Mechanical and chemical characteristics of mineral produced by basic fibroblast growth factor-treated bone marrow stromal cells in vitro

Hierarchically Porous Calcium Carbonate Scaffolds for Bone Tissue Engineering

Hierarchically porous CaCO₃ scaffolds comprised of micro- (diameter = 2.0 ± 0.3 μm) and nano-sized (diameter = 50.4 ± 14.4 nm) pores were fabricated on silicon substrates using a supercritical CO₂-based process. Differentiated human THP-1 monocytes exposed to the CaCO₃ scaffolds produced negligible levels of the inflammatory cytokine tumor necrosis factor-alpha (TNF-α), confirming the lack of immunogenicity of the scaffolds. Extracellular matrix (ECM) proteins, vitronectin and fibronectin, displayed enhanced adsorption to the scaffolds compared to the silicon controls. ECM protein-coated CaCO₃ scaffolds promoted adhesion, growth, and proliferation of osteoblast MC3T3 cells. MC3T3 cells grown on the CaCO₃ scaffolds produced substantially higher levels of transforming growth factor-beta and vascular endothelial growth factor A, which regulate osteoblast differentiation, and exhibited markedly increased alkaline phosphatase activity, a marker of early osteoblast differentiation, compared to controls. Moreover, the CaCO₃ scaffolds stimulated matrix mineralization (calcium deposition), an end point of advanced osteoblast differentiation and an important biomarker for bone tissue formation. Taken together, these results demonstrate the significant potential of the hierarchically porous CaCO₃ scaffolds for bone tissue engineering applications.

Extracellular matrix mineralization in murine MC3T3-E1 osteoblast cultures: an ultrastructural, compositional and comparative analysis with mouse bone

Bone cell culture systems are essential tools for the study of the molecular mechanisms regulating extracellular matrix mineralization. MC3T3-E1 osteoblast cell cultures are the most commonly used in vitro model of bone matrix mineralization. Despite the widespread use of this cell line to study biomineralization, there is as yet no systematic characterization of the mineral phase produced in these cultures. Here we provide a comprehensive, multi-technique biophysical characterization of this cell culture mineral and extracellular matrix, and compare it to mouse bone and synthetic apatite mineral standards, to determine the suitability of MC3T3-E1 cultures for biomineralization studies. Elemental compositional analysis by energy-dispersive X-ray spectroscopy (EDS) showed calcium and phosphorus, and trace amounts of sodium and magnesium, in both biological samples. X-ray diffraction (XRD) on resin-embedded intact cultures demonstrated that similar to 1-month-old mouse bone, apatite crystals grew with preferential orientations along the (100), (101) and (111) mineral planes indicative of guided biogenic growth as opposed to dystrophic calcification. XRD of crystals isolated from the cultures revealed that the mineral phase was poorly crystalline hydroxyapatite with 10 to 20nm-sized nanocrystallites. Consistent with the XRD observations, electron diffraction patterns indicated that culture mineral had low crystallinity typical of biological apatites. Fourier-transform infrared spectroscopy (FTIR) confirmed apatitic carbonate and phosphate within the biological samples. With all techniques utilized, cell culture mineral and mouse bone mineral were remarkably similar. Scanning (SEM) and transmission (TEM) electron microscopy showed that the cultures had a dense fibrillar collagen matrix with small, 100nm-sized, collagen fibril-associated mineralization foci which coalesced to form larger mineral aggregates, and where mineralized sites showed the accumulation of the mineral-binding protein osteopontin. Light microscopy, confocal microscopy and three-dimensional reconstructions showed that some cells had dendritic processes and became embedded within the mineral in an osteocyte-like manner. In conclusion, we have documented characteristics of the mineral and matrix phases of MC3T3-E1 osteoblast cultures, and have determined that the structural and compositional properties of the mineral are highly similar to that of mouse bone.

Introduction to cell-hydrogel mechanosensing

The development of hydrogel-based biomaterials represents a promising approach to generating new strategies for tissue engineering and regenerative medicine. In order to develop more sophisticated cell-seeded hydrogel constructs, it is important to understand how cells mechanically interact with hydrogels. In this paper, we review the mechanisms by which cells remodel hydrogels, the influence that the hydrogel mechanical and structural properties have on cell behaviour and the role of mechanical stimulation in cell-seeded hydrogels. Cell-mediated remodelling of hydrogels is directed by several cellular processes, including adhesion, migration, contraction, degradation and extracellular matrix deposition. Variations in hydrogel stiffness, density, composition, orientation and viscoelastic characteristics all affect cell activity and phenotype. The application of mechanical force on cells encapsulated in hydrogels can also instigate changes in cell behaviour. By improving our understanding of cell-material mechano-interactions in hydrogels, this should enable a new generation of regenerative medical therapies to be developed.

Surface energy effects on osteoblast spatial growth and mineralization

While short-term surface energy effects on cell adhesion are relatively well known, little is revealed as regards its later stage effects on cell behavior. We examined surface energy effects on osteoblastic cell growth and mineralization by using human fetal osteoblastic (hFOB) cells cultured on plasma-treated quartz (contact angle, theta=0 degrees) and octadecyltrichlorosilane (OTS)-treated quartz (theta=113 degrees). hFOB cells formed a homogeneous cell layer on plasma-treated quartz, while those cultured on OTS-treated quartz produced randomly distributed clump-like structures that were filled with cells (confirmed by confocal microscopy). Mineral deposition by hFOB cells was spatially homogeneous when cultured on hydrophilic surfaces. Furthermore, cells on hydrophilic surfaces exhibited increased mineralized area as well as enhanced mineral-to-matrix ratio (assessed by Fourier transform infrared spectroscopy), relative to cells on hydrophobic surfaces. Experiments using other types of osteoblast-like cells (MC3T3-E1, MG63, and SAOS-2) revealed more or less similar effects in spatial growth morphology. It was concluded that hydrophilic surfaces induce homogeneous spatial osteoblastic cell growth and mineral deposition and enhance the quantity (e.g., area) and quality (e.g., mineral-to-matrix ratio) of mineralization relative to hydrophobic surfaces. Our data suggest that surface energy effects on osteoblastic cell differentiation, especially mineralization, may be correlated with surface energy dependent changes in spatial cell growth.

Human iliac crest cancellous bone elastic modulus and hardness differ with bone formation rate per bone surface but not by existence of prevalent vertebral fracture

The goals of this study were to measure tissue-level elastic moduli and hardness of human cancellous bone using nanoindentation, and determine the relationship between nanoindentation results and previously measured bone histomorphometric variables and bone mineralization. Forty iliac crest biopsies were used in this study, which were collected from Caucasian females with vertebral fracture or from a normal healthy female Caucasian population. They were also categorized into two groups according to high or low bone formation rate per bone surface (BFR/BS). Thirty-two sites were randomly selected on each specimen for nanoindentation with a Berkovich diamond indenter. Two sets of elastic moduli and hardness were calculated using the continuous stiffness measurement method and the Oliver-Pharr method, respectively. Relationships between nanoindentation results and donor age, bone mineralization, and histomorphometric variables were examined. No difference in elastic moduli or hardness was observed between the normal and fracture groups. Significantly lower elastic moduli were observed in the high BFR/BS group. The elastic moduli and hardness measurements were not significantly correlated with the bone mineralization measured independently in a previous study. Linear correlation between elastic modulus and hardness calculated using the Oliver-Pharr method was not different between the normal and fracture groups or between the high and low BFR/BS groups. Nanoindentation hardness was a very good predictor of bone tissue elastic modulus for both normal and osteoporotic bone tissues. Osteoporosis may not change the relationship between bone tissue elastic modulus, bone hardness, and bone mineralization.

Effects ofin vitro bone formation on the mechanical properties of a trabeculated hydroxyapatite bone substitute

This study was designed to test the hypothesis that the mechanical properties of a trabecular bone substitute can be enhanced through in vitro tissue formation. Our specific objectives were to (1) determine the effects of in vitro marrow stromal cell-mediated tissue deposition upon a trabeculated hydroxyapatite scaffold on the strength and toughness of the resulting bone substitute; and (2) identify and characterize regions of newly deposited matrix and mineral. This work provides a basis for future investigations aimed at transforming a brittle hydroxyapatite scaffold into an osteoinductive, biomechanically functional implant through in vitro bone deposition. As hypothesized, the mechanical properties of the trabecular bone substitutes were significantly enhanced by in vitro tissue formation. As a result of cell seeding and a 5 week culture protocol, mean strength increased by 85% (p = 0.008) and energy to fracture increased by 130% (p = 0.003). Accompanying the enhancement of mechanical properties was the deposition of significant amounts of bone matrix and mineral. Fluorescence imaging, scanning electron microscopy, electron probe microanalysis, and nanoindentation confirmed the presence of bonelike mineral with Ca/P ratio, modulus, and hardness similar to that within human and rat trabecular bone tissue. This new mineralization was found to exist within a newly deposited parallel-fibered matrix both encasing and bridging between scaffold trabeculae. Taken as a whole, our results establish the feasibility of the production of an osteoinductive hydroxyapatite-based trabecular bone substitute with mechanical properties enhanced through in vitro bone deposition.

Comparison ofin VitroMineralization by Murine Embryonic and Adult Stem Cells Cultured in an Osteogenic Medium

Nearly half a million bone-grafting procedures occurred in the United States in the year 2000. Tissue-engineered bone substitutes may mitigate difficulties associated with current grafting options. Embryonic stem cells (ESCs) could be a potential cell source for bone substitutes; however, direct comparisons between ESCs and other cell sources are lacking. Here we provide a direct, long-term, in vitro comparison of mineralization processes in adult, marrow-derived, mesenchymal stem cells (MSCs) and ESCs from the 129/Sv+c/+p mouse strain. MSCs were observed to grow at a slower rate than ESCs. MSCs expressed seven times more alkaline phosphatase (AP) per cell than did ESCs and immediately showed type I collagen and osteocalcin production. ESCs also produced type I collagen and osteocalcin, but production was delayed. Mineral deposition by ESCs was nearly 50 times higher than by MSCs. Spectroscopic analysis showed the calcium-to-phosphorus ratio (Ca:P) of the ESC mineral (1.26:1) to be significantly higher than that of the MSCs (0.29:1), but still 25% lower than hydroxyapatite (1.67:1). Addition of basic fibroblast growth factor significantly inhibited AP expression, mineral deposition, and Ca:P ratios in MSCs and had little effect on ESCs. These functional characteristics may assist with cell selection for purposes of bone tissue engineering.

Nanoindentation of soft hydrated materials for application to vascular tissues

Soft hydrated materials, such as vascular tissues and other biomaterials, provide a number of challenges in the field of nanoindentation. However, the ability of nanoindentation to probe local, nanoscale mechanical properties of heterogeneous materials makes it desirable to adapt this technique for application to biologic tissues. To develop the field of nanoindentation for the analysis of soft hydrated materials, the goals of this study were fourfold: develop a sample hydration system, select an appropriate tip for soft material indentation, identify a substrate to be used for blunt tip alignment, and determine an appropriate control material for the development of future indentation protocols. A hydration system was developed that maintained sample hydration for over 8 h without completely submerging the sample. Further, a 100-microm radius of curvature conospherical tip was shown to be a suitable tip for indenting a variety of soft hydrated materials and back-illuminated agarose gel was found to be an effective material for use in tip alignment. Finally, agarose gel demonstrated similar qualitative and quantitative nanomechanical behavior to vascular tissue, suggesting that it will be an appropriate control material for the development of future indentation protocols for soft biologic tissues.