compressive unconfined creep and stress relaxation measurements, we systematically measured the viscoelastic features sensitive to pH by way of time-domain models and complex modulus analysis. These results are used to determine the sensitivity of quasi-static ultrasonic elasticity imaging to pH. We found a strong elastic response of the polymer network to pH, such that the matrix stiffness decreases as pH was reduced; however, the viscous response of the medium to pH was negligible. While physiological features of breast stroma such as proteoglycans and vascular networks are not included in our hydrogel model, observations in this study provide insight into viscoelastic features specific to pH changes in the collagenous stromal network. These observations suggest that the large contrast common in breast tumours with desmoplasia may be reduced under acidic conditions, and that viscoelastic features are unlikely to improve discriminability.

Prelude to corneal tissue engineering - gaining control of collagen organization

By most standard engineering practice principles, it is premature to credibly discuss the "engineering" of a human cornea. A professional design engineer would assert that we still do not know what a cornea is (and correctly so), therefore we cannot possibly build one. The proof resides in the fact that there are no clinically viable corneas based on classical tissue engineering methods available. This is possibly because tissue engineering in the classical sense (seeding a degradable scaffolding with a population synthetically active cells) does not produce conditions which support the generation of organized tissue. Alternative approaches to the problem are in their infancy and include the methods which attempt to recapitulate development or to produce corneal stromal analogs de novo which require minimal remodeling. Nonetheless, tissue engineering efforts, which have been focused on producing the fundamental functional component of a cornea (organized alternating arrays of collagen or "lamellae"), may have already provided valuable new insights and tools relevant to development, growth, remodeling and pathologies associated with connective tissue in general. This is because engineers ask a fundamentally different question (How can that be done?) than do biological scientists (How is that done?). The difference in inquiry has prompted us to closely examine (and to mimic) development as well as investigate collagen physicochemical behavior so that we may exert control over organization both in cell culture (in vitro) and on the benchtop (de novo). Our initial results indicate that reproducing corneal stroma-like local and long-range organization of collagen may be simpler than we anticipated while controlling spacing and fibril morphology remains difficult, but perhaps not impossible in the (reasonably) near term.