Twisted liquid crystalline supramolecular arrangements in morphogenesis

Mineralized Collagen Fibrils: An Essential Component in Determining the Mechanical Behavior of Cortical Bone

Bone comprises mechanically different materials in a specific hierarchical structure. Mineralized collagen fibrils (MCFs), represented by tropocollagen molecules and hydroxyapatite nanocrystals, are the fundamental unit of bone. The mechanical characterization of MCFs provides the unique adaptive mechanical competence to bone to withstand mechanical load. The structural and mechanical role of MCFs is critical in the deformation mechanisms of bone and the marvelous strength and toughness possessed by bone. However, the role of MCFs in the mechanical behavior of bone across multiple length scales is not fully understood. In the present study, we shed light upon the latest progress regarding bone deformation at multiple hierarchical levels and emphasize the role of MCFs during bone deformation. We propose the concept of hierarchical deformation of bone to describe the interconnected deformation process across multiple length scales of bone under mechanical loading. Furthermore, how the deterioration of bone caused by aging and diseases impairs the hierarchical deformation process of the cortical bone is discussed. The present work expects to provide insights on the characterization of MCFs in the mechanical properties of bone and lays the framework for the understanding of the multiscale deformation mechanics of bone.

Gelatin as It Is: History and Modernity

The data concerning the synthesis and physicochemical characteristics of one of the practically important proteins-gelatin, as well as the possibilities of its practical application, are systematized and discussed. When considering the latter, emphasis is placed on the use of gelatin in those areas of science and technology that are associated with the specifics of the spatial/molecular structure of this high-molecular compound, namely, as a binder for the silver halide photographic process, immobilized matrix systems with a nano-level organization of an immobilized substance, matrices for creating pharmaceutical/dosage forms and protein-based nanosystems. It was concluded that the use of this protein is promising in the future.

Mechanochemistry of collagen

The collagen molecular family is the result of nearly one billion years of evolution. It is a unique family of proteins, the majority of which provide general mechanical support to biological tissues. Fibril forming collagens are the most abundant collagens in vertebrate animals and are generally found in positions that resist tensile loading. In animals, cells produce fibril-forming collagen molecules that self-assemble into larger structures known as collagen fibrils. Collagen fibrils are the fundamental, continuous, load-bearing elements in connective tissues, but are often further aggregated into larger load-bearing structures, fascicles in tendon, lamellae in cornea and in intervertebral disk. We know that failure to form fibrillar collagen is embryonic lethal, and excessive collagen formation/growth (fibrosis) or uncontrolled enzymatic remodeling (type II collagen: osteoarthritis) is pathological. Collagen is thus critical to vertebrate viability and instrumental in maintaining efficient mechanical structures. However, despite decades of research, our understanding of collagen matrix formation is not complete, and we know still less about the detailed mechanisms that drive collagen remodeling, growth, and pathology. In this perspective, we examine the known role of mechanical force on the formation and development of collagenous structure. We then discuss a mechanochemical mechanism that has the potential to unify our understanding of collagenous tissue assembly dynamics, which preferentially deposits and grows collagen fibrils directly in the path of mechanical force, where the energetics should be dissuasive and where collagen fibrils are most required. We term this mechanism: Mechanochemical force-structure causality. STATEMENT OF SIGNIFICANCE: Our mechanochemical-force structure causality postulate suggests that collagen molecules are components of mechanochemically-sensitive and dynamically-responsive fibrils. Collagen molecules assemble preferentially in the path of applied strain, can be grown in place by mechanical extension, and are retained in the path of force through strain-stabilization. The mechanisms that drive this behavior operate at the level of the molecules themselves and are encoded into the structure of the biomaterial. The concept might change our understanding of structure formation, enhance our ability to treat injuries, and accelerate the development of therapeutics to prevent pathologies such as fibrosis. We suggest that collagen is a mechanochemically responsive dynamic element designed to provide a substantial "material assist" in the construction of adaptive carriers of mechanical signals.

Identification of the Collagen Types Essential for Mammalian Breast Acinar Structures

Modeling human breast tissue architecture is essential to study the pathophysiological conditions of the breast. We report that normal mammary epithelial cells grown in human breast extracellular matrix (ECM) hydrogel formed acini structurally similar to those of human and pig mammary tissues. Type I, II, III and V collagens were commonly identified in human, pig, and mouse breast ECM. Mammary epithelial cells formed acini on certain types or combinations of the four collagens at normal levels of breast tissue elasticity. Comparison of the collagen species in mouse normal breast and breast tumor ECM revealed common and distinct sets of collagens within the two types of tissues. Elevated expression of collagen type I alpha 1 chain (Col1a1) was found in mouse and human breast cancers. Collagen type XXV alpha 1 chain (Col25a1) was identified in mouse breast tumors but not in normal breast tissues. Our data provide strategies for modeling human breast pathophysiological structures and functions using native tissue-derived hydrogels and offer insight into the potential contributions of different collagen types in breast cancer development.

Segment-Long-Spacing (SLS) and the Polymorphic Structures of Fibrillar Collagen

The diverse and complex functions of collagen during the development of an organism are closely related to the polymorphism of its supramolecular structures in the extracellular matrix. SLS (segment-long-spacing) is one of the best understood alternative structures of collagen. SLS played an instrumental role in the original studies of collagen more than half a century ago that laid the foundation of nearly everything we know about collagen today. Despite being used mostly under in vitro conditions, the natural occurrence of SLS in tissues has also been reported. Here we will provide a brief overview of the major findings of the SLS and other structures of collagen based on a wealth of work published starting from the 1940s. We will discuss the factors that determine the stability and the structural specificity of the different molecular assemblies of collagen in light of the new studies using designed fibril forming collagen peptides. At the end of the chapter, we will summarize some recent discoveries of the alternative structures of collagen in tissues, especially those involved in pathogenic states. A revisit of SLS will likely inspire new understandings concerning the range of critical roles of fibrillar collagen in terms of its organizational diversity in the extracellular matrix.

Collagen Scaffolds Treated by Hydrogen Peroxide for Cell Cultivation

Collagen in the body is exposed to a range of influences, including free radicals, which can lead to a significant change in its structure. Modeling such an effect on collagen fibrils will allow one to get a native structure in vitro, which is important for modern tissue engineering. The aim of this work is to study the effect of free radicals on a solution of hydrogen peroxide with a concentration of 0.006–0.15% on the structure of collagen fibrils in vitro, and the response of cells to such treatment. SEM measurements show a decrease in the diameter of the collagen fibrils with an increase in the concentration of hydrogen peroxide. Such treatment also leads to an increase in the wetting angle of the collagen surface. Fourier transform infrared spectroscopy demonstrates a decrease in the signal with wave number 1084 cm⁻¹ due to the detachment of glucose and galactose linked to hydroxylysine, connected to the collagen molecule through the -C-O-C- group. During the first day of cultivating ASCs, MG-63, and A-431 cells, an increase in cell adhesion on collagen fibrils treated with H₂O₂ (0.015, 0.03%) was observed. Thus the effect of H₂O₂ on biologically relevant extracellular matrices for the formation of collagen scaffolds was shown.

Helical twist direction in the macrofibrils of keratin fibres is left handed

Macrofibrils, the main structural features within the cortical cells of mammalian hair shafts, are long composite bundles of keratin intermediate filaments (KIFs) embedded in a matrix of keratin-associated proteins. The KIFs can be helically arranged around the macrofibril central axis, making a cylinder within which KIF helical angle relative to macrofibril axis increases approximately linearly from macrofibril centre to edge. Mesophase-based self-assembly has been implicated in the early formation of macrofibrils, which first appear as liquid-crystal tactoids in the bulb of hair follicles. Formation appears to be driven initially by interactions between pre-keratinized KIFs. Differences in the nature of these KIF-KIF interactions could result in all macrofibrils being internally twisted in a single handedness, or a 50:50 mixture of handedness within each cortical cell. We data-mined 41 electron tomograms containing three-dimensional macrofibril data from previously published studies of hair and wool. In all 644 macrofibrils examined we found that within each tomogram all macrofibrils had the same handedness. We concluded that earlier reports of left- and right-handed macrofibrils were due to artefacts of imaging or data processing. A handedness marker was used to confirm (using re-imaged sections from earlier studies) that, in both human and sheep, all macrofibrils are left-handed around the macrofibril axis. We conclude that this state is universal within mammalian hair. This also supports the conclusion that the origin of macrofibril twist is the expression of chiral twisting forces between adjacent KIFs, rather than mesophase splay and bending forces relaxing to twisting forces acting within a confined space.

Development of Functional Bionanocomposites Using Cyanobacterial Polysaccharides

Cyanobacteria are regarded as very eco-friendly microreactors for the production of various biomolecules such as polysaccharides by fixing not only carbon but also nitrogen in water. Cyanobacterial polysaccharides having various functional groups such as hydroxyls, carboxyls, sulfates, etc. have the ability to interact with metals or inorganics, to create bionanocomposites. Sacran, a supergiant cyanobacterial anionic polysaccharide extracted from the extracellular matrix of Aphanothece sacrum which is mass-cultivated in freshwater, is mainly used to create functional bionanocomposites. Gel-type bionanocomposites of sacran with various metal cations are formed and showed photoresponsive functions. Metal recovery is performed from the sacran bionanocomposite gels. Sacran chains are complexed with multi-wall carbon nanotubes (MWCNT) to give viscose dispersion from which MWCNT bionanocomposites can be collected by electrophoresis. The MWCNT/sacran dispersion retains the capability of adsorbing various metal ions to form hardened hydrogel beads. Finally, natural inorganic sepiolite can be used for sacran bionanocomposites which show an efficient neodymium ion adsorption ability. Thus, cyanobacterial polysaccharides are useful for preparing eco-friendly and functional bionanocomposites with various hard materials.

Cholesteric liquid crystals in living matter

Liquid crystals play an important role in biology because the combination of order and mobility is a basic requirement for self-organisation and structure formation in living systems. Cholesteric liquid crystals are omnipresent in living matter under both in vivo and in vitro conditions and address the major types of molecules essential to life. In the animal and plant kingdoms, the cholesteric structure is a recurring design, suggesting a convergent evolution to an optimised left-handed helix. Herein, we review the recent advances in the cholesteric organisation of DNA, chromatin, chitin, cellulose, collagen, viruses, silk and cholesterol ester deposition in atherosclerosis. Cholesteric structures can be found in bacteriophages, archaea, eukaryotes, bacterial nucleoids, chromosomes of unicellular algae, sperm nuclei of many vertebrates, cuticles of crustaceans and insects, bone, tendon, cornea, fish scales and scutes, cuttlebone and squid pens, plant cell walls, virus suspensions, silk produced by spiders and silkworms, and arterial wall lesions. This article specifically aims at describing the consequences of the cholesteric geometry in living matter, which are far from being fully defined and understood, and discusses various perspectives. The roles and functions of biological cholesteric liquid crystals include maximisation of packing efficiency, morphogenesis, mechanical stability, optical information, radiation protection and evolution pressure.

Three-dimensional aspects of matrix assembly by cells in the developing cornea

Cell-directed deposition of aligned collagen fibrils during corneal embryogenesis is poorly understood, despite the fact that it is the basis for the formation of a corneal stroma that must be transparent to visible light and biomechanically stable. Previous studies of the structural development of the specialized matrix in the cornea have been restricted to examinations of tissue sections by conventional light or electron microscopy. Here, we use volume scanning electron microscopy, with sequential removal of ultrathin surface tissue sections achieved either by ablation with a focused ion beam or by serial block face diamond knife microtomy, to examine the microanatomy of the cornea in three dimensions and in large tissue volumes. The results show that corneal keratocytes occupy a significantly greater tissue volume than was previously thought, and there is a clear orthogonality in cell and matrix organization, quantifiable by Fourier analysis. Three-dimensional reconstructions reveal actin-associated tubular cell protrusions, reminiscent of filopodia, but extending more than 30 µm into the extracellular space. The highly extended network of these membrane-bound structures mirrors the alignment of collagen bundles and emergent lamellae and, we propose, plays a fundamental role in dictating the orientation of collagen in the developing cornea.

In the beginning there were soft collagen-cell gels: towards better 3D connective tissue models?

In the 40 years since Elsdale and Bard's analysis of fibroblast culture in collagen gels we have moved far beyond the concept that such 3D fibril network systems are better models than monolayer cultures. This review analyses key aspects of that progression of models, against a background of what exactly each model system tries to mimic. This story tracks our increasing understanding of fibroblast responses to soft collagen gels, in particularly their cytoskeletal contraction, migration and integrin attachment. The focus on fibroblast mechano-function has generated models designed to directly measure the overall force generated by fibroblast populations, their reaction to external loads and the role of the matrix structure. Key steps along this evolution of 3D collagen models have been designed to mimic normal skin, wound repair, tissue morphogenesis and remodelling, growth and contracture during scarring/fibrosis. As new models are developed to understand cell-mechanical function in connective tissues the collagen material has become progressively more important, now being engineered to mimic more complex aspects of native extracellular matrix structure. These have included collagen fibril density, alignment and hierarchical structure, controlling material stiffness and anisotropy. But of these, tissue-like collagen density is key in that it contributes to control of the others. It is concluded that across this 40 year window major progress has been made towards establishing a family of 3D experimental collagen tissue-models, suitable to investigate normal and pathological fibroblast mechano-functions.

Disorganized collagen scaffold interferes with fibroblast mediated deposition of organized extracellular matrix in vitro

Many tissue engineering applications require the remodeling of a degradable scaffold either in vitro or in situ. Although inefficient remodeling or failure to fully remodel the temporary matrix can result in a poor clinical outcome, very few investigations have examined in detail, the interaction of regenerative cells with temporary scaffoldings. In a recent series of investigations, randomly oriented collagen gels were directly implanted into human corneal pockets and followed for 24 months. The resulting remodeling response exhibited a high degree of variability which likely reflects differing regenerative/synthetic capacity across patients. Given this variability, we hypothesize that a disorganized, degradable provisional scaffold could be disruptive to a uniform, organized reconstruction of stromal matrix. In this investigation, two established corneal stroma tissue engineering culture systems (collagen scaffold-based and scaffold-free) were compared to determine if the presence of the disorganized collagen gel influenced matrix production and organizational control exerted by primary human corneal fibroblast cells (PHCFCs). PHCFCs were cultured on thin disorganized reconstituted collagen substrate (RCS--five donors: average age 34.4) or on a bare polycarbonate membrane (five donors: average age 32.4 controls). The organization and morphology of the two culture systems were compared over the long-term at 4, 8, and 11/12 weeks. Construct thickness and extracellular matrix organization/alignment was tracked optically with bright field and differential interference contrast (DIC) microscopy. The details of cell/matrix morphology and cell/matrix interaction were examined with standard transmission, cuprolinic blue and quick-freeze/deep-etch electron microscopy. Both the scaffold-free and the collagen-based scaffold cultures produced organized arrays of collagen fibrils. However, at all time points, the amount of organized cell-derived matrix in the scaffold-based constructs was significantly lower than that produced by scaffold-free constructs (controls). We also observed significant variability in the remodeling of RCS scaffold by PHCFCs. PHCFCs which penetrated the RCS scaffold did exert robust local control over secreted collagen but did not appear to globally reorganize the scaffold effectively in the time period of the study. Consistent with our hypothesis, the results demonstrate that the presence of the scaffold appears to interfere with the global organization of the cell-derived matrix. The production of highly organized local matrix by fibroblasts which penetrated the scaffold suggests that there is a mechanism which operates close to the cell membrane capable of controlling fibril organization. Nonetheless, the local control of the collagen alignment produced by cells within the scaffold was not continuous and did not result in overall global organization of the construct. Using a disorganized scaffold as a guide to produce highly organized tissue has the potential to delay the production of useful matrix or prevent uniform remodeling. The results of this study may shed light on the recent attempts to use disorganized collagenous matrix as a temporary corneal replacement in vivo which led to a variable remodeling response.

Order versus Disorder: in vivo bone formation within osteoconductive scaffolds

In modern biomaterial design the generation of an environment mimicking some of the extracellular matrix features is envisaged to support molecular cross-talk between cells and scaffolds during tissue formation/remodeling. In bone substitutes chemical biomimesis has been particularly exploited; conversely, the relevance of pre-determined scaffold architecture for regenerated bone outputs is still unclear. Thus we aimed to demonstrate that a different organization of collagen fibers within newly formed bone under unloading conditions can be generated by differently architectured scaffolds. An ordered and confined geometry of hydroxyapatite foams concentrated collagen fibers within the pores, and triggered their self-assembly in a cholesteric-banded pattern, resulting in compact lamellar bone. Conversely, when progenitor cells were loaded onto nanofibrous collagen-based sponges, new collagen fibers were distributed in a nematic phase, resulting mostly in woven isotropic bone. Thus specific biomaterial design relevantly contributes to properly drive collagen fibers assembly to target bone regeneration.

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.

Mechanical strain enhances survivability of collagen micronetworks in the presence of collagenase: implications for load-bearing matrix growth and stability

There has been great interest in understanding the methods by which collagen-based load-bearing tissue is constructed, grown and maintained in vertebrate animals. To date, the responsibility for this process has largely been placed with mesenchymal fibroblastic cells that are thought to fully control the morphology of load-bearing extracellular matrix (ECM). However, given clear limitations in the ability of fibroblastic cells to precisely place or remove single collagen molecules to sculpt tissue, we have hypothesized that the material itself must play a critical role in the determination of the form of structural ECM. We here demonstrate directly, using live, dynamic, differential interference contrast imaging, that mechanically strained networks of collagen fibrils, exposed to collagenase (Clostridium histolyticum), degrade preferentially. Specifically, unstrained fibrils are removed 'quickly', while strained fibrils persist significantly longer. The demonstration supports the idea that collagen networks are mechanosensitive in that they are stabilized by mechanical strain. Thus, collagen molecules (together with their complement enzymes) may comprise the basis of a smart, load-adaptive, structural material system. This concept has the potential to drastically simplify the assumed role of the fibroblast, which would need only to provide ECM molecules and mechanical force to sculpt collagenous tissue.

Development of the corneal stroma, and the collagen-proteoglycan associations that help define its structure and function

The cornea of the eye is a unique, transparent connective tissue. It is comprised predominantly of collagen fibrils, remarkably uniform in diameter and regularly spaced, organized into an intricate lamellar array. Its establishment involves a precisely controlled sequence of developmental events in which the embryonic cornea undergoes major structural transformations that ultimately determine tissue form and function. In this article, we will review corneal developmental dynamics from a structural perspective, consider the roles and interrelationships of collagens and proteoglycans, and comment on contemporary concepts and current challenges pertinent to developmental processes that result in an optically clear, mature cornea.

Influence of matrix processing on the optical and biomechanical properties of a corneal stroma equivalent

Interest in developing tissue-engineered cornea has increased with the decrease in the supply of donor tissue; however, the high strength and transparency of the cornea present a challenge. Both the collagen processing and crosslinking methods were hypothesized to influence the optical and biomechanical properties of collagen matrices, while regular surface topography was hypothesized to align stromal fibroblasts. Improved transparency and strength were observed when soluble tropocollagen was added to the insoluble collagen and when glucose-mediated ultraviolet (UV) crosslinking as opposed to dehydrothermal crosslinking was used. The fraction of transmittance of the collagen films fabricated from insoluble collagen and soluble tropocollagen and glucose-mediated UV crosslinking was initially 0.91 +/- 0.02 and 0.98 +/- 0.01 for the smooth films and 0.90 +/- 0.02 and 0.97 +/- 0.02 for the microgrooved films at 400 and 700 nm and was comparable to that of the native cornea, while the relaxed modulus and ultimate tensile strength ranged from 0.9 to 9.4 MPa and from 0.7 to 4.1 MPa, respectively, over the 3 weeks of culture and were initially at or below the range of values for the native cornea. These collagen scaffolds were significantly stronger and more transparent than previous scaffolds, and aligned stromal fibroblasts were observed on microgrooved surfaces.

Morphologic characterization of organized extracellular matrix deposition by ascorbic acid-stimulated human corneal fibroblasts

PURPOSE

To characterize the structure and morphology of extracellular matrix (ECM) synthesized by untransformed, cultured human corneal fibroblasts in long-term cultures.

METHODS

Human corneal stromal keratocytes were expanded in transwell culture in the presence of fetal bovine serum and a stable derivative of vitamin C. The cells were allowed to synthesize a fibrillar ECM for up to 5 weeks. Constructs were assessed by light (phase-contrast and differential interference-contrast) and transmission (standard and quick freeze/deep etch) microscopy.

RESULTS

Electron micrographs revealed stratified constructs with multiple parallel layers of cells and an extracellular matrix comprising parallel arrays of small, polydisperse fibrils (27-51 nm) that often alternate in direction. Differential interference contrast images demonstrated oriented ECM fibril arrays parallel to the plane of the construct, whereas quick-freeze, deep-etch micrographs showed the details of the matrix interaction with fibroblasts through arrays of membrane surface structures.

CONCLUSIONS

Human keratocytes, cultured in a stable vitamin C derivative, are capable of assembling extracellular matrix, which comprises parallel arrays of ECM fibrils. The resultant constructs, which are highly cellular, are morphologically similar to the developing mammalian stroma, where organized matrix is derived. The appearance of arrays of structures on the cell membranes suggests a role in the local organization of synthesized ECM. This model could provide critical insight into the fundamental processes that govern the genesis of organized connective tissues such as the cornea and may provide a scaffolding suitable for tissue engineering a biomimetic stroma.

Biomineralization of limpet teeth: A cryo-TEM study of the organic matrix and the onset of mineral deposition

The continuously growing limpet radula contains teeth at various stages of maturity and thus provides an excellent opportunity for studying the processes and mechanisms of their mineralization. We report here on our structural investigations of the pre-formed chitin matrix and the initial deposition and growth of goethite (alpha-FeOOH) crystals within the matrix. By using cryo-techniques, in which unstained sections of the teeth are examined in a frozen-hydrated state in a transmission electron microscope (TEM), we were able to characterize the process without introducing artifacts normally associated with the staining, dehydration, and embedding required for conventional TEM. The unmineralized matrix consists of relatively well ordered, densely packed arrays of chitin fibers, with only a few nanometers between adjacent fibers. There are clearly no pre-formed compartments that control goethite crystal size and shape; rather, crystals must push aside or engulf the fibers as they grow. By examining teeth nearly row-by-row around the onset of mineralization, we were able to image the first-formed mineral within the chitin matrix. These linear deposits of goethite appear to nucleate on the chitin fibers, which thus control the orientation of the crystals. Crystal growth, on the other hand, is apparently not influenced by the matrix, in contrast to many other biomineralization systems.

Irregular helicoids in leech cocoon membranes

Helicoids in the cocoon membrane of leeches Theromyzon tessulatum and Erpobdella punctata comprise a twisted superposition of layers, each containing a variable number of planes formed by unidirectional fibrils. Straight fibrils intersecting at different angles were displayed in tangential sections through the cocoon wall of each species. When the sectioning angle was below a certain value (i.e., the critical angle), bow-shaped lines apparent in oblique sections were replaced by a succession of layers containing straight fibrils, permitting a direct measurement of step-angle change between successive layers in a helicoid. By this methodology, we determined that no regularities exist in the succession of step-angles or in layer thicknesses within the cocoon membranes, but that the distribution of step-angles between layers was unique for each cocoon type.

Combining soft matter and soft chemistry: integrative chemistry towards designing novel and complex multiscale architectures

There is today a strong and emerging commitment toward designing complex and hierarchical architectures. In this context, great interest is appearing in the combination of soft chemistry and complex fluids. In any circumstance we can see that there is a strong affinity between and as they can interact together without disrupting their own function. For instance, a continuous network should emerge from the former while the latter should promote patterning effects. In this general context, the new concept of is proposed as well as its principle and main perspectives.

Two-dimensional ultrastructural elements lead to three-dimensional reconstruction of protuberances on the cocoon membrane of the leech, Theromyzon tessulatum

Protuberances on the cocoon surface of the leech, Theromyzon tessulatum, are roughly parallel rows of triangular prisms arranged equidistantly to each other on the outer surface of the cocoon membrane. The distance between neighboring protuberances is approximately 1.6 microm, the height approximately 0.5 microm and the semi-width approximately 0.3 microm. The fibrillar arrangement within the protuberance maintains some elements of the helicoids found within the cocoon membrane but a high proportion of large holes disrupt the symmetry of the protuberance ultrastructure. A procedure for 3D reconstruction of the protuberance using the complementarity between the paratangential and normal sections through the cocoon is presented. Our results demonstrate that the ultrastructure of protuberances show elements of a twisted fibrillar arrangement, but the demands of filling a narrow space ruled by acute angles appears to cause a high degree of ultrastructural disorganization.

Dense tissue-like collagen matrices formed in cell-free conditions

A new protocol was developed to produce dense organized collagen matrices hierarchically ordered on a large scale. It consists of a two stage process: (1) the organization of a collagen solution and (2) the stabilization of the organizations by a sol-gel transition that leads to the formation of collagen fibrils. This new protocol relies on the continuous injection of an acid-soluble collagen solution into glass microchambers. It leads to extended concentration gradients of collagen, ranging from 5 to 1000 mg/ml. The self-organization of collagen solutions into a wide array of spatial organizations was investigated. The final matrices obtained by this procedure varied in concentration, structure and density. Changes in the liquid state of the samples were followed by polarized light microscopy, and the final stabilized gel states obtained after fibrillogenesis were analyzed by both light and electron microscopy. Typical organizations extended homogeneously by up to three centimetres in one direction and several hundreds of micrometers in other directions. Fibrillogenesis of collagen solutions of high and low concentrations led to fibrils spatially arranged as has been described in bone and derm, respectively. Moreover, a relationship was revealed between the collagen concentration and the aggregation of and rotational angles between lateral fibrils. These results constitute a strong base from which to further develop highly enriched collagen matrices that could lead to substitutes that mimic connective tissues. The matrices thus obtained may also be good candidates for the study of the three-dimensional migration of cells.

Autonomous changes in the orientation of cortical microtubules underlying the helicoidal cell wall of the sunflower hypocotyl epidermis: spatial variation translated into temporal changes

Angles (lambda) at which parallel cortical microtubules (cMTs) were oriented with respect to the longitudinal direction were measured in Helianthus annuus hypocotyl epidermal cells. Histograms showing lambda frequencies in cell populations at the instant of epidermis fixation were obtained. Analysis of the histograms indicates that, in a particular position within a cell, the angle lambda changes periodically with time, i.e., there is a cycle of lambda change at that position. This cycle is most likely rotational rather than oscillatory, i.e., the change in lambda has a defined chirality (clockwise or counterclockwise). The full diversity of histograms can be consistently explained by rotational cycles with a variable velocity of lambda change, and with a cMT rebuilding stage taking place at a different phase of the cycle. The rotational cycles also provide the simplest explanation of cMT arrays in which the angle lambda changes along a cell (fixed) and no parallel orientation of cMTs is apparent at a certain position. This explanation assumes a gradient in the phase of the rotational cycle along the cell. The symmetry of the angular characteristics of the rotational cycle, with respect to the morphological directions in cells, leads to the concept that these directions typically represent the principal directions of a certain tensor quantity, which may control the cycling. Possible interactions between the rotational cycle of cMT reorientation and the helicoidal cycle during cell wall formation are discussed.

Bone matrix like assemblies of collagen: from liquid crystals to gels and biomimetic materials

Skeletal tissues associate in close interaction, a dense organic matrix and a mineral network. In bone, the major structural protein is type I collagen, associated with inorganic crystals of hydroxyapatite. The three-dimensional arrangement of collagen fibrils in compact bone forms regularly ordered networks and a parallel was evidenced between these structures and molecular assemblies described in liquid crystals. Similar structures are now obtained in vitro. Indeed, when purified type I collagen is highly concentrated in an acid soluble state, the protein spontaneously assembles into ordered liquid crystalline phases. After a sol/gel transition triggered by pH increase, biomimetic materials are formed which resemble the exact compact bone matrix architecture over distances reaching centimetres and more. The properties of these highly ordered materials will be reviewed recalling their supramolecular arrangement and the corresponding patterns when visualised in polarised light microscopy (birefringence) and transmission electron microscopy (TEM). The association of inorganic phases (amorphous silica) to form chiral hybrid materials will also be described so as the behaviour of cells (fibroblast adhesion and migration) when seeded on these dense biomimetic matrices.

Fibroblast populated dense collagen matrices: cell migration, cell density and metalloproteinases expression

Dense collagen matrices obtained by using the property of type I collagen to form liquid crystals at high concentrations, were shown to be colonized by human dermal fibroblasts (Biomaterials 23 (2002) 27). In order to evaluate them as possible tissue substitutes, we investigated in this study the mechanism of cell colonization. Fibroblasts were seeded at the surface of collagen matrices at concentrations of 5 and 40 b mg/ml. Cell density and migration were estimated from histological sections over 28 days within 500 microm thick matrices. At day 14, migration started in the 40 mg/ml matrices, attaining 320 microm in distance and 5500 cell/mm(3) in density at day 28. As zymography and western blot techniques demonstrated production of collagenase 1 (MMP1) and gelatinase A (MMP2) in culture medium, collagen hydrolysis was required for cells to penetrate the collagen network. Furthermore, the presence of MMP1 and MMP2 and their tissue inhibitors TIMP1 and TIMP2 was revealed by immunohistochemistry. We presently show that 40 mg/ml collagen matrices are colonized by human dermal fibroblasts and reach, at day 28, a density close to that measured in human dermis.

Biomimetism and bioinspiration as tools for the design of innovative materials and systems

Materials found in nature combine many inspiring properties such as sophistication, miniaturization, hierarchical organizations, hybridation, resistance and adaptability. Elucidating the basic components and building principles selected by evolution to propose more reliable, efficient and environment-respecting materials requires a multidisciplinary approach.

Collagen fibril form and function

The majority of collagen in the extracellular matrix is found in a fibrillar form, with long slender filaments each displaying a characteristic approximately 67?nm D-repeat. Here they provide the stiff resilient part of many tissues, where the inherent strength of the collagen triple helix is translated through a number of hierarchical levels to endow that tissue with its specific mechanical properties. A number of collagen types have important structural roles, either comprising the core of the fibril or decorating the fibril surface to give enhanced functionality. The architecture of subfibrillar and suprafibrillar structures (such as microfibrils), lateral crystalline and liquid crystal ordering, interfibrillar interactions, and fibril bundles is described. The fibril surface is recognized as an area that contains a number of intimate interactions between different collagen types and other molecular species, especially the proteoglycans. The interplay between molecular forms at the fibril surface is discussed in terms of their contribution to the regulation of fibril diameter and their role in interfibrillar interactions.

Phase transformations in a model mesenchymal tissue

Connective tissues, the most abundant tissue type of the mature mammalian body, consist of cells suspended in complex microenvironments known as extracellular matrices (ECMs). In the immature connective tissues (mesenchymes) encountered in developmental biology and tissue engineering applications, the ECMs contain varying amounts of randomly arranged fibers, and the physical state of the ECM changes as the fibers secreted by the cells undergo fibril and fiber assembly and organize into networks. In vitro composites consisting of assembling solutions of type I collagen, containing suspended polystyrene latex beads ( approximately 6 microm in diameter) with collagen-binding surface properties, provide a simplified model for certain physical aspects of developing mesenchymes. In particular, assembly-dependent topological (i.e., connectivity) transitions within the ECM could change a tissue from one in which cell-sized particles (e.g., latex beads or cells) are mechanically unlinked to one in which the particles are part of a mechanical continuum. Any particle-induced alterations in fiber organization would imply that cells could similarly establish physically distinct microdomains within tissues. Here we show that the presence of beads above a critical number density accelerates the sol-gel transition that takes place during the assembly of collagen into a globally interconnected network of fibers. The presence of this suprathreshold number of beads also dramatically changes the viscoelastic properties of the collagen matrix, but only when the initial concentration of soluble collagen is itself above a critical value. Our studies provide a starting point for the analysis of phase transformations of more complex biomaterials including developing and healing tissues as well as tissue substitutes containing living cells.

Liquid crystalline assemblies of collagen in bone and in vitro systems

Precise descriptions of the three-dimensional arrangements of collagen in bone are essential to understand the mechanical properties of this complex tissue. Transmission electron microscopy (TEM) analysis of decalcified human compact bone in section reveals characteristic patterns forming regular series of nested arcs. Such patterns are a direct consequence of an organization described as a twisted plywood and relate the distribution of collagen fibrils in osteons with that of molecules in cholesteric liquid crystals. The hypothesis that liquid crystalline properties are involved in the morphogenesis of dense collagen matrices was supported by data obtained in vitro. At a molecular level, acid-soluble collagen molecules spontaneously assemble, at concentrations of 50mg/ml or more, in precholesteric-banded patterns and cholesteric phases, identified by polarized light microscopy. In a more physiological context, these results were conforted, with the precursor molecule of collagen, procollagen, soluble at neutral pH. This protein spontaneously forms liquid crystalline precholesteric phases corresponding to banded patterns and birefringent cords. Stabilization of the liquid crystalline collagen, induced by pH modification and fibril formation, shows characteristic morphologies in TEM, which directly mimic arrays described in vivo. Undulating fibrils are indeed similar to crimp morphologies described in tendons and continuously twisting fibrils, and give rise to arced patterns similar to supra-molecular architectures identified in compact bone.

Chiral front propagation in liquid-crystalline materials: Formation of the planar monodomain twisted plywood architecture of biological fibrous composites

Biological fibrous composites commonly exhibit an architecture known as twisted plywood, which is similar to that of the cholesteric liquid-crystalline mesophases. The explanation for the structural similarity is that biological fibrous composites adopt a lyotropic cholesteric liquid-crystalline phase during their formation process. In this work, a mathematical model based on the Landau-de Gennes theory of liquid crystals has been developed to reproduce the process by which long chiral fibrous molecules form the twisted plywood structures observed in biological composites. The dynamics of the process was then further investigated by analytically solving a simplified version of the governing equations. Results obtained from the model are in good qualitative agreement with the theory of Neville [Biology of Fibrous Composites (Cambridge University Press, Cambridge, England, 1993)] who hypothesized the necessity of a constraining layer to lock the direction of the helical axis of the plywood in order to create a monodomain structure. Computational results indicate that the plywood architecture is obtained by a chiral front propagation process with a fully relaxed wake. The effects of chirality and concentration on the formation process kinetics are characterized.

Monodomain and polydomain helicoids in chiral liquid-crystalline phases and their biological analogues

Many natural composites exhibit an architecture known as twisted plywood which imparts to them a superior set of physical properties. The origin of this structure is complex and not yet understood. However, it is thought to involve a lyotropic chiral nematic liquid-crystalline mesophase. Indeed, striking structural similarities have been observed and reported between biological fibrous composites and ordered fluids. In this work, a mathematical model based on the Landau-de Gennes theory has been developed to investigate the role played by constraining surfaces in the structural development of a composite material that experiences a liquid-crystalline state during the early steps of its morphogenesis. The goal of this study is to verify the need for an initial constraining surface in the formation of monodomain twisted plywoods as hypothesized by Neville (Tissue & Cell 20, 133 (1988); Biology of Fibrous Composites (Cambridge University Press, 1993)). The numerical simulations qualitatively confirm this theory and highlight the important role that modelling of liquid-crystalline self-assembly plays in the study of tissue morphogenesis.

Assembly of collagen matrices as a phase transition revealed by structural and rheologic studies

We have studied the structural and viscoelastic properties of assembling networks of the extracellular matrix protein type-I collagen by means of phase contrast microscopy and rotating disk rheometry. The initial stage of the assembly is a nucleation process of collagen monomers associating to randomly distributed branched clusters with extensions of several microns. Eventually a sol-gel transition takes place, which is due to the interconnection of these clusters. We analyzed this transition in terms of percolation theory. The viscoelastic parameters (storage modulus G' and loss modulus G") were measured as a function of time for five different frequencies ranging from omega = 0.2 rad/s to 6.9 rad/s. We found that at the gel point both G' and G" obey a scaling law, with the critical exponent Delta = 0.7 and a critical loss angle being independent of frequency as predicted by percolation theory. Gelation of collagen thus represents a second order phase transition.

Polyamine structural effects on the induction and stabilization of liquid crystalline DNA: potential applications to DNA packaging, gene therapy and polyamine therapeutics

DNA undergoes condensation, conformational transitions, aggregation and resolubilization in the presence of polyamines, positively charged organic molecules present in all cells. Under carefully controlled environmental conditions, DNA can also transform to a liquid crystalline state in vitro. We undertook the present work to examine the ability of spermidine, N4-methylspermidine, spermine, N1-acetylspermine and a group of tetramine, pentamine and hexamine analogs of spermine to induce and stabilize liquid crystalline DNA. Liquid crystalline textures were identified under a polarizing microscope. In the absence of polyamines, calf thymus DNA assumed a diffused, planar cholesteric phase with entrapped bubbles when incubated on a glass slide at 37 degrees C. In the presence of spermidine and spermine, the characteristic fingerprint textures of the cholesteric phase, adopting a hexagonal order, were obtained. The helical pitch was 2.5 micro m. The final structures were dendrimeric and crystalline when DNA was treated with spermine homologs and bis(ethyl) derivatives. A cholesteric structure was observed when DNA was treated with a hexamine at 37 degrees C. This structure changed to a hexagonal dendrimer with fluidity on prolonged incubation. These data show a structural specificity effect of polyamines on liquid crystalline phase transitions of DNA and suggest a possible physiological function of natural polyamines.

Production of ordered collagen matrices for three-dimensional cell culture

The aim of this study was to produce collagen gels with controlled fibrillar order as matrices for cell culture. Their structural characterization and colonization by human dermal fibroblasts arc presently reported. Ordered matrices are obtained by using the property of type I collagen monomers to self-assemble in liquid crystalline arrays by slow evaporation of acidic solutions at high concentrations. Induction of fibrillogenesis concomittent with the stabilization of the supramolecular order is then obtained, within petri dishes, by gelation of the viscous preparations under ammoniac vapours. For comparison, dermal equivalents, in which collagen compaction depends on fibroblasts contraction, are made according to the method of Bell et al. (Proc. Natl. Acad. Sci. 76(3) (1979) 1274). The fibrillar arrangement of the collagen network in the samples is determined by polarizing optical microscopy and by transmission electron microscopy. Whereas dermal equivalents exhibit heterogeneous distributions of fibrils, two differents types of order are obtained in the stabilized liquid crystalline collagen samples, namely aligned, i.e. nematic, at 20 mg/ml, or crimped, i.e. precholesteric, at 40 mg/ml. The morphology and behaviour of fibroblasts seeded on the surface of the matrices are analysed from day 1 to day 21. The cells are viable, proliferate at the surface of ordered matrices and migrate up to 400 microm in depth. Production of concentrated and ordered collagen matrices provides new perspectives to study the behaviour of cells in a valorized three-dimensional context where the fibrillar organization becomes close to in vivo situations.

Building collagen molecules, fibrils, and suprafibrillar structures

Fibril-forming collagens are synthesized in precursor form, procollagens, with N- and C-terminal propeptide extensions. The C-propeptides direct chain association during intracellular assembly of the procollagen molecule from its three constituent polypeptide chains. Following or during secretion into the extracellular matrix, propeptides are cleaved by specific procollagen proteinases, thereby triggering fibril formation. The recent determination of the low-resolution structure of the C-propeptide trimer gives insights into the mechanism of procollagen chain association. In the extracellular matrix, the procollagen C-propeptides ensure procollagen solubility, while persistence of the N-propeptides controls fibril shape. Mechanisms for the control of fibril diameter are reviewed in terms of the radial packing model for collagen fibril structure. Finally, procollagen molecules have recently been shown to undergo liquid crystalline ordering in solution, prior to fibril assembly. This may provide an explanation for the liquid crystal-like suprafibrillar architectures of different connective tissues.