Topographically induced bone formation in vitro: implications for bone implants and bone grafts

Biomechanical, biophysical and biochemical modulators of cytoskeletal remodelling and emergent stem cell lineage commitment

Across complex, multi-time and -length scale biological systems, redundancy confers robustness and resilience, enabling adaptation and increasing survival under dynamic environmental conditions; this review addresses ubiquitous effects of cytoskeletal remodelling, triggered by biomechanical, biophysical and biochemical cues, on stem cell mechanoadaptation and emergent lineage commitment. The cytoskeleton provides an adaptive structural scaffold to the cell, regulating the emergence of stem cell structure-function relationships during tissue neogenesis, both in prenatal development as well as postnatal healing. Identification and mapping of the mechanical cues conducive to cytoskeletal remodelling and cell adaptation may help to establish environmental contexts that can be used prospectively as translational design specifications to target tissue neogenesis for regenerative medicine. In this review, we summarize findings on cytoskeletal remodelling in the context of tissue neogenesis during early development and postnatal healing, and its relevance in guiding lineage commitment for targeted tissue regeneration. We highlight how cytoskeleton-targeting chemical agents modulate stem cell differentiation and govern responses to mechanical cues in stem cells' emerging form and function. We further review methods for spatiotemporal visualization and measurement of cytoskeletal remodelling, as well as its effects on the mechanical properties of cells, as a function of adaptation. Research in these areas may facilitate translation of stem cells' own healing potential and improve the design of materials, therapies, and devices for regenerative medicine.

Dental Implants: Enhancing Biological Response Through Surface Modifications

Surface characteristics are an important factor for long-term clinical success of dental implants. Alterations of implant surface characteristics accelerate or improve osseointegration by interacting with the physiology of bone healing. Dental implant surfaces have been traditionally modified at the microlevel. Recently, researchers have actively investigated nano-modifications in dental implants. This review explores implant surface modifications that enhance biological response at the interface between a bone and the implant.

Three interfaces of the dental implant system and their clinical effects on hard and soft tissues

Anatomically, the human tooth has structures both embedded within and forming part of the exterior surface of the human body. When a tooth is lost, it is often replaced by a dental implant, to facilitate the chewing of food and for esthetic purposes. For successful substitution of the lost tooth, hard tissue should be integrated into the implant surface. The microtopography and chemistry of the implant surface have been explored with the aim of enhancing osseointegration. Additionally, clinical implant success is dependent on ensuring that a barrier, comprising strong gingival attachment to an abutment, does not allow the infiltration of oral bacteria into the bone-integrated surface. Epithelial and connective tissue cells respond to the abutment surface, depending on its surface characteristics and the materials from which it is made. In particular, the biomechanics of the implant-abutment connection structure (i.e., the biomechanics of the interface between implant and abutment surfaces, and the screw mechanics of the implant-abutment assembly) are critical for both the soft tissue seal and hard tissue integration. Herein, we discuss the clinical importance of these three interfaces: bone-implant, gingiva-abutment, and implant-abutment.

Novel insights into the coupling of osteoclasts and resorption to bone formation

Bone remodeling consists of resorption by osteoclasts (OCs) and formation by osteoblasts (OBs). Precise coordination of these activities is required for the resorbed bone to be replaced with an equal amount of new bone in order to maintain skeletal mass throughout the lifespan. This coordination of remodeling processes is referred to as the "coupling" of resorption to bone formation. In this review, we discuss the essential role for OCs in coupling resorption to bone formation, mechanisms for this coupling, and how coupling becomes less efficient or disrupted in conditions of bone loss. Lastly, we provide perspectives on targeting coupling to treat human bone disease.

Recent Advances in Osteoclast Biological Behavior

With the progress of the aging population, bone-related diseases such as osteoporosis and osteoarthritis have become urgent problems. Recent studies have demonstrated the importance of osteoclasts in bone homeostasis, implying these will be an important mediator in the treatment of bone-related diseases. Up to now, several reviews have been performed on part of osteoclast biological behaviors such as differentiation, function, or apoptosis. However, few reviews have shown the complete osteoclast biology and research advances in recent years. Therefore, in this review, we focus on the origin, differentiation, apoptosis, behavior changes and coupling signals with osteoblasts, providing a simple but comprehensive overview of osteoclasts for subsequent studies.

Aspects of intercellular communication in bone and implications in therapy

Communication processes among the cells of bone are essential for the structure and function of the organ. After it was proposed that communication from the osteoblast lineage to hemopoietic cells initiated osteoclastogenesis, the molecular controls were identified to be the tumour necrosis factor ligand and receptor families. This was followed by revelation of very many signalling processes among the cells of bone that regulate the three phases of bone remodelling, the resorption, reversal and formation phases. In many instances the ways in which these mechanisms operate can determine how drugs act on bone, whether they be inhibitors of resorption or promoters of formation.

Exploring Macroporosity of Additively Manufactured Titanium Metamaterials for Bone Regeneration with Quality by Design: A Systematic Literature Review

Additive manufacturing facilitates the design of porous metal implants with detailed internal architecture. A rationally designed porous structure can provide to biocompatible titanium alloys biomimetic mechanical and biological properties for bone regeneration. However, increased porosity results in decreased material strength. The porosity and pore sizes that are ideal for porous implants are still controversial in the literature, complicating the justification of a design decision. Recently, metallic porous biomaterials have been proposed for load-bearing applications beyond surface coatings. This recent science lacks standards, but the Quality by Design (QbD) system can assist the design process in a systematic way. This study used the QbD system to explore the Quality Target Product Profile and Ideal Quality Attributes of additively manufactured titanium porous scaffolds for bone regeneration with a biomimetic approach. For this purpose, a total of 807 experimental results extracted from 50 different studies were benchmarked against proposed target values based on bone properties, governmental regulations, and scientific research relevant to bone implants. The scaffold properties such as unit cell geometry, pore size, porosity, compressive strength, and fatigue strength were studied. The results of this study may help future research to effectively direct the design process under the QbD system.

Osteoclasts Provide Coupling Signals to Osteoblast Lineage Cells Through Multiple Mechanisms

Bone remodeling is essential for the repair and replacement of damaged and old bone. The major principle underlying this process is that osteoclast-mediated resorption of a quantum of bone is followed by osteoblast precursor recruitment; these cells differentiate to matrix-producing osteoblasts, which form new bone to replace what was resorbed. Evidence from osteopetrotic syndromes indicate that osteoclasts not only resorb bone, but also provide signals to promote bone formation. Osteoclasts act upon osteoblast lineage cells throughout their differentiation by facilitating growth factor release from resorbed matrix, producing secreted proteins and microvesicles, and expressing membrane-bound factors. These multiple mechanisms mediate the coupling of bone formation to resorption in remodeling. Additional interactions of osteoclasts with osteoblast lineage cells, including interactions with canopy and reversal cells, are required to achieve coordination between bone formation and resorption during bone remodeling.

The role of nitrogen off-stoichiometry in the osteogenic behavior of silicon nitride bioceramics

The surface chemistry of silicon nitride plays an important role in stimulating osteoblasts to proliferate and produce bone tissue with improved efficiency. This property, which is advantageous in spinal fusion surgery has a chemical origin and is a direct consequence of the cleavage of covalent SN bonds in an aqueous environment. Building upon a wealth of published research on the stimulation of osteoblastic activity by silicon, the aim of this paper is to explore the role of nitrogen and, more specifically, the N/Si atomic ratio on the osteogenic response of Si₃N₄. The surface stoichiometry of Si₃N₄ was gradually altered toward a silicon-rich composition by systematically treating the Si₃N₄ surface with a high-power pulsed laser in an Ar gas atmosphere (i.e., operated at different pulse times, spot sizes, and voltages). Different analytical probes were used to characterize the surface including X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and energy dispersive X-ray spectroscopy (EDS). Osteoconductivity was tested in vitro using SaOS-2 osteosarcoma cells, and samples with different surface stoichiometry were compared for their osteogenic response. These experiments clearly indicated a fundamental role for nitrogen off-stoichiometry in osteogenesis, and showed that both cell proliferation and growth of bone tissue diminished with decreasing nitrogen content.

Eldecalcitol effects on osteoblastic differentiation and function in the presence or absence of osteoclastic bone resorption

Eldecalcitol (ELD) is an active vitamin D3 analog, possesses anti-resorption properties and is an approved therapeutic drug for the treatment of osteoporosis in Japan. However, the effect of ELD on osteoblasts in a distinct cell microenvironment, including in the presence or absence of osteoclastic bone resorption, is undetermined. In the current study, the effect of bone resorption supernatant on the ELD-mediated regulation of viability, differentiation and receptor activator of ΝF-κB ligand/osteoprotegerin (RANKL/OPG) expression was assessed in MC3T3-E1 pre-osteoblast cells. The murine macrophage-like cell line RAW 264.7 was induced to differentiate into functional osteoblasts. Bone resorption supernatant was prepared by culturing osteoclast with a bovine cortical bone specimen. Mouse MC3T3-E1 cells were subsequently treated with ELD combined with differentiated osteoclast cell culture (OCS) or osteoclast bone resorption model supernatants. Cell counting kit-8, alkaline phosphatase (ALP) activity, reverse transcription-quantitative (RT-q) PCR and western blot analysis were used to assess cell viability, osteogenic activity and RANKL and OPG expression in MC3T3-E1 cells. The OCS and OCS + ELD treatment exhibited significantly increased MC3T3-E1 cell viability when compared with the control group. However, ELD, bone resorption culture supernatant (BRS) and ELD + BRS treatments significantly decreased MC3T3-E1 cell viability. The results of ALP activity analysis, RT-qPCR and western blot analysis demonstrated that ELD treatment and OCS decreased the osteogenic markers (ALP and RUNX2), however, BRS increased them. All treatments enhanced the expression of RANKL and RANKL/OPG ratio. The results of the current study revealed that ELD inhibits osteoblastic differentiation in vitro. However, in the presence of BRS, which mimics the local bone microenvironment in vivo, the net effect on osteogenesis was positive. Furthermore, osteoclasts and bone matrix-derived factors increased the RANKL/OPG ratio, thereby potentiating osteoclastic activity.

Cell Morphology on Poly(methyl methacrylate) Microstructures as Function of Surface Energy

Whilst the significance of substrate topography as a regulator of cell function is well established, a systematic analysis of the principles underlying this is still unavailable. Here we evaluate the hypothesis that surface energy plays a decisive role in substrate-mediated modulation of cell phenotype by evaluation of cell behaviour on synthetic microstructures exhibiting pronounced differences in surface energy. These microstructures, specifically cubes and walls, were fabricated from a biocompatible base polymer, poly(methyl methacrylate), by variotherm injection molding. The dimensions of the cubes were 1 μm x 1 μm x 1 μm (height x width x length) with a periodicity of 1:1 and 1:5 and the dimensions of the walls 1 μm x 1 μm x 15 mm (height x width x length) with a periodicity of 1:1 and 1:5. Mold inserts were made by lithography and electroplating. The surface energy of the resultant microstructures was determined by static contact angle measurements. Light scanning microscopy of the morphology of NT2/D1 and MC3T3-E1 preosteoblast cells cultured on structured PMMA samples in both cases revealed a profound surface energy dependence. “Walls” appeared to promote significant cell elongation, whilst a lack of cell adhesion was observed on “cubes” with the lowest periodicity. Contact angle measurements on walls revealed enhanced surface energy anisotropy (55 mN/m max., 10 mN/m min.) causing a lengthwise spreading of the test liquid droplet, similar to cell elongation. Surface energy measurements for cubes revealed increased isotropic hydrophobicity (87° max., H₂O). A critical water contact angle of ≤ 80° appears to be necessary for adequate cell adhesion. A “switch” for cell adhesion and subsequently cell growth could therefore be applied by, for example, adjusting the periodicity of hydrophobic structures. In summary cell elongation on walls and a critical surface energy level for cell adhesion could be produced for NT2/D1 and MC3T3-E1 cells by symmetrical and asymmetrical energy barrier levels. We, furthermore, propose a water-drop model providing a common physicochemical cause regarding similar cell/droplet geometries and cell adhesion on the investigated microstructures.

The Cells of Bone and Their Interactions

Bone tissue is comprised of a collagen-rich matrix containing non-collagenous organic compounds, strengthened by mineral crystals. Bone strength reflects the amount and structure of bone, as well as its quality. These qualities are determined and maintained by osteoblasts (bone-forming cells) and osteoclasts (bone-resorbing cells) on the surface of the bone and osteocytes embedded within the bone matrix. Bone development and growth also involves cartilage cells (chondrocytes). These cells do not act in isolation, but function in a coordinated manner, including co-ordination within each lineage, between the cells of bone, and between these cells and other cell types within the bone microenvironment. This chapter will briefly outline the cells of bone, their major functions, and some communication pathways responsible for controlling bone development and remodeling.

Sacrificial template method for the synthesis of three-dimensional nanofibrous 58S bioglass scaffold and its in vitro bioactivity and cell responses

Three-dimensional nanofibrous scaffolds that morphologically mimic natural extracellular matrices hold great promises in tissue engineering and regenerative medicine due to their increased cell attachment and differentiation compared with block structure. In this work, for the first time, three-dimensional porous nanofibrous 58S bioglass scaffolds have been fabricated using a sacrificial template method. During the process, a natural three-dimensional nanofibrous bacterial cellulose was used as the sacrificial template on which precursor 58S glass was deposited via a sol-gel route. SEM and TEM results verify that the as-prepared 58S scaffolds can inherit the three-dimensional nanofibrous feature of bacterial cellulose. Pore structure characterizations by nitrogen adsorption-desorption and mercury intrusion porosimetry demonstrate that the 58S scaffolds are highly porous with a porosity of 75.1% and contain both mesopores (39.4 nm) and macropores (60 µm) as well as large BET surface area (127.4 m² g^-1). In vitro cell studies suggest that the 58S scaffold is bioactive and biocompatible with primary mouse osteoblast cells, suggesting that the nanofibrous structure of 58S is able to provide an appropriate environment for cellular functioning. These results strongly suggest that the three-dimensional nanofibrous 58S scaffold has great potential for application in bone tissue engineering and regenerative medicine.

Osteoinduction of bone grafting materials for bone repair and regeneration

Regeneration of bone defects caused by trauma, infection, tumours or inherent genetic disorders is a clinical challenge that usually necessitates bone grafting materials. Autologous bone or autograft is still considered the clinical "gold standard" and the most effective method for bone regeneration. However, limited bone supply and donor site morbidity are the most important disadvantages of autografting. Improved biomaterials are needed to match the performance of autograft as this is still superior to that of synthetic bone grafts. Osteoinductive materials would be the perfect candidates for achieving this task. The aim of this article is to review the different groups of bone substitutes in terms of their most recently reported osteoinductive properties. The different factors influencing osteoinductivity by biomaterials as well as the mechanisms behind this phenomenon are also presented, showing that it is very limited compared to osteoinductivity shown by bone morphogenetic proteins (BMPs). Therefore, a new term to describe osteoinductivity by biomaterials is proposed. Different strategies for adding osteoinductivity (BMPs, stem cells) to bone substitutes are also discussed. The overall objective of this paper is to gather the current knowledge on osteoinductivity of bone grafting materials for the effective development of new graft substitutes that enhance bone regeneration.

Calcium ions and osteoclastogenesis initiate the induction of bone formation by coral-derived macroporous constructs

Coral-derived calcium carbonate/hydroxyapatite macroporous constructs of the genus Goniopora with limited hydrothermal conversion to hydroxyapatite (7% HA/CC) initiate the induction of bone formation. Which are the molecular signals that initiate pattern formation and the induction of bone formation? To evaluate the role of released calcium ions and osteoclastogenesis, 7% HA/CC was pre-loaded with either 500 μg of the calcium channel blocker, verapamil hydrochloride, or 240 μg of the osteoclast inhibitor, biphosphonate zoledronate, and implanted in the rectus abdominis muscle of six adult Chacma baboons Papio ursinus. Generated tissues on days 15, 60 and 90 were analysed by histomorphometry and qRT-PCR. On day 15, up-regulation of type IV collagen characterized all the implanted constructs correlating with vascular invasion. Zoledronate-treated specimens showed an important delay in tissue patterning and morphogenesis with limited bone formation. Osteoclastic inhibition yielded minimal, if any, bone formation by induction. 7% HA/CC pre-loaded with the Ca⁺⁺ channel blocker verapamil hydrochloride strongly inhibited the induction of bone formation. Down-regulation of bone morphogenetic protein-2 (BMP-2) together with up-regulation of Noggin genes correlated with limited bone formation in 7% HA/CC pre-loaded with either verapamil or zoledronate, indicating that the induction of bone formation by coral-derived macroporous constructs is via the BMPs pathway. The spontaneous induction of bone formation is initiated by a local peak of Ca⁺⁺ activating stem cell differentiation and the induction of bone formation.

Selective and non-selective cyclooxygenase inhibitors delay stress fracture healing in the rat ulna

Anti-inflammatory drugs are widely used to manage pain associated with stress fractures (SFxs), but little is known about their effects on healing of those injuries. We hypothesized that selective and non-selective anti-inflammatory treatments would retard the healing of SFx in the rat ulna. SFxs were created by cyclic loading of the ulna in Wistar rats. Ulnae were harvested 2, 4 or 6 weeks following loading. Rats were treated with non-selective NSAID, ibuprofen (30 mg/kg/day); selective COX-2 inhibition, [5,5-dimethyl-3-3 (3 fluorophenyl)-4-(4 methylsulfonal) phenyl-2 (5H)-furanone] (DFU) (2.0 mg/kg/day); or the novel c5a anatagonist PMX53 (10 mg/kg/day, 4 and 6 weeks only); with appropriate vehicle as control. Quantitative histomorphometric measurements of SFx healing were undertaken. Treatment with the selective COX-2 inhibitor, DFU, reduced the area of resorption along the fracture line at 2 weeks, without affecting bone formation at later stages. Treatment with the non-selective, NSAID, ibuprofen decreased both bone resorption and bone formation so that there was significantly reduced length and area of remodeling and lamellar bone formation within the remodeling unit at 6 weeks after fracture. The C5a receptor antagonist PMX53 had no effect on SFx healing at 4 or 6 weeks after loading, suggesting that PMX53 would not delay SFx healing. Both selective COX-2 inhibitors and non-selective NSAIDs have the potential to compromise SFx healing, and should be used with caution when SFx is diagnosed or suspected.

Reciprocal osteoblastic and osteoclastic modulation in co-cultured MG63 osteosarcoma cells and human osteoclast precursors

Osteosarcoma is usually associated with a disturbed bone metabolism. The aim of this work was to characterize the reciprocal interactions between MG63 osteosarcoma cells and osteoclasts, in a co-culture system. Co-cultures were characterized throughout 21 days for the osteoclastogenic response and the expression of osteoblastic markers. Monocultures of MG63 cells and peripheral blood mononuclear cell (PBMC) and co-cultures of PBMC + human bone marrow cells (hBMC) were also performed. Compared to PBMC cultures, co-cultures yielded significantly increased gene expression of osteoclast-related markers, tartarate-acid resistant phosphatase (TRAP) activity, TRAP-positive multinucleated cells, cells with actin rings and vitronectin receptors (VNR) and calcitonin receptors (CTR) and calcium phosphate resorbing ability. Results showed that the development of functional osteoclasts required a very low number of MG63 cells, suggesting a high osteoclastogenic-triggering capacity of this cell line. Subjacent mechanisms involved the pathways MEK and NF-kB, although with a lower relevance than that observed on PBMC monocultures or co-cultures of hBMC + PBMC; PGE2 production also had a contribution. Compared to MG63 cell monocultures, the co-culture expressed lower levels of COL1 and ALP, and higher levels of BMP-2, suggesting that PBMC also modulated the osteoblastic behavior. While M-CSF appeared to be involved in the osteoclastogenic response on the MG63 + PBMC co-cultures, RANKL does not seem to be a key player in the process. On the other hand, sphingosine-1-phosphate production might contribute to the modulation of the osteoblastic behavior. Results suggest that the reciprocal modulation between osteosarcoma and osteoclastic cells might contribute to the disturbed bone metabolism associated with bone tumors.

An overview of the regulation of bone remodelling at the cellular level

OBJECTIVES

To review the current literature on the regulation of bone remodelling at the cellular level.

DESIGN AND METHODS

The cellular activities of the cells in the basic multicellular unit (BMU) were evaluated.

RESULTS

Bone remodelling requires an intimate cross-talk between osteoclasts and osteoblasts and is tightly coordinated by regulatory proteins that interact through complex autocrine/paracrine mechanisms. Osteocytes, bone lining cells, osteomacs, and vascular endothelial cells also regulate bone remodelling in the BMU via cell signalling networks of ligand-receptor complexes. In addition, through secreted and membrane-bound factors in the bone microenvironment, T and B lymphocytes mediate bone homeostasis in osteoimmunology.

CONCLUSIONS

Osteoporosis and other bone diseases occur because multicellular communication within the BMU is disrupted. Understanding the cellular and molecular basis of bone remodelling and the discovery of novel paracrine or coupling factors, such as RANKL, sclerostin, EGFL6 and semaphorin 4D, will lay the foundation for drug development against bone diseases.

Osseointegration--communication of cells

BACKGROUND

The article provides the scientific documentation for the 3D animated film - "Osseointegration - Communication of cells".

AIM

The aim of this article and of the film is to visualise the molecular and cellular events during the healing of an osseous wound after installation of a dental implant with special emphasis on the process of osseointegration.

MATERIAL AND RESULTS

In this review article for didactic reasons the concept of the four phases of a healing soft tissue wound was transferred to a bone wound after insertion of a dental implant: haemostasis, inflammatory phase, proliferative phase and remodelling phase. Wound healing throughout these phases is the result of a coordinated action of different cell types which communicate with each other by their interaction using signalling molecules like cytokines, extracellular matrix proteins and small molecules. A regular sequence of cell types controlled by adequate concentrations of signalling molecules results in undisturbed healing. Disturbed healing is associated with a continuation of the early inflammatory phase and the development of a toxic wound environment. The latter is characterized by high counts of polymorphnuclear cells, high concentrations of toxic radicals and proteolytic enzymes and low concentrations of growth factors and extracellular matrix molecules. Clinically the development of a toxic wound environment should be avoided, e.g. by antibacterial measures.

DISCUSSION AND CONCLUSION

Experiencing implant osseointegration as a biological process may provide the clinician new targets to improve the therapy with dental implants.

New directions in nanofibrous scaffolds for soft tissue engineering and regeneration

This review focuses on the role of nanostructure and nanoscale materials for tissue engineering applications. We detail a scaffold production method (electrospinning) for the production of nanofiber-based scaffolds that can approximate many critical features of the normal cellular microenvironment, and so foster and direct tissue formation. Further, we describe new and emerging methods to increase the applicability of these scaffolds for in vitro and in vivo application. This discussion includes a focus on methods to further functionalize scaffolds to promote cell infiltration, methods to tune scaffold mechanics to meet in vivo demands and methods to control the release of pharmaceuticals and other biologic agents to modulate the wound environment and foster tissue regeneration. This review provides a perspective on the state-of-the-art production, application and functionalization of these unique nanofibrous structures, and outlines future directions in this growing field.

Spatial and temporal changes in the morphology of preosteoblastic cells seeded on microstructured tantalum surfaces

It has been widely reported that surface morphology on the micrometer scale affects cell function as well as cell shape. In this study, we have systematically compared the influence of 13 topographically micropatterned tantalum surfaces on the temporal development of morphology, including spreading, and length of preosteoblastic cells (MC3T3-E1). Cells were examined after 0.5, 1, 4, and 24 h on different Ta microstructures with vertical dimensions (heights) of 0.25 and 1.6 mum. Cell morphologies depended upon the underlying surface topography, and the length and spreading of cells varied as a function of time with regard to the two-dimensional pattern and vertical dimension of the structure. Microstructures of parallel grooves/ridges caused elongated cell growth after 1 and 4 h in comparison to a flat, nonstructured, reference surface. For microstructures consisting of pillars, cell spreading was found to depend on the distance between the pillars with one specific pillar structure exhibiting a decreased spreading combined with a radical change in morphology of the cells. Interestingly, this morphology on the particular pillar structure was associated with a markedly different distribution of the actin cytoskeleton. Our results provide a basis for further work toward topographical guiding of cell function.

Local communication on and within bone controls bone remodeling

Bone remodeling is required for healthy calcium homeostasis and for repair of damage occurring with stress and age. Osteoclasts resorb bone and osteoblasts form bone. These processes normally occur in a tightly regulated sequence of events, where the amount of formed bone equals the amount of resorbed bone, thereby restoring the removed bone completely. Osteocytes are the third cell type playing an essential role in bone turnover. They appear to regulate activation of bone remodeling, and they exert both positive and negative regulation on both osteoclasts and osteoblasts. In this review, we consider the intricate communication between these bone cells in relation to bone remodeling, reviewing novel data from patients with mutations rendering different cell populations inactive, which have shown that these interactions are more complex than originally thought. We highlight the high probability that a detailed understanding of these processes will aid in the development of novel treatments for bone metabolic disorders, i.e. we discuss the possibility that bone resorption can be attenuated pharmacologically without a secondary reduction in bone formation.

Stereo imaging and cytocompatibility of a model dental implant surface formed by direct laser fabrication

Direct laser fabrication (DLF) allows solids with complex geometry to be produced by sintering metal powder particles in a focused laser beam. In this study, 10 Ti6Al4V alloy model dental root implants were obtained by DLF, and surface characterization was carried out using stereo scanning electron microscopy to produce 3D reconstructions. The surfaces were extremely irregular, with approximately 100 microm deep, narrow intercommunicating crevices, shallow depressions and deep, rounded pits of widely variable shape and size, showing ample scope for interlocking with the host bone. Roughness parameters were as follows: R(t), 360.8 microm; R(z), 358.4 microm; R(a), 67.4 microm; and R(q), 78.0 microm. Disc specimens produced by DLF with an identically prepared surface were used for biocompatibility studies with rat calvarial osteoblasts: After 9 days, cells had attached and spread on the DLF surface, spanning across the crevices, and voids. Cell density was similar to that on a commercial rough microtextured surface but lower than on commercial smooth machined and smooth-textured grit-blasted, acid-etched surfaces. Human fibrin clot extension on the DLF surface was slightly improved by inorganic acid etching to increase the microroughness. With further refinements, DLF could be an economical means of manufacturing implants from titanium alloys.

In vivo low-density bone apposition on different implant surface materials

During osseointegration, new bone may be laid down on the implant surface and/or on the old bone surface; the former is known as contact osteogenesis and the latter as distance osteogenesis. Implant surface topography and material composition affect this process. The present study evaluates Ca and P apposition onto three different dental implant material surfaces (carbon monoxide (CO) ion implantation on Ti6Al4V, sand blasting and acid etching on commercially pure titanium and untreated Ti6Al4V) on the mandibles of beagles after healing periods of 3 and 6 months. Energy dispersive spectroscopy is useful for identifying low-density bone relative to surrounding mature bone, allowing for discrimination of the osteogenesis source. Low-density bone was only found at the apical end; there was none on the surface of untreated implants. Low-density bone arising from mature bone towards the implant at month 3 (i.e. distance osteogenesis) was only present on the CO ion implanted samples, due to the modification of the surface nano-topography and the chemistry and structure of the material.

Communication between ephrinB2 and EphB4 within the osteoblast lineage

Members of the ephrin and Eph family are local mediators of cell function through largely contact-dependent processes in development and in maturity. Production of ephrinB2 mRNA and protein are increased by PTH and PTHrP in osteoblasts. Both a synthetic peptide antagonist of ephrinB2/EphB4 receptor interaction and recombinant soluble extracellular domain of EphB4 (sEphB4), which is an antagonist of both forward and reverse EphB4 signaling, were able to inhibit mineralization and the expression of several osteoblast genes involved late in osteoblast differentiation. The findings are consistent with ephrinB2/EphB4 signaling within the osteoblast lineage having a paracrine role in osteoblast differentiation, in addition to the proposed role of osteoclast-derived ephrinB2 in coupling of bone formation to resorption. This local regulation might contribute to control of osteoblast differentiation and bone formation at remodeling sites, and perhaps also in modeling.

Bone remodeling: Multiple cellular interactions required for coupling of bone formation and resorption

The dynamic nature of the skeleton is achieved by a process called "remodeling" which involves the co-ordinated actions of osteoclasts, osteoblasts, osteocytes within the bone matrix and osteoblast-derived lining cells that cover the surface of bone. Remodeling commences with signals that initiate osteoclast formation followed by osteoclast-mediated bone resorption, a reversal period, and then a long period of bone matrix formation mediated by osteoblasts, followed by mineralisation of the matrix. This review will discuss each of these steps with particular emphasis on the communication pathways between each cell type involved and the roles of ephrins, sclerostin, RANKL and PTHrP.

Mechanisms of the anabolic effects of teriparatide on bone: insight from the treatment of a patient with pycnodysostosis

Pycnodysostosis is an extremely rare genetic osteosclerosis caused by cathepsin K deficiency. We hypothesized that teriparatide, a potent anabolic agent used in the treatment of osteoporosis, might reduce skeletal fragility by activating bone turnover. We studied a typical case of pycnodysostosis in a 37-yr-old woman who exhibited short stature, skull and thorax deformities, and a history of severe fragility fractures. Cathepsin K gene sequencing was performed. Before and after 6 mo of 20 microg/d teriparatide, biochemical markers of bone turnover were measured, and 3D bone structure and microarchitecture was assessed in vivo by HR-pQCT. Qualitative and quantitative analysis of transiliac bone biopsies were performed, and the degree of mineralization was evaluated by quantitative microradiography. In vitro assessment of bone resorption was performed after separation and differentiation of CD14(+) monocytes from peripheral blood. Bone structure assessed by HR-pQCT on the radius and tibia showed augmentation of cortical and trabecular density. Transiliac bone biopsy showed highly increased bone mass (+63% versus age- and sex-matched controls), a decrease in bone remodeling without evidence of active osteoblasts, and a severe decrease in the dynamic parameters of bone formation (mineralizing surfaces, -90% and bone formation rate, -93% versus age- and sex-matched controls). This depressed bone turnover probably explained the increased degree of mineralization. The presence of a novel missense mutation leading to an A141V amino acid substitution confirmed a genetic defect of cathepsin K as the cause of the disease. The deficiency of active osteoclasts was confirmed by an in vitro study that showed a decreased concentration of CD14(+) monocytes (the precursor of osteoclasts) in blood. These osteoclasts had low resorptive activity when incubated on bone slices. After 6 mo of teriparatide, the structure, microarchitecture, and turnover of bone--assessed by HR-pQCT, histology, and bone turnover markers--remained unchanged. Our data strongly suggest that some features of the osteoclastic phenotype--that are absent in pycnodysostosis--are a prerequisite for the anabolic effect of PTH on osteoblasts.

Bioglass-derived glass-ceramic scaffolds: study of cell proliferation and scaffold degradation in vitro

Cell support function as well as cell proliferation on highly porous Bioglass(R)-derived glass-ceramic scaffolds (designed for bone tissue engineering) have been assessed in vitro using osteoblast-like cells (MG 63) cultured for up to 6 days. The biodegradation and mechanical stability of the scaffolds in the cell-culture medium have also been investigated. It was found that the scaffolds had excellent cell supporting ability, with cells effectively infiltrating into and surviving at the center of the scaffolds. A quantitative study using the AlamarBlue assay revealed that the proliferation of cells on the glass-ceramic materials was comparable to that on the noncrystallized Bioglass. While the crystalline phase in the glass-ceramic scaffolds transformed into a biodegradable amorphous calcium phosphate phase during cell culture, the mechanical strength of the scaffolds was maintained when compared with that of scaffolds incubated in simulated body fluid or immersed in cell-free culture medium. It is believed that the attached cells and collagen secreted by cells could fill the micropores and microcracks on the surface of the foam struts, thus contributing to the mechanical stability of the degrading scaffolds. In summary, the developed glass-ceramic scaffolds possess the most essential features of a scaffold for bone tissue engineering: they are capable to support and foster relevant cells, able to provide temporary mechanical function, and biodegradable.

Osteoclast-osteoblast communication

Cells in osteoclast and osteoblast lineages communicate with each other through cell-cell contact, diffusible paracrine factors and cell-bone matrix interaction. Osteoclast-osteoblast communication occurs in a basic multicellular unit (BMU) at the initiation, transition and termination phases of bone remodeling. At the initiation phase, hematopoietic precursors are recruited to the BMU. These precursors express cell surface receptors including c-Fms, RANK and costimulatory molecules, such as osteoclast-associated receptor (OSCAR), and differentiate into osteoclasts following cell-cell contact with osteoblasts, which express ligands. Subsequently, the transition from bone resorption to formation is mediated by osteoclast-derived 'coupling factors', which direct the differentiation and activation of osteoblasts in resorbed lacunae to refill it with new bone. Bidirectional signaling generated by interaction between ephrinB2 on osteoclasts and EphB4 on osteoblast precursors facilitates the transition. Such interaction is likely to occur between osteoclasts and lining cells in the bone remodeling compartment (BRC). At the termination phase, bone remodeling is completed by osteoblastic bone formation and mineralization of bone matrix. Here, we describe molecular communication between osteoclasts and osteoblasts at distinct phases of bone remodeling.

Chemical and topographical modification of PHBV surface to promote osteoblast alignment and confinement

Proper cell attachment and distribution, and thus stronger association in vivo between a bone implant and native tissue will improve the success of the implant. In this study, the aim was to achieve promotion of attachment and uniform distribution of rat mesenchymal stem cell-derived osteoblasts by introducing chemical and topographical cues on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) film surfaces. As the chemical cues, either alkaline phosphatase was covalently immobilized on the film surface to induce deposition of calcium phosphate minerals or fibrinogen was adsorbed to improve cell adhesion. Microgrooves and micropits were introduced on the film surface by negative replication of micropatterned Si wafers. Both chemical cues improved cell attachment and even distribution on the PHBV films, but Fb was more effective especially when combined with the micropatterns. Cell alignment (<10 degrees deviation angle) parallel to chemically modified microgrooves (1, 3, or 8 microm groove width) and on 10 microm-thick Fb lines printed on the unpatterned films was achieved. The cells on unpatterned and 5 microm-deep micropitted films were distributed and oriented randomly. Results of this study proved that microtopographies on PHBV can improve osseointegration when combined with chemical cues, and that microgrooves and cell adhesive protein lines on PHBV can guide selective osteoblast adhesion and alignment.

Expansion of human bone marrow stromal cells on poly-(dl-lactide-co-glycolide) (PDLLGA) hollow fibres designed for use in skeletal tissue engineering

Strategies to expand human bone marrow stromal cells (HBMSC) for bone tissue engineering are a key to revolutionising the processes involved in three-dimensional skeletal tissue reconstruction. To facilitate this process we believe the use of biodegradable porous poly(DL-lactide-co-glycolide) (PDL LGA) hollow fibres as a scaffold used in combination with HBMSC to initiate natural bone repair and regeneration offers a potential solution. In this study, the biocompatibility of 75:25 PDL LGA fibres with HBMSC and the capacity of a PDL LGA fibre-associated HBMSC-monolayer to establish an osteogenic phenotype in vivo was examined. A high proportion of HBMSC survived when expanded on PDL LGA fibres for 6 days, with only 10% of the propidium iodide (pI)-labelled population represented in the sub-G1 DNA peak on analysis by flow cytometry. Tracking carboxy-fluorescein diacetate, succinimidyl ester (CFSE)-labelled HBMSC by flow cytometry indicated that HBMSC attachment to the P(DL)LGA fibres does not interfere with their rate of proliferation. Furthermore, in response to osteogenic stimuli, HBMSC expanded on PDL LGA fibres can differentiate, as expected, along the osteogenic lineage with associated alkaline phosphatase activity. Following implantation into SCID mice, osteogenic-conditioned PDL LGA fibre-HBMSC graft resulted in type I collagen deposition and associated bone mineralisation and osteoid formation, as evidenced by immunohistochemistry and histology. These studies provide evidence that porous PDL LGA hollow fibre-HBMSC graft is an innovative biomaterial that offers new approaches to mesenchymal cell expansion, which could be utilised as a scaffold for skeletal tissue generation.

Fabrication of nanostructured hydroxyapatite and analysis of human osteoblastic cellular response

Nano-sized hydroxyapatite (HA) powders were produced by a hydrothermal method and a precipitation method. Spark plasma sintering (SPS) was used to fabricate nanostructured HA (NHA) using nano-sized HA powders as a precursor. Conventional sintering was employed to produce microstructured HA (MHA). Characteristics of HA powders and HA bulk ceramics after sintering were investigated by XRD, FTIR, SEM, TEM, particle size distribution, and AFM. Dense compacts consisting of equiaxed grains with an average grain size of approximately 100 nm were obtained by SPS. Human osteoblasts were cultured on both NHA and MHA and cell attachment, proliferation, and mineralization were evaluated. After 90 min incubation, the cell density on NHA surface was significantly higher than that of MHA and glass control, whereas average cell area of a spread cell was significantly lower on NHA surface compared to MHA and glass control after 4 h incubation. Matrix mineralization was determined after 7 and 14 days incubation by using alizarin red assay combined with cetylpyridinium chloride extraction. NHA shows significant enhancement (p < 0.05) in mineralization compared to MHA. Results from this study suggest that NHA may be a much better candidate for clinical use in terms of bioactivity.

Early Tissue Response to Modified Implant Surfaces Using Back Scattered Imaging

PURPOSE

It is now well known that implant surface properties affect osseointegration. Grit-blasting with abrasives and coating by plasma are methods to modify implant surfaces. This study aimed to compare the direction of new bone formation associated with three types of surfaces.

MATERIALS AND METHODS

Titanium (Ti) alloy rods grit-blasted with alumina abrasive (Group 1, G1), with apatitic abrasive (Group 2, G2), and with apatitic abrasive and plasma-sprayed with hydroxyapatite (Group 3, G3) were implanted in surgically created defects in tibias of New Zealand white rabbits for 2 and 4 weeks. After sacrifice, the implants and surrounding bones were obtained and analyzed using back scattered imaging.

RESULTS

Differences in patterns of bone formation among the groups were observed: originating from the cortical bone towards the implant surface (Type A), surrounding the implant (Type B) and originating from the medullary cavity (Type C). G1 and G3 showed Types A and B while G2 exhibited Types A, B and C. After 4 weeks, greater amount of new bone was observed in G2 group compared with those in G1 and G3 groups.

CONCLUSIONS

This study demonstrated that patterns of bone formation are influenced by methods of surface modification.

Soluble and insoluble signals and the induction of bone formation: molecular therapeutics recapitulating development

The osteogenic molecular signals of the transforming growth factor-beta (TGF-beta) superfamily, the bone morphogenetic/osteogenic proteins (BMPs/OPs) and uniquely in primates the TGF-beta isoforms per se, pleiotropic members of the TGF-beta supergene family, induce de novo endochondral bone formation as a recapitulation of embryonic development. Naturally derived BMPs/OPs and gamma-irradiated human recombinant osteogenic protein-1 (hOP-1) delivered by allogeneic and xenogeneic insoluble collagenous matrices initiate de novo bone induction in heterotopic and orthotopic sites of the primate Papio ursinus, culminating in complete calvarial regeneration by day 90 and maintaining the regenerated structures by day 365. The induction of bone by hOP-1 in P. ursinus develops as a mosaic structure with distinct spatial and temporal patterns of gene expression of members of the TGF-beta superfamily that singly, synergistically and synchronously initiate and maintain tissue induction and morphogenesis. The temporal and spatial expressions of TGF-beta1 mRNA indicate a specific temporal transcriptional window during which expression of TGF-beta1 is mandatory for successful and optimal osteogenesis. Highly purified naturally derived bovine BMPs/OPs and hOP-1 delivered by human collagenous bone matrices and porous hydroxyapatite, respectively, induce bone formation in mandibular defects of human patients. By using healthy body sites as bioreactors it is possible to recapitulate embryonic developments by inducing selected biomaterials combined with recombinant proteins to transform into custom-made prefabricated bone grafts for human reconstruction. The osteogenic proteins of the TGF-beta superfamily, BMPs/OPs and TGF-betas, the last endowed with the striking prerogative of inducing endochondral bone formation in primates only, are helping to engineer skeletal reconstruction in molecular terms.

Osteoclast-derived activity in the coupling of bone formation to resorption

The cells of bone and the immune system communicate by means of soluble and membrane-bound cytokines and growth factors. Through local signalling mechanisms, cells of the osteoblast lineage control the formation and activity of osteoclasts and, therefore, the resorption of bone. Both T and B lymphocytes produce activators and inhibitors of osteoclast formation. A local 'coupling factor' linking bone resorption to subsequent formation in the bone multicellular unit (BMU) has long been proposed as the key regulator of the bone remodelling process, but never identified. There is evidence in support of the view that the coupling mechanism is dependent on growth factors released from the bone matrix during resorption, or is generated from maturing osteoblasts. We argue that osteoclasts contribute in important ways to the transiently activated osteoclast, and stimulate osteoblast lineage cells to begin replacing the resorbed bone in each BMU.

Osteoblast attachment and mineralized nodule formation on rough and smooth 45S5 bioactive glass monoliths

Human primary osteoblast responses to smooth and roughened bioactive glass of 45S5 (Bioglass trade mark ) composition (46.1% SiO(2), 26.9% CaO, 2.6% P(2)O(5), 24.4% Na(2)O) were analysed in vitro. The smooth and rough surfaces had R(a) values and peak to valley distances of 0.04, 4.397, 2.027, and 21.328 microm, respectively. Cell attachment and morphology was observed using phalloidin staining of the actin cytoskeleton and revealed significant differences between smooth and rough surfaces. Cells that were spiky in appearance on the rough compared to the smooth surface formed an organized actin matrix much later on the rough surface. Scanning electron microscopy revealed many cell filipodia extending from more rounded cell bodies on the rough surface. A significantly greater number of nodules on the rough surface was observed, and these were shown to mineralize when supplemented with beta-glycerophosphate and dexamethasone. Raman spectroscopy confirmed the presence of hydroxyapatite in the mineralized cultures showing a definite peak at 964 cm(-1). FTIR analysis showed hydroxyapatite formation occurred more rapidly on the rough surface. This study demonstrates that although initial cell morphology was less advanced on the roughened surface, the cells were able to form mineralized nodules in greater numbers. This may have implications to bone tissue engineering using bioactive glasses.

Short-term load bearing capacity of osteochondral autografts implanted by the mosaicplasty technique: an in vitro porcine model

Articular surface congruency and graft stability are considered essential factors in the success of osteochondral grafting; however, quantitative measures of short-term load bearing capacity of grafts implanted by the mosaicplasty technique have not been reported. The purpose of this study was to develop a live tissue in vitro model to examine short-term fixation strength of mosaicplasty autografts immediately after and 1 week following graft implantation. Cylindrical osteochondral autografts were implanted in vitro by the mosaicplasty technique on five pairs of porcine femoral condyles within one and a half hours of animal sacrifice. Immediately following the surgical procedure, graft push-in and pull-out strength tests as well as indentation tests to determine modulus of the surrounding cancellous bone were performed on half of the specimens from the distal femurs of each animal. The remaining specimens, matched for location in the contralateral leg, were incubated in culture medium for 7 days prior to performing the same set of mechanical tests. Averaged push-in and pull-out graft fixation strength decreased 44% from 135.7 to 75.5N over the 7-day period, while no change in modulus was detected in the surrounding cancellous bone. These in vitro results demonstrate a substantial deterioration of short-term fixation strength of mosaicplasty grafts from the immediate post-operative state. Such a reduction in short-term graft load bearing capacity may pose a threat to the surgically established articular surface congruency and blood vessels formed during the early stages of the healing response.

Fibroblast reaction to island topography: changes in cytoskeleton and morphology with time

In order to develop next-generation tissue engineering materials, the understanding of cell responses to novel material surfaces needs to be better understood. Topography presents powerful cues for cells, and it is becoming clear that cells will react to nanometric, as well as micrometric, scale surface features. Polymer-demixing of polystyrene and polybromostyrene has been found to produce nanoscale islands of reproducible height, and is very cheap and fast compared to techniques such as electron beam lithography. This study observed temporal changes in cell morphology and actin and tubulin cytoskeleton using scanning electron and fluorescence microscopy. The results show large differences in cell response to 95 nm high islands from 5 min to 3 weeks of culture. The results also show a change in cell response from initial fast organisation of cytoskeleton in reaction to the islands, through to lack of cell spreading and low recruitment of cell numbers on the islands.

A phage display technique identifies a novel regulator of cell differentiation

The formation of new bone during the process of bone remodeling occurs almost exclusively at sites of prior bone resorption. In an attempt to discover what regulatory pathways are utilized by osteoblasts to effect this site-specific formation event we probed components of an active bone resorption surface with an osteoblast phage expression library. In these experiments primary cultures of rat osteoblasts were used to construct a phage display library in T7 phage. Tartrate-resistant acid phosphatase (type V) (TRAP) was used as the bait in a biopanning procedure. 40 phage clones with very high affinity for TRAP were sequenced, and of the clones with multiple consensus sequences we identified a regulatory protein that modulates osteoblast differentiation. This protein is the TGFbeta receptor-interacting protein (TRIP-1). Our data demonstrate that TRAP activation of TRIP-1 evokes a TGFbeta-like differentiation process. Specifically, TRIP-1 activation increases the activity and expression of osteoblast alkaline phosphatase, osteoprotegerin, collagen, and Runx2. Moreover, we show that TRAP interacts with TRIP intracellularly, that activation of the TGFbeta type II receptor by TRIP-1 occurs in the presence of TRAP and that the differentiation process is mediated through the Smad2/3 pathway. A final experiment demonstrates that osteoblasts, when cultured in osteoclast lacunae containing TRAP, rapidly and specifically differentiate into a mature bone-forming phenotype. We hypothesize that binding to TRAP may be one mechanism by which the full osteoblast phenotype is expressed during the process of bone remodeling.

Nonadhesive nanotopography: Fibroblast response to poly(n-butyl methacrylate)-poly(styrene) demixed surface features

It is becoming clear that cells do not only respond to micrometric scale topography, but may also respond to topography at the nanometric scale. Nano-fabrication methods such as electron beam lithography are, however, expensive and time consuming. Polymer demixing of poly(styrene) and poly(4-bromostyrene) has been found to produce nano-scale islands of reproducible height, and the islands have been previously shown to effect cell events such as adhesion, spreading, proliferation, and differentiation. This study uses demixed poly(styrene) and poly(n-butyl methacrylate) to produce nano-islands with closer packing and narrower widths compared with those previously studied. Observations have been made of morphological and cytoskeletal changes in human fibroblasts interacting with 10- and 50-nm-high islands. The methods used included scanning electron microscopy, fluorescent microscopy, and optical microscopy. The results indicated that the cells do not respond differently to the 10-nm islands compared with planar samples but, in contrast, the 50-nm islands are nonadhesive.