A continuous damage random thresholds model for simulating the fracture behavior of nacre

This study investigates the fracture properties of nacre using a discrete lattice model based on continuous damage random threshold fuse network. The discrete lattice topology of the model is based on nacre's unique brick and mortar microarchitecture. The mechanical behavior of each of the bonds in the discrete lattice model is governed by the characteristic modular damage evolution of the organic matrix and the mineral bridges between the aragonite platelets. The numerical results obtained using this simple discrete lattice model are in very good agreement with the previously obtained experimental results, such as nacre's stiffness, tensile strength, and work of fracture. The analysis indicates that nacre's superior toughness is a direct consequence of ductility (maximum shear strain) of the organic matrix in terms of repeated unfolding of protein molecules, and its fracture strength is a result of its ordered brick and mortar architecture with significant overlap of the platelets, and shear strength of the organic matrix.     

Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction analyses of mineral and organic matrix during heating of mother of pearl (nacre) from the shell of the mollusc Pinctada maxima.

Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction patterns were used to analyze the mineral structure and organic matrix composition and thermal behavior of the internal nacreous layer (mother of pearl or nacre) of the shell of the giant oyster Pinctada maxima. Nacre is a natural biomaterial with osteogenic properties. The mineral of nacre is calcium carbonate crystallized as aragonite and it is highly crystallized. The FT-IR spectra showed amide, amine, and carboxylic acid groups in the organic matrix of the whole (organic and mineral) nacreous layer, with the HCO(-)(3) groups possibly at the organic-mineral interface. The insoluble organic matrix remaining after decalcification contained amide, amine, and carboxylic groups. The heated aragonite mineral structure of nacre underwent two transformations (X-ray diffraction), aragonite to calcite at 300-400 degrees C, and calcite to calcium oxide (CaO) at 500-600 degrees C. The organic matrix of nacre was destroyed around 550-600 degrees C, the same temperature as the calcite to CaO transformation, revealing the great thermal stability of the organic matrix and the organic-mineral bonding. This could be an useful feature for the in vivo use of this natural biomaterial as an implant.     

Organic interlamellar layers,mesolayers and mineral nanobridges: Contribution to strength in abalone (Haliotis rufescence) nacre

The contributions of mesolayers, organic interlamellar layers and nanoasperities/mineral bridges to the strength of nacre from red abalone (Haliotis rufescens) shell nacre are investigated. Samples were demineralized and deproteinized to separate the organic and mineral components, respectively. Tensile tests were performed on both the isolated organic constituent and the isolated mineral. The strength of the isolated organic component suggests that growth bands play an important role in the mechanical behavior as they are thick regions of protein that are a significant fraction (∼0.4) of the total organic content. The thickness variation of the nacre tablets was measured and found to be a small fraction of the mean tablet thickness (0.568 μm); the standard deviation is 26 nm, indicating that the wedge mechanism of toughening does not operate in the nacre investigated. Results obtained from the isolated mineral validate the importance of the organic constituent as the mechanical properties decline greatly when the organic component is removed. The results presented herein add to the understanding of the mechanical response of the organic interlayers and growth bands and their effect on the toughness of the abalone nacre.     

Structural and mechanical properties of the organic matrix layers of nacre

The type of nanostructure referred to in biomineralization as a mineral bridge has been directly observed and measured in the organic matrix layers of nacre by transmission electron microscopy and scanning electron microscopy. Statistical analysis provides the geometric characteristics and a distribution law of the mineral bridges in the organic matrix layers. Experiments reveal that the nanostructures significantly influences the mechanical properties of the organic matrix layers. In addition, the mechanical analysis illustrates the effects of the nanostructures on the behaviors of the organic matrix layers, and the analytical results explain the corresponding experimental phenomena fairly well. The present study shows that the mineral bridges play a key role in the mechanical performances of the organic matrix layers of nacre. The results obtained provide a guide to the interfacial design of synthetic materials.     

Morphologie der segmentierten polyetheresteramide auf der basis von polyamid 12 und oligotetrahydrofuran

Polyetheresteramides (PEEA) on the basis of polyamide 12 and oligotetrahydrofuran (OTHF) may be referred to as intrinsically plasticized and intrinsically impact strength toughened thermoplastic elastomers. This is due to their special 4-phase morphology consisting of two pure crystalline and two mixed amorphous phases, which was completely analysed by various TEM preparation techniques. PEEA containing more than 50% PA 12 hard segments exhibit a space filling dendritic superstructure of lamellar crystallized hard segments. In the range of 50 – 30% PA 12, the dendrites become more and more isolated, and below 30% PA 12 only single lamellae can be observed. The matrix in all PEEA is an amorphous, PA 12-rich mixed phase (“OTHF-plasticized PA 12”) located between the hard segment lamellae. An amorphous OTHF-rich mixed phase is very finely dispersed in PEEA and acts as an intrinsically impact strength toughening modifier. Lamellar crystallized soft segments could be imaged by TEM in this dispersed phase, but only in PEEA products with a higher molecular OTHF and at sufficient low temperatures.     

Mechanical properties and osteoconductivity of new bioactive composites consisting of partially crystallized glass beads and poly(methyl methacrylate)

New bioactive composites consisting of partially crystallized glass beads as inorganic fillers and poly(methyl methacrylate) (PMMA) as an organic matrix were developed. Two kinds of partially crystallized glass beads, designated Cry820 and Cry850, were newly prepared by the heating of MgO-CaO-SiO(2)-P(2)O(5) glass at 820 and 850 degrees C, respectively. The glass beads were mixed with PMMA to form two new composites designated Cry820C and Cry850C, respectively. The goal of this study was to produce a highly osteoconductive and mechanically strong composite cement with these new fillers. A previously reported composite cement designated AWC, which was composed of apatite- and wollastonite-containing glass ceramic (AW-GC) as a powder filler and the same PMMA polymer used in the new composites, was used as a reference material. The quantity of filler added to each composite was 70 wt %. The bending strength of Cry820C was significantly higher than that of Cry850C. Composites were packed into intramedullary canals of rat tibiae to evaluate their osteoconductivity, as determined by an affinity index. The affinity index, which equaled the length of bone in direct contact with the composite surface expressed as a percentage of the total length of the composite surface, was calculated for each composite. The rats were euthanized at 4, 8, and 25 weeks after implantation. At each time interval studied, Cry820C showed a significantly higher affinity index than AWC up to 25 weeks after implantation. Cry850C showed a significantly higher affinity index than AWC up to 8 weeks and a higher affinity index than AWC at 25 weeks, although the difference was not significant. The values for each composite increased significantly with time up to 25 weeks. Our study revealed that the higher osteoconductivity of the new composites was due to the larger quantity of the glassy phase of the crystallized glass beads at the composite surface and the lower solubility of the PMMA powder to methyl methacrylate monomer. In addition, the spherical shape of the crystallized glass beads gave the new composites strong enough mechanical properties to be useful under weight-bearing conditions. The new composites show promise as alternatives, with improved properties, to conventional PMMA bone cement.     

Influence of Crystallization Conditions on the Crystal Order and Dynamic Thermal Behavior of Nylon 1010

Nylon 1010 crystals with different size and perfection were prepared under various crystallization conditions. Temperature‐varied X‐ray diffraction and transmission electron microscopy were used to investigate the structure and morphology of these crystals. The pseudo‐hexagonal phase (also known as γ modification) of nylon 1010 was obtained by heating the quenched sample to a high temperature, where this phase is stable. Cooling of the γ modification sample immediately resulted in a Brill transition into the triclinic form. The WAXD and TEM results showed that the crystal structure at room temperature as well as its thermal behavior on heating is strongly influenced by the crystallization conditions. No Brill transition temperature was observed for large or perfect crystals in both the heating and the cooling process, i. e. for melt‐crystallized spherulitic crystals and lamellar single crystals grown from dilute solution. This shows that the Brill transition depends on the size and perfection of the crystal.     

N-terminal amino-sequencing of camel (Camelus dromedarius) luteinizing-hormone alpha and beta-subunits

The subunits of luteinizing hormone from the Camelus dromedarius (CamLH) have been separated by reverse phase HPLC after reduction and alkylation of their disulfide bridges. The N-terminal amino-acid sequencing of the alpha and beta subunits has been performed up to the 53rd and 67th residue respectively (i.e. more than half of each polypeptide chain). These sequences have been compared to those of LH from other mammalian species in order to estimate the phylogenetic divergence of LH in this species and in order to point out characteristic features of its primary structure that can be related to its physico-chemical properties.     

In situ study of partially crystallized Bioglass and hydroxylapatite in vitro bioactivity using atomic force microscopy

The present work investigates, in situ, the in vitro bioactivity of partially crystallized 45S5 Bioglass (BG) as a function of immersion time in a simulated body fluid (SBF) using atomic force microscopy (AFM). The results obtained for the crystallized BG were compared to those of hydroxyapatite c- and a-faces. The calcium phosphate layer grows on the crystallized 45S5 B by multiple two-dimensional nucleation and fusion of these two-dimensional islands, which is essentially the same mode as for the hydroxyapatite c-face. The surface of the crystallized 45S5 BG was almost fully covered with a dense and compact calcium phosphate layer after 24 h. The calcium phosphate formation on the crystallized BG arises from a low surface energy of the surface layer and/or an effect of the layer to lower the resistance when the growth units of calcium phosphate incorporate into the growing island. These results indicate that the crystallized 45S5 BG is suitable to be used as a filler for polymeric matrix bioactive composites, as it maintains a high bioactivity associated with a stiffer behavior (as compared to standard BG).     

Study of the elastic fibres' framework of the rat lung

A combination of intravascular resin injection and formic acid incubation was used to study the three-dimensional organization of the elastic fibres of the adult rat lung by scanning electron microscopy (SEM). After SEM observations, the same samples were further processed for transmission electron microscopy (TEM) in order to confirm the presence of the elastic fibres and to complement some aspects of its surface morphology observed under the SEM. Complementary studies by light microscopy (LM) and TEM using specific histochemical methods for the elastic fibres were also performed. The SEM study clearly demonstrated that the cast of the microvasculature acted as a scaffold to preserve the in vivo arrangement of the easily collapsible elastic tissue. The methodology used allowed the observation of a fine framework of elastic fibres representing remnants of the alveolar walls in close association with the capillaries interwoven with the network of elastin. Each thick elastic fibre was composed of a bundle of thin fibres. Some of these thin fibres separated from the main fibre, join other fibres, giving the appearance of an anastomosing net. The interwoven network of elastin and its proximity with the capillaries suggests that the distensibility of the alveolar wall should contribute to the subtle rhythmical change of the alveolar microcirculation at each respiratory movement. On the sub-pleural region of the lung, the elastic fibres were observed forming a continuous and fine mesh network. The elastic fibres linking the walls of the intrapulmonary conducting airways, the vessels wall and the alveolar and sub-pleural elastic network establish an interrelated and interlaced continuous framework, certainly with great physiological implications to the overall process of the mechanics of the lung respiratory function. The methodology applied was a useful tool in order to study the spatial organization of the pulmonary elastic fibres, its branching and close relation with the other lung structures.     

Dentin matrix proteins: composition and possible functions in calcification

Dentin may be regarded as a mineralized connective tissue. In its composition as well as its mode of formation, dentin exhibits several similarities with bone, but also definite differences. The dentin organic phase, the matrix, determines its morphology and is believed to be instrumental in the formation of the mineral phase. A fibrous web of collagen type I dominates the organic matrix. Also, minor amounts of other collagen types may be present. The noncollagenous proteins (NCPs), which constitute about 10% of the matrix, fall into several categories: phosphoproteins, Gla-proteins of the osteocalcin type as well as matrix Gla-protein, proteoglycans, different acidic glycoproteins, and serum proteins. Some of these NCPs have unique chemical compositions that give them specific properties. Dentinogenesis occurs by two simultaneous processes: the formation of a collagenous web in predentin, which is followed by the formation of the inorganic phase at the mineralization front. The composition of the predentin organic matrix differs from that of dentin, as some NCP components are secreted extracellularly just in advance of the mineralization front. In addition, some constituents of predentin seem to be metabolized. The NCPs may be important to several processes during dentinogenesis. Much evidence indicates that noncollagenous components in the matrix are instrumental in mineral formation. New data show that polyanionic NCPs, such as phosphoprotein and proteoglycans, when immobilized on a solid support, induce apatite formation under physiological conditions. These data indicate that polyanionic NCPs may function as mineral nucleators in vivo. They may also act as size and rate regulators for crystallization and promote calcium ion diffusion in the tissue. In addition, NCPs may regulate collagen fibrillogenesis.     

Matrix crystals in cytologic urine specimens: Observations on their mineral composition by energy dispersive X-ray microanalysis and morphologic scanning electron microscopy

Crystals consisting by light microscopy of organic matrix (matrix crystals) encountered in cytologic urine specimens of 8 patients were examined for mineral phase components by scanning electron microscopy with energy dispersive X-ray microanalysis (SEM-EDX) and by morphologic scanning electron microscopy (SEM) performed separately in four of the eight cases. Whenever possible (three cases) mineralized crystals present in these specimens were examined separately by SEM-EDX for comparison of mineral phase composition with that of the corresponding matrix forms. Although by SEM-EDX components of matrix, glass and slide preparation media interfere with the precise estimation of the mineral phase components, the results of this method supported by the SEM morphology suggest that crystals consisting of organic matrix include a mineral phase, the lattice structure of which provides them from the early stages of formation with the characteristic morphology of the fully mineralized forms. This also suggests that organic matrix plays a role in the nucleation of minerals during the formation of certain urinary crystals. Diagn Cytopathol 1994; 11:38–46. © 1994 Wiley-Liss, Inc.     

Immunohistochemical identification of MMP-2 and MMP-9 in human dentin: correlative FEI-SEM/TEM analysis

Matrix metalloproteinases (MMPs) are a family of peptidases trapped within mineralized dentin matrix and involved with degradation of the extracellular matrix components in hybrid layers and caries. Despite their identification through indirect evidences and biochemical assays, MMP-2 and -9 have not been localized within the human dentin extracellular organic matrix. Thus, this study aimed to assess the localization and distribution of MMP-2 and -9 in human dentin organic matrix by employing a correlative field emission in-lens-scanning electron microscopy (FEI-SEM) and transmission electron microscopy (TEM) immunohistochemical approach. Dentin specimens were submitted either to a preembedding or to a postembedding immunolabeling technique using primary monoclonal antibodies anti-MMP-2 and anti-MMP-9 and exposed to a secondary antibody conjugated with gold nanoparticles. MMP-2 and -9 labelings were identified in the demineralized dentin matrix as highly electron-dense gold particles dispersed on the collagen fibrils. Correlative FEI-SEM/TEM observations confirmed that MMP-2 and MMP-9 are endogenous components of the human dentin organic matrix and revealed the three-dimensional relationship between these proteinases and the collagen fibrils, showing that both antibodies yielded a similar labeling pattern. In conclusion, the results of the study contribute to reveal distinct distribution pattern of gelatinases and support the hypothesis that these enzymes are intrinsic constituents of the dentin organic matrix after decalcification.     

Homoepitaxial meso- and microscale crystal co-orientation and organic matrix network structure in Mytilus edulis nacre and calcite

New developments in high-resolution, low accelaration voltage electron backscatter diffraction (EBSD) enable us to resolve and quantify the co-orientation of nanocrystals constituting biological carbonate crystals with a scan step resolution of 125 nm. This allows the investigation of internal structures in carbonate tablets and tower biocrystals in the nacre of mollusc shells, and it provides details on the calcite–aragonite polymorph interface in bivalves. Within the aragonite tablets of Mytilus edulis nacre we find a mesoscale crystallographic mosaic structure with a misorientation distribution of 2° full width at half maximum. Selective etching techniques with critical point drying reveal an organic matrix network inside the nacre tablets. The size scales of the visible aragonite tablet subunits and nanoparticles correspond to those of the open pore system in the organic matrix network. We further observe by EBSD that crystal co-orientation spans over tablet boundaries and forms composite crystal units of up to 20 stacked co-oriented tablets (tower crystals). Statistical evaluation of the misorientation data gives a probability distribution of grain boundary misorientations with two maxima: a dominant peak for very-small-angle grain boundaries and a small maximum near 64°, the latter corresponding to {1 1 0} twinning orientations. However, the related twin boundaries are typically the membrane-lined {0 0 1} flat faces of the tablets and not {1 1 0} twin walls within tablets. We attribute this specific pattern of misorientation distribution to growth by particle accretion and subsequent semicoherent homoepitaxial crystallization. The semicoherent crystallization percolates between the tablets through mineral bridges and across matrix membranes surrounding the tablets. In the “prismatic” calcite layer crystallographic co-orientation of the prisms reaches over more than 50 micrometers.     

Nucleation and growth of aragonite crystals at the growth front of nacres in pearl oyster,Pinctada fucata

The growth front of nacreous layer, which lies just above the outer prismatic layer, is one of the crucial areas to comprehend the formation of nacreous aragonite. The crystallographic properties of aragonite crystals at the growth front in pearl oyster, Pinctada fucata, were investigated using scanning electron microscopy with electron back-scattered diffraction, and transmission electron microscopy with focused ion beam sample preparation technique. Nano-sized aragonite crystals nucleate with random crystallographic orientation inside the dimples on the surface of the organic matrix that covers the outer prismatic columns. The dimples are filled with horn-like aragonite crystals, which enlarge from the bottom to the upper surface to form hemispheric domes. The domes grow concentrically and coalesce together to become the initial nacreous layer. The c-axes of aragonite at the top surface of the domes are preferentially oriented perpendicular to the surface. The horn-like aragonite and its crystallographic orientation are probably attained by geometrical selection with the fastest growth rate of aragonite along the c-axis, until organic sheets are continuously formed and interrupt the crystal growth of aragonite. The further crystal growth along the shell thickness is attained via mineral bridges through discontinuity or holes in the organic sheets. These results indicate that the crystal growth of aragonite at the growth front results from not only biotic process but also inorganic ones such as geometrical selection and mineral bridges.     

Nanostructure and Shape Control in Polymer‐Ceramic Hybrids from Poly(ethylene oxide)‐block‐Poly(hexyl methacrylate) and Aluminosilicates Derived from Them

Summary: The present study describes the use of poly(ethylene oxide)‐block‐poly(hexyl methacrylate) diblock copolymers (PEO‐b‐PHMA) as structure‐directing agents for the synthesis of nanostructured polymer‐inorganic hybrid materials from (3‐glycidylpropyl)trimethoxysilane and aluminium sec‐butoxide as precursors and organic, volatile solvents. Four different morphologies, i.e., inorganic spheres, cylinders, lamellae, and organic cylinders in an inorganic matrix, are obtained confirmed by a combination of small‐angle X‐ray scattering (SAXS) and transmission electron microscopy (TEM). The composites are further characterized by differential scanning calorimetry (DSC) and solid‐state ¹³C, ²⁹Si, and ²⁷Al NMR. It is demonstrated that the change in the hydrophobic block from polyisoprene (PI) to poly(hexyl methacrylate) (PHMA) has no significant effect on the local structure of the inorganic rich phase. By the dissolution of the composites rich in poly(hexyl methacrylate), nano‐particles of different shapes, i.e., spheres, cylinders, and lamellae, are obtained as demonstrated by atomic force microscopy (AFM) and TEM. Finally, calcination of composites with the inverse hexagonal structure at elevated temperatures up to 600 °C results in nanostructured aluminosilicates that retain their structure as evidenced through a combination of SAXS and TEM. The study opens pathways towards tailoring filler‐matrix interactions in model nanocomposites and builds the bases for the preparation of composites from multiblock copolymers with polyisoprene (PI), poly(ethylene oxide) (PEO), and poly(hexyl methacrylate) (PHMA) as building blocks.

Bright field TEM micrograph of composite T55/1 with inverse hexagonal morphology after calcination.     

Morphology and Crystallization Kinetics of Melt Miscible Polyolefin Blends

Blends of copolymers of ethylene/hexene (9 CH₃/1 000 C) and ethylene/butene (77 CH₃/1 000 C) synthesized with metallocene catalysts were prepared by co‐precipitation from solution. A phase diagram for this blend had been obtained in a preceding work, where the blends were found to be miscible in the melt with a characteristic upper critical solution temperature (UCST). In this work, the successive self‐nucleation and annealing (SSA) thermal fractionation method revealed that both pure copolymers, even though they had been prepared by metallocene catalysis, displayed a very broad distribution of short chain branching. The thermal behavior of the pure copolymers and their blends revealed that co‐crystallization effects were present within each phase and that molecular fractionation during crystallization can prevail when the materials are slow cooled from the melt. A crystallization kinetics study was performed by differential scanning calorimetry (DSC) and it revealed a strong competition between crystallization and phase segregation. Upon cooling from a one phase melt, phase segregation precedes crystallization if the crystallization temperature (T_c) is high. On the other hand when low crystallization temperatures are employed, the amount of phase segregation that can be achieved before crystallization is limited. Transmission electron microscopy (TEM) allowed the observation of the lamellar morphology and their aggregation state. In specially prepared samples, the relative lamellar frequency of isothermally crystallized material was employed to ascertain whether the blend under consideration was within the one or the two phase region of the blends phase diagram.

Crystallization half‐time for isothermally crystallized PEB/PEH blends quenched to the crystallization temperature from 170 °C.     

Microstructural characterization of plasma-sprayed hydroxyapatite-10 wt% ZrO2 composite coating on titanium.

Previous research showed that the concept of adding ZrO2 as second phase to hydroxyapatite (HA) significantly increased the bonding strength of plasma-sprayed composite material. The present work aimed to investigate the microstructural characteristics of plasma-sprayed hydroxyapatite-10 wt% ZrO2 composite coating on titanium using X-ray diffractometry (XRD) and transmission electron microscopy (TEM). In TEM, phases such as HA, amorphous calcium phosphate, alpha-TCP, ZrO2 and minor transformed CaZrO3 are identified. The cubic phase of ZrO2 in HA-10 wt% ZrO2 powders before coating maintains during plasma spraying, and zirconia particle apparently bonds well to the calcium phosphate matrix with local crystallographic relationship. The transformed CaZrO3 does not exist as interface interphase between calcium phosphate matrix and zirconia particle. Instead, reaction of calcium phosphate and zirconia occurs rapidly to transform ZrO2 into CaZrO3. The toughening mechanism of the material studied and its biological implication of the system are discussed.     

The nature and function of mononuclear cells on the resorbed surfaces of bone in the reversal phase during remodeling.

In a reversal phase of bone remodeling many mononuclear cells appear on the resorbed surfaces of bone with characteristic reversal lines as revealed by transmission electron microscopy (TEM). However, these mononuclear cells have been variously hypothesized or reported. The present study examined the TEM features on the resorbed surfaces of three calcified connective tissues, and aimed to clarify the nature and function of the mononuclear cells in a reversal phase. Dentine slices cultured with isolated osteoclasts, human deciduous teeth, and rat mandibles were used in this study. Specimens were fixed, decalcified, and then embedded in Epon 812, and sectioned into 0.1-microm-thick ultrathin sections. The ultrathin sections were stained with uranyl acetate and lead citrate, and then examined by TEM. Many sharply pointed collagen fibrils with striation were observed exposed on the resorbed surfaces of cultured dentine slices, but there were neither cells nor reversal lines. The same features were observed on the root dentine surfaces of human deciduous teeth. Under many mononuclear cells in a reversal phase of remodeling, reversal lines were seen on the resorbed surfaces of rat mandibles, but there were no striated collagen fibrils exposed on the bone surfaces. The alternation of the TEM features on the resorbed surfaces before and after the participation of mononuclear cells in a reversal phase of remodeling suggests the nature and function of these cells: they participate in both degrading the demineralized and disrupted matrix left on the resorbed surfaces and forming reversal lines there.     

Hypoxia preconditioning of tissue-engineered mucosa enhances its angiogenic capacity in vitro

Improving vascularization of tissue-engineered oral mucosa (TEM) is a major challenge in the field of plastic surgery. Hypoxia is a stimulator of angiogenesis through a number of mechanisms. Therefore, hypoxia is a critical parameter that can be controlled in an effort to improve angiogenesis. In the present study we studied the secretion of a number of angiogenic factors during hypoxia exposure and evaluated the effect of TEM conditioned medium on endothelial cells. TEM was constructed by seeding human oral mucosa keratinocytes and fibroblasts on acellular human donor skin. TEM was exposed to hypoxia during 6, 12, and 24?h. Cellular hypoxia was assessed by immunolocalization of the hypoxia-inducible factor-1α. Secretion of vascular endothelial growth factor, placental growth factor (PlGF), tissue inhibitors of matrix metalloproteinases-1 and -2, and the activity of matrix metalloproteinase-9 significantly increased during hypoxia exposure. Moreover, conditioned medium from hypoxic TEM strongly enhanced endothelial cell proliferation and migration. In vitro exposure of TEM to hypoxia improves its capacity to support endothelial cell proliferation and migration, which suggests that hypoxia preconditioning of TEM potentially improves angiogenic responses for in vivo implantation.