Investigations into the polymorphism of rat tail tendon fibrils using atomic force microscopy

New insight into the shortening of the collagen fibril D-period in human cornea

Collagen fibrils type I display a typical banding pattern, so-called D-periodicity, of about 67 nm, when visualized by atomic force or electron microscopy imaging. Herein we report on a significant shortening of the D-period for human corneal collagen fibrils type I (21 ± 4 nm) upon air-drying, whereas no changes in the D-period were observed for human scleral collagen fibrils type I (64 ± 4 nm) measured under the same experimental conditions as the cornea. It was also found that for the corneal stroma fixed with glutaraldehyde and air-dried, the collagen fibrils show the commonly accepted D-period of 61 ± 8 nm. We used the atomic force microscopy method to image collagen fibrils type I present in the middle layers of human cornea and sclera. The water content in the cornea and sclera samples was varying in the range of .066-.085. Calculations of the D-period using the theoretical model of the fibril and the FFT approach allowed to reveal the possible molecular mechanism of the D-period shortening in the corneal collagen fibrils upon drying. It was found that both the decrease in the shift and the simultaneous reduction in the distance between tropocollagen molecules can be responsible for the experimentally observed effect. We also hypothesize that collagen type V, which co-assembles with collagen type I into heterotypic fibrils in cornea, could be involved in the observed shortening of the corneal D-period.

Single-pulse CARS based multimodal nonlinear optical microscope for bioimaging

Noninvasive label-free imaging of biological systems raises demand not only for high-speed three-dimensional prescreening of morphology over a wide-field of view but also it seeks to extract the microscopic functional and molecular details within. Capitalizing on the unique advantages brought out by different nonlinear optical effects, a multimodal nonlinear optical microscope can be a powerful tool for bioimaging. Bringing together the intensity-dependent contrast mechanisms via second harmonic generation, third harmonic generation and four-wave mixing for structural-sensitive imaging, and single-beam/single-pulse coherent anti-Stokes Raman scattering technique for chemical sensitive imaging in the finger-print region, we have developed a simple and nearly alignment-free multimodal nonlinear optical microscope that is based on a single wide-band Ti:Sapphire femtosecond pulse laser source. Successful imaging tests have been realized on two exemplary biological samples, a canine femur bone and collagen fibrils harvested from a rat tail. Since the ultra-broad band-width femtosecond laser is a suitable source for performing high-resolution optical coherence tomography, a wide-field optical coherence tomography arm can be easily incorporated into the presented multimodal microscope making it a versatile optical imaging tool for noninvasive label-free bioimaging.

Generation of spatially aligned collagen fiber networks through microtransfer molding

The unique biomechanical properties of native tissue are governed by the organization and composition of integrated collagen and elastin networks. An approach for fabricating spatially aligned, fiber-reinforced composites with adjustable collagen fiber dimensions, layouts, and distribution within an elastin-like protein matrix yielding a biocomposite with controllable mechanical responses is reported. Microtransfer molding is employed for the fabrication of hollow and solid collagen fibers with straight or crimped fiber geometries. Collagen fibers (width: 2-50 μm, thickness: 300 nm to 3 μm) exhibit a Young's modulus of 126 ± 61 MPa and an ultimate tensile strength of 7 ± 3.2 MPa. As fiber networks within composite structures, straight fiber layouts display orthotropic responses with Young's modulus ranging from 0.95 ± 0.35 to 10.4 ± 0.5 MPa and tensile strength from 0.22 ± 0.08 to 0.87 ± 0.5 MPa with increasing fraction of collagen fibers (1-10%, v/v). In contrast, composites based on crimped fiber layouts exhibit strain-dependent stiffness with an increase in Young's modulus from 0.7 ± 0.14 MPa to 3.15 ± 0.49 MPa, at a specific transition strain. Through controlling the microstructure of engineered collagen fiber networks, a facile means is established to control macroscale mechanical responses of composite protein-based materials.

Short-term effect of cryotherapy on human scleral tissue by atomic force microscopy

This study investigated the inflammatory effect of cryotherapy application on collagen matrix network in human infant sclera. Donor scleral tissues taken from three infant patients divided into five groups: control group, sham-treated group, and three cryotreated groups. In the cryotherapy groups, the sclera was treated for 5 s, 10 s, and 20 s with -80°C freezing by a cryosurgical system. The cryotreated reactions were examined using double histological analysis with hematoxylin-eosin and Masson's trichrome, and atomic force microscopy analysis to quantify the diameter and D-banding of collagen fibrils. The infant scleral tissues treated with cryotherapy showed a significantly increased collagen density associated with inflammatory response (p < 0.05), increased fibril diameter (p < 0.005) compared to the scleral tissues in the control group. The results directly suggest that the cryotherapy affects the morphology of scleral collagen.

Effects of various salts on structural polymorphism of reconstituted type I collagen fibrils

Even though the behavior of collagen monomers self-assembling into fibrils is commonly understood in terms of hydrophobic and electrostatic interactions, the mechanisms that drive their ordered, longitudinal alignment to form a characteristic periodicity are still unclear. By introducing various salts into the collagen fibrillogenesis system, the intermolecular interactions of fibril formation were studied. We found that the pH and ion species play a critical role in forming native fibrils. Turbidity and electron microscopy revealed that collagen self-assembled into fibrils with a native banding pattern in the presence of multivalent ions. The isoelectric point of collagen in 12mM of NaCl is pH 8.9; it shifted to pH 9.4 and pH 7.0 after adding 10mM CaCl2 and Na2SO4, respectively. Native fibrils were reconstituted at pH 7.4 in salts with divalent anions and at pH 9.0 in salts with divalent cations. Circular dichroism spectroscopy showed a loss of helicity in the conditions where fibrillogenesis was unable to be achieved. The multivalent ions not only change the surface charge of collagen, but also facilitate the formation of fibrils with the native D-periodic banding pattern. It is likely that the binding multivalent ions induce the like-charge attraction and facilitate monomers' longitudinal registration to form fibrils with the native banding.

Micromechanical model of a surrogate for collagenous soft tissues: development, validation and analysis of mesoscale size effects

Aligned, collagenous tissues such as tendons and ligaments are composed primarily of water and type I collagen, organized hierarchically into nanoscale fibrils, microscale fibers and mesoscale fascicles. Force transfer across scales is complex and poorly understood. Since innervation, the vasculature, damage mechanisms and mechanotransduction occur at the microscale and mesoscale, understanding multiscale interactions is of high importance. This study used a physical model in combination with a computational model to isolate and examine the mechanisms of force transfer between scales. A collagen-based surrogate served as the physical model. The surrogate consisted of extruded collagen fibers embedded within a collagen gel matrix. A micromechanical finite element model of the surrogate was validated using tensile test data that were recorded using a custom tensile testing device mounted on a confocal microscope. Results demonstrated that the experimentally measured macroscale strain was not representative of the microscale strain, which was highly inhomogeneous. The micromechanical model, in combination with a macroscopic continuum model, revealed that the microscale inhomogeneity resulted from size effects in the presence of a constrained boundary. A sensitivity study indicated that significant scale effects would be present over a range of physiologically relevant inter-fiber spacing values and matrix material properties. The results indicate that the traditional continuum assumption is not valid for describing the macroscale behavior of the surrogate and that boundary-induced size effects are present.

Tailored laminin-332 alpha3 sequence is tethered through an enzymatic linker to a collagen scaffold to promote cellular adhesion

Surface modification techniques have been used to develop biomimetic scaffolds by incorporating cell adhesion peptides, which facilitates cell adhesion, migration and proliferation. In this study, we evaluated the cell adhesion properties of a tailored laminin-332 alpha3 chain tethered to a type I collagen scaffold using microbial transglutaminase (mTGase) by incorporating transglutaminase substrate peptide sequences containing either glutamine (peptide A: PPFLMLLKGSTREAQQIVM) or lysine (peptide B: PPFLMLLKGSTRKKKKG). The degree of cross-linking was studied by amino acid analysis following proteolytic digestion and the structural changes in the modified scaffold further investigated using Fourier transform infrared spectroscopy and atomic force microscopy. Fibroblasts were used to evaluate the cellular behaviour of the functionalized collagen scaffold. mTGase supports cell growth but tethering of peptide A and peptide B to the mTGase cross-linked collagen scaffold caused a significant increase in cell proliferation when compared with native and mTGase cross-linked collagen scaffolds. Both peptides enabled cell-spreading, attachment and normal actin cytoskeleton organization with slight increase in the cell proliferation was observed in peptide A when compared with the peptide B and mTGase cross-linked scaffold. An increase in the amount of epsilon(gamma-glutamyl) lysine isopeptide was observed in peptide A conjugated scaffolds when compared with peptide B conjugated scaffolds, mTGase cross-linked scaffold without peptide. Changes in D-spacing were observed in the cross-linked scaffolds with tethered peptides. These results demonstrate that mTGase can play a bifunctional role in both conjugation of the glutamine and lysine containing peptide sequences and also in the cross-linking of the collagen scaffold, thus providing a suitable substrate for cell growth.

Supramolecular structure of human aortic valve and pericardial xenograft material: atomic force microscopy study

Pericardial tissue (bovine or porcine), chemically stabilized with glutaraldehyde (GA), is widely used in cardiovascular surgery in the form of bioprosthetic valves. GA reacts with tissue proteins and creates inter- and intra-molecular cross-links, resulting in improved durability. However, tissue calcification and mechanical damage are still unresolved problems. The purpose of this study was to examine the surface topography of normal human aortic valve and GA-stabilized porcine pericardium tissue in order to gain comparative insight into supramolecular structure of both tissues. The analysis was focused on morphologic evaluation of collagen constituents of the tissues. Atomic force microscopy working in the contact mode in air was employed in the study. Considerable diversity in the spatial orientation of collagen fibrils for the human aortic valve and pericardial tissue were observed. It was found that different forms of collagen fibril packing, i.e. dense and "in phase" or loose, could have an impact on the collagen D-banding pattern. Stabilization with GA introduced significant changes in the surface topography of collagen fibrils and in their spatial organization on the tissue surface. Strong disturbance in the fibril's D-spacing was observed. It was also suggested, that the observed structural changes at the supramolecular level might make an important contribution to the progressive damage and calcification of the tissue. The presented results demonstrate that the AFM method can be useful for non-destructive structural characterization of heart valves and bioprosthetic heart valve material.

Micromechanical bending of single collagen fibrils using atomic force microscopy

A new micromechanical technique was developed to study the mechanical properties of single collagen fibrils. Single collagen fibrils, the basic components of the collagen fiber, have a characteristic highly organized structure. Fibrils were isolated from collagenous materials and their mechanical properties were studied with atomic force microscopy (AFM). In this study, we determined the Young's modulus of single collagen fibrils at ambient conditions from bending tests after depositing the fibrils on a poly(dimethyl siloxane) (PDMS) substrate containing micro-channels. Force-indentation relationships of freely suspended collagen fibrils were determined by loading them with a tip-less cantilever. From the deflection-piezo displacement curve, force-indentation curves could be deduced. With the assumption that the behavior of collagen fibrils can be described by the linear elastic theory of isotropic materials and that the fibrils are freely supported at the rims, a Young's modulus of 5.4 +/- 1.2 GPa was determined. After cross-linking with glutaraldehyde, the Young's modulus of a single fibril increases to 14.7 +/- 2.7 GPa. When it is assumed that the fibril would be fixed at the ends of the channel the Young's moduli of native and cross-linked collagen fibrils are calculated to be 1.4 +/- 0.3 GPa and 3.8 +/- 0.8 GPa, respectively. The minimum and maximum values determined for native and glutaraldehyde cross-linked collagen fibrils represent the boundaries of the Young's modulus.

Analysis of type I and IV collagens by FT-IR spectroscopy and imaging for a molecular investigation of skeletal muscle connective tissue

Many muscular diseases result from abnormal organization of connective tissue and/or collagen network formation. Only a few molecular imaging techniques are able to analyze this collagen network by differentiating collagen types. In this study, FT-IR spectroscopy was used to analyze type I and IV collagens, the most important compounds of which are perimysium and endomysium, respectively. Secondary structure of collagen types was determined by curve-fitting the 1,700-1,480 cm(-1) spectral interval. Type I collagen could be differentiated from type IV by its higher amounts of triple helix and alpha-helix, but lower amounts of beta-sheets (P < 0.01). FT-IR imaging was then used to determine structural features of perimysium and endomysium collagen network in bovine Flexor carpi radialis muscle. Secondary structure of proteins contained in perimysium and endomysium was found to be very close to type I and IV collagens, respectively. FT-IR spectroscopy and imaging are thus analytical tools that might be used for investigating biodistribution and assembly of collagen types in connective tissues.

Tannic acid-stabilized pericardium tissue: IR spectroscopy, atomic force microscopy, and dielectric spectroscopy investigations

Infrared (IR) spectroscopy, atomic force microscopy (AFM), and dielectric spectroscopy methods were employed to study structural and dynamic changes in the tannic acid (TA)-stabilized pericardium tissue. Chemically stabilized pericardium tissue is widely used in construction of the tissue derived bioprostheses. IR spectra recorded in the range 400-4000 cm-1 allowed us to recognize different types of TA-collagen interactions. Formation of hydrogen bonds between amine as well as amide NH groups from collagen and hydroxyl groups of TA was analyzed. The AFM imaging showed that the stabilization procedure with TA introduces considerable changes in both surface topography and thickness of collagen fibrils as well as in fibril arrangement on the tissue surface. It was found, that these structural changes have an impact on the dielectric behavior of the TA-stabilized tissue. The dielectric spectra for the native and TA-stabilized tissues were measured in the frequency and temperature ranges of 10(-1) -10(7) Hz and 120-270 K, respectively. The dielectric spectra revealed the relaxation process due to orientation of bound water supplemented by the fluctuation of collagen polar side groups. At the temperatures above approximately 210 K, the relaxation due to ion migration process was observed. It was found that both relaxation processes were influenced by the TA-collagen interaction.

Microfibrillar structure of type I collagen in situ

The fibrous collagens are ubiquitous in animals and form the structural basis of all mammalian connective tissues, including those of the heart, vasculature, skin, cornea, bones, and tendons. However, in comparison with what is known of their production, turnover and physiological structure, very little is understood regarding the three-dimensional arrangement of collagen molecules in naturally occurring fibrils. This knowledge may provide insight into key biological processes such as fibrillo-genesis and tissue remodeling and into diseases such as heart disease and cancer. Here we present a crystallographic determination of the collagen type I supermolecular structure, where the molecular conformation of each collagen segment found within the naturally occurring crystallographic unit cell has been defined (P1, a approximately 40.0 A, b approximately 27.0 A, c approximately 678 A, alpha approximately 89.2 degrees , beta approximately 94.6 degrees , gamma approximately 105.6 degrees ; reflections: 414, overlapping, 232, and nonoverlapping, 182; resolution, 5.16 A axial and 11.1 A equatorial). This structure shows that the molecular packing topology of the collagen molecule is such that packing neighbors are arranged to form a supertwisted (discontinuous) right-handed microfibril that interdigitates with neighboring microfibrils. This interdigitation establishes the crystallographic superlattice, which is formed of quasihexagonally packed collagen molecules. In addition, the molecular packing structure of collagen shown here provides information concerning the potential modes of action of two prominent molecules involved in human health and disease: decorin and the Matrix Metallo-Proteinase (MMP) collagenase.

Impact of fibronectin assembly on platelet thrombus formation in response to type I collagen and von Willebrand factor

Plasma fibronectin enhances platelet thrombus formation on surfaces coated with collagen. We investigated the role of fibronectin assembly in this process. Platelets adherent to fibrillar type I collagen, but not platelets adherent to von Willebrand factor (VWF), supported assembly of plasma fibronectin under static conditions. At a shear rate of 1250 s(-1), platelets adherent to collagen assembled coperfused plasma fibronectin and formed larger thrombi in a fibronectin-concentration-dependent manner, with a maximum effect at 250 mug/mL. Enhanced thrombus formation on collagen was blocked by a peptide that binds to the N-terminal region of fibronectin and inhibits fibronectin assembly. Cross-linking of fibronectin to collagen prior to exposure to platelets had no effect on thrombus formation. Collagen-induced platelet thrombus formation at a shear rate of 5000 s(-1) required coperfusion with VWF and did not result in assembly of coperfused fibronectin. VWF-mediated increase in platelet thrombi on collagen was not enhanced and indeed was somewhat attenuated by coperfused fibronectin at a shear rate of 5000 s(-1). These results indicate that, at moderately high but not very high shear rates, fibronectin assembly in platelet aggregates that form in response to collagen enhances thrombus formation and serves as an alternative to VWF-mediated enhancement.

The mechanical function and structure of aortic microfibrils in the lobster Homarus americanus

Marfan syndrome, a connective tissue disorder affecting the cardiovascular system, is caused by mutations of fibrillin-based microfibrils. These mutations often affect the calcium-binding domains, resulting in structural changes to the proteins. It is hypothesized that these Ca+2 binding sites regulate the structure and mechanical properties of the microfibrils. The mechanical properties of fresh and extracted lobster aortic rings in calcium solutions (1, 13 and 30 mM Ca+2) were measured. Samples underwent amino acid compositional analysis. Antibodies were produced against the material comprising extracted aortic rings. The ultrastructure of strained and unstrained samples was examined using transmission electron microscopy. Calcium level altered the tangent modulus of fresh vessels. These rings were significantly stiffer when tested at 30 mM Ca+2 compared to rings tested at 1 mM Ca+2. Amino acid comparisons between extracted samples, porcine and human fibrillin showed compositional similarity. Immunohistochemical analysis showed that antibodies produced against the material in extracted samples localized to the known microfibrillar elements in the lobster aorta and cross-reacted with fibrillin microfibrils of mammalian ciliary zonules. Ultrastructurally, vessels incubated in low calcium solutions showed diffuse interbead regions while those incubated in physiological or high calcium solutions showed interbead regions with more defined lateral edges.

Atomic Force and Confocal Microscopic Studies of Collagen-Cell-Based Scaffolds for Vascular Tissue Engineering

Collagen is the most used naturally occurring scaffold material. It’s a structural protein ubiquitous among mammalian. The ability of collagen type I to host different cell phenotype in vitro and its low antigenecity in vivo are well known. However, the principal drawback of collagenbased materials consists in their low mechanical properties. For vascular tissue engineering this represents a major limit, as the aim is to mimic the structure of a native vessel, which is known to be resistant and viscoelastic. Moreover, vascular cells are known to be susceptible in vivo to reorganize the matrix in which they proliferate. Therefore, the aim of this project is to study the micro structural organization of collagen-based scaffolds, and to assess the interactions between collagen and smooth muscle cells during regeneration. This knowledge will then allow the development of appropriate strategies to tailor the microstructure of the scaffold and its properties. Smooth muscle cells (SMCs) were selected to study the interactions between cells and matrix during the proliferation. Atomic Force Microscopy (AFM) in dry state in tapping mode and Confocal Laser Scanning Microscopy (CLSM) in reflection mode were used to investigate the microstructure of the scaffold. For the former technique cells were seeded on top of the collagen gel after jellification, while for the latter, cells were embedded into the collagen gel and stained with Rhodamine. The contact points between matrix and cells were investigated, as well as the capacity of vascular cells to induce a structural reorganization of collagen fibrils in the scaffold.

Atomic force microscopy on human trabecular bone from an old woman with osteoporotic fractures

AFM images were taken of the exterior surface of a single trabecula, extracted from a human femoral head removed during surgery for a hip fracture in an old women with former fractures. The images showed a dense structure of bundled collagen fibrils banded with 67 nm periodicity. Bundles were seen to run in parallel in layers confirming the collagen structure seen by other techniques. Single collagen fibrils were seen to cross the bundles, thus forming cross-links between neighboring bundles of collagen fibrils. Some of these crossing fibrils did not have the 67 nm band pattern and their dimensions were about half compared to the neighboring collagen fibrils. Very little mineral was found on the surface of the trabecula. An AFM image of a fracture plane was also displayed. The trabecula was extracted from a region close to the hip fracture. However, there were in this case no obvious features in the images that could be linked directly to osteoporosis, but altered collagen banding and collagen protrusions may alter mechanical competence. A path to extensive studies of the nanometer scale structure of bone was demonstrated.

Atomic force microscopy investigation of chemically stabilized pericardium tissue

Native and chemically stabilized porcine pericardium tissue was imaged by the contact mode atomic force microscopy (AFM), in air. Chemically stabilized pericardium is used as a tissue-derived biomaterial in various fields of the reconstructive and replacement surgery. Collagen type I is the main component of the fibrous layer of the pericardium tissue. In this study, the surface topography of collagen fibrils in their native state in tissue and after chemical stabilization with different cross-linking reagents: glutaraldehyde (GA), dimethyl suberimidate (DMS) and tannic acid (TA) was investigated. It has been found that chemical stabilization causes considerable changes in the surface topography of collagen fibrils as well as in the spatial organization of the fibrils within the tissue. The observed changes in the D-spacing pattern of the collagen fibril correspond to the formation of intrafibrilar cross-links, whereas formation of interfibrilar cross-links is mainly responsible for the observed tangled spatial arrangement of fibrils and crimp structure of the tissue surface. The crimp structure was distinctly seen for the GA cross-linked tissue. Surface heterogeneity of the cross-linking process was observed for the DMS-stabilized tissue. SDS-PAGE electrophoresis was performed in order to evaluate the stabilization effect of the tissues treated with the cross-linking reagents. It has been found that stabilization with DMS, GA or TA enhances significantly the tissue resistance to SDS/NaCl extraction. The relation between the tissue stability and changes in the topography of the tissue surface was interpreted in terms of different nature of cross-links formed by DMS, GA and TA with collagen.

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.

Force spectroscopy of collagen fibers to investigate their mechanical properties and structural organization

Tendons are composed of collagen and other molecules in a highly organized hierarchical assembly, leading to extraordinary mechanical properties. To probe the cross-links on the lower level of organization, we used a cantilever to pull substructures out of the assembly. Advanced force probe technology, using small cantilevers (length <20 microm), improved the force resolution into the sub-10 pN range. In the force versus extension curves, we found an exponential increase in force and two different periodic rupture events, one with strong bonds (jumps in force of several hundred pN) with a periodicity of 78 nm and one with weak bonds (jumps in force of <7 pN) with a periodicity of 22 nm. We demonstrate a good correlation between the measured mechanical behavior of collagen fibers and their appearance in the micrographs taken with the atomic force microscope.

Influence of the degradation of the organic matrix on the microscopic fracture behavior of trabecular bone

In recent years, the important role of the organic matrix for the mechanical properties of bone has become increasingly apparent. It is therefore of great interest to understand the interactions between the organic and inorganic constituents of bone and learn the mechanisms by which the organic matrix contributes to the remarkable properties of this complex biomaterial. In this paper, we present a multifaceted view of the changes of bone's properties due to heat-induced degradation of the organic matrix. We compare the microscopic fracture behavior (scanning electron microscopy; SEM), the topography of the surfaces (atomic force microscopy; AFM), the condition of bone constituents [X-ray diffraction (XRD), thermogravimetric analysis (TGA), and gel electrophoresis], and the macromechanical properties of healthy bovine trabecular bone with trabecular bone that has a heat-degraded organic matrix. We show that heat treatment changes the microfracture behavior of trabecular bone. The primary failure mode of untreated trabecular bone is fibril-guided delamination, with mineralized collagen filaments bridging the gap of the microcrack. In contrast, bone that has been baked at 200 degrees C fractures nondirectionally like a brittle material, with no fibers spanning the microcracks. Finally, bone that has been boiled for 2 h in PBS solution fractures by delamination with many small filaments spanning the microcracks, so that the edges of the microcracks become difficult to distinguish. Of the methods we used, baking most effectively weakens the mechanical strength of bone, creating the most brittle material. Boiled bone is stronger than baked bone, but weaker than untreated bone. Boiled bone is more elastic than untreated bone, which is in turn more elastic than baked bone. These studies clearly emphasize the importance of the organic matrix in affecting the fracture mechanics of bone.

High-resolution AFM imaging of intact and fractured trabecular bone

Nanoscale structural analyses of biomineralized materials can frequently help elucidate important structure-function relationships in these complex organic-inorganic composites. Atomic force microscope (AFM) imaging of the exterior surface of trabecular bone reveals a densely woven structure of collagen fibrils, banded with a 67-nm periodicity, and densely packed mineral plates. The mineral plates on the collagen fibrils overlap and exhibit a large range of plate diameters from 30 to 200 nm. On the collagen fibrils, small nodular features, spaced 20-30 nm, run perpendicular to the fibrils. In some cases, these nodules are also seen on filaments extending between collagen fibrils. We hypothesize that these protrusions are noncollagenous proteins such as proteoglycans and may have collapsed into compact structures when the sample was dried. AFM images of pristine fractured surfaces reveal a dense array of mineral plates. In a few isolated locations, short sections of bare collagen fibrils are visible. In other regions, the existence of the underlying collagen fibrils can be inferred from the linear patterns of the mineral plates. Fractured samples, rinsed to remove mineral plates, reveal separated collagen fibrils on the fractured surfaces. These fibrils are often covered with protrusions similar to those observed on the exterior surfaces but are less organized. In addition, as on the exterior surfaces, there are sometimes small filaments extending between neighboring collagen fibrils. These studies provide important insights into the nanostructured architecture of this complex biocomposite.