Methodology and Characterization of a 3D Bone Organoid Model Derived from Murine Cells

Here, we report on the development of a cost-effective, well-characterized three-dimensional (3D) model of bone homeostasis derived from commonly available stocks of immortalized murine cell lines and laboratory reagents. This 3D murine-cell-derived bone organoid model (3D-mcBOM) is adaptable to a range of contexts and can be used in conjunction with surrogates of osteoblast and osteoclast function to study cellular and molecular mechanisms that affect bone homeostasis in vitro or to augment in vivo models of physiology or disease. The 3D-mcBOM was established using a pre-osteoblast murine cell line, which was seeded into a hydrogel extracellular matrix (ECM) and differentiated into functional osteoblasts (OBs). The OBs mineralized the hydrogel ECM, leading to the deposition and consolidation of hydroxyapatite into bone-like organoids. Fourier-transform infrared (FTIR) spectroscopy confirmed that the mineralized matrix formed in the 3D-mcBOM was bone. The histological staining of 3D-mcBOM samples indicated a consistent rate of ECM mineralization. Type I collagen C-telopeptide (CTX1) analysis was used to evaluate the dynamics of OC differentiation and activity. Reliable 3D models of bone formation and homeostasis align with current ethical trends to reduce the use of animal models. This functional model of bone homeostasis provides a cost-effective model system using immortalized cell lines and easily procured supplemental compounds, which can be assessed by measuring surrogates of OB and OC function to study the effects of various stimuli in future experimental evaluations of bone homeostasis.

Effect of high cyclic hydrostatic pressure on osteogenesis of mesenchymal stem cells cultured in liquefied micro-compartments

Bone resident cells are constantly subjected to a range of distinct mechanical loadings, which generates a complex microenvironment. In particular, hydrostatic pressure (HP) has a key impact on modulation of cell function and fate determination. Although HP is a constant mechanical stimulus, its role in regulating the osteogenesis process within a defined 3D microenvironment has not been comprehensively elucidated. Perceiving how environmental factors regulate the differentiation of stem cells is essential for expanding their regenerative potential. Inspired by the mechanical environment of bone, this study attempted to investigate the influence of different ranges of cyclic HP on human adipose-derived mesenchymal stem cells (MSCs) encapsulated within a compartmentalized liquefied microenvironment. Taking advantage of the liquefied environment of microcapsules, MSCs were exposed to cyclic HP of 5 or 50 MPa, 3 times/week at 37 °C. Biological tests using fluorescence staining of F-actin filaments showed a noticeable improvement in cell-cell interactions and cellular network formation of MSCs. These observations were more pronounced in osteogenic (OST) condition, as confirmed by fluorescent staining of vinculin. More interestingly, there was a significant increase in alkaline phosphatase activity of MSCs exposed to 50 MPa magnitude of HP, even in the absence of osteoinductive factors. In addition, a greater staining area of both osteopontin and hydroxyapatite was detected in the 50 MPa/OST group. These findings highlight the benefit of hydrostatic pressure to regulate osteogenesis of MSCs as well as the importance of employing simultaneous biochemical and mechanical stimulation to accelerate the osteogenic potential of MSCs for biomedical purposes.

The intricate mechanism of PLS3 in bone homeostasis and disease

Since our discovery in 2013 that genetic defects in PLS3 lead to bone fragility, the mechanistic details of this process have remained obscure. It has been established that PLS3 variants cause syndromic and nonsyndromic osteoporosis as well as osteoarthritis. PLS3 codes for an actin-bundling protein with a broad pattern of expression. As such, it is puzzling how PLS3 specifically leads to bone-related disease presentation. Our review aims to summarize the current state of knowledge regarding the function of PLS3 in the predominant cell types in the bone tissue, the osteocytes, osteoblasts and osteoclasts. This is related to the role of PLS3 in regulating mechanotransduction, calcium regulation, vesicle trafficking, cell differentiation and mineralization as part of the complex bone pathology presented by PLS3 defects. Considering the consequences of PLS3 defects on multiple aspects of bone tissue metabolism, our review motivates the study of its mechanism in bone diseases which can potentially help in the design of suitable therapy.

Role of Polyphenols in the Metabolism of the Skeletal System in Humans and Animals – A Review

Polyphenols are a group of compounds arousing enormous interest due to their multiple effects on both human and animal health and omnipresence in plants. A number of in vitro and animal model studies have shown that all polyphenols exhibit anti-inflammatory and antioxidant activities, and play a significant role against oxidative stress-related pathologies. They also exert gut promotory effects and prevent chronic degenerative diseases. However, less attention has been paid to the potential influence of polyphenols on bone properties and metabolism. It is well known that proper growth and functioning of the organism depend largely on bone growth and health. Therefore, understanding the action of substances (including polyphenols) that may improve the health and functioning of the skeletal system and bone metabolism is extremely important for the health of the present and future generations of both humans and farm animals. This review provides a comprehensive summary of literature related to causes of bone loss during ageing of the organism (in both humans and animals) and possible effects of dietary polyphenols preventing bone loss and diseases. In particular, the underlying cellular and molecular mechanisms that can modulate skeletal homeostasis and influence the bone modeling and remodeling processes are presented.

On the Use of Black Ti as a Bone Substituting Biomaterial: Behind the Scenes of Dual-Functionality

Despite the potential of small-scale pillars of black titanium (bTi) for killing the bacteria and directing the fate of stem cells, not much is known about the effects of the pillars' design parameters on their biological properties. Here, three distinct bTi surfaces are designed and fabricated through dry etching of the titanium, each featuring different pillar designs. The interactions of the surfaces with MC3T3-E1 preosteoblast cells and Staphylococcus aureus bacteria are then investigated. Pillars with different heights and spatial organizations differently influence the morphological characteristics of the cells, including their spreading area, aspect ratio, nucleus area, and cytoskeletal organization. The preferential formation of focal adhesions (FAs) and their size variations also depend on the type of topography. When the pillars are neither fully separated nor extremely tall, the colocalization of actin fibers and FAs as well as an enhanced matrix mineralization are observed. However, the killing efficiency of these pillars against the bacteria is not as high as that of fully separated and tall pillars. This study provides a new perspective on the dual-functionality of bTi surfaces and elucidates how the surface design and fabrication parameters can be used to achieve a surface topography with balanced bactericidal and osteogenic properties.

Bio-artificial bone formation model with a radial-flow bioreactor for implant therapy-comparison between two cell culture carriers: porous hydroxyapatite and β-tricalcium phosphate beads

Bone grafting is necessary before dental implant treatment in patients with jaw bone defects. Currently, autologous bone grafting is a major burden on the patient. However, it is impossible to form a sufficient foundation for the implant with a bone-filling agent alone. It is, therefore, necessary to prepare hybrid artificial bone tissue containing osteoblasts and osteoclasts. In this study, mouse MC3T3-E1 pre-osteoblast cells and human embryonic-derived osteoblastic cell line hFOB1.19 were cultured in radial-flow bioreactors (RFB) to form three-dimensional artificial bone filled with porous beads of β-tricalcium phosphate (β-TCP) or hydroxyapatite (HA)—which are clinically used as bone-filling agents—as cell culture carriers. When circulation culturing was performed in the growth medium for the first 10–12 days, glucose consumption was increased in the cultures with HA beads in comparison to the cultures with β-TCP beads. When cultured in the differentiation culture medium during the second half of the culture period, the glucose consumption decreased in the culture with HA beads. A DNA microarray analysis suggested that osteogenesis progressed fast in three-dimensional culture filled with HA beads and that partly differentiation into osteoblasts was prominent in cultures with β-TCP beads. In the growth process of MC3T3-E1 cells, the vitamin A metabolism was also activated, the synthesis and degradation of retinoic acid was enhanced, and the metabolism of the same process decreased at the end of differentiation in three-dimensional cultures. Three-dimensional circulation culture in RFB is considered to be useful for the formation of hybrid bio-artificial bone tissue.

Enhancement of synthesis of extracellular matrix proteins on retinoic acid loaded electrospun scaffolds

Electrospinning is a renowned technique for the generation of ultrafine, micro- and nanoscale fibres due to its simplicity, versatility and tunability. Owing to its adaptability to a wide selection of materials and scaffold architectures, electrospun meshes have been developed as biocompatible scaffolds and drug delivery systems for tissue engineering. Here, we developed a drug delivery scaffold by electrospinning poly(ε-caprolactone) (PCL) directly blended with a therapeutic agent, retinoic acid (RA), at different concentrations. The release profile, DNA, and elastin analysis of direct and transwell seeded RA-loaded PCL electrospun scaffolds showed desirable controlled release at 15 kV fabrication, with 0.01% RA as the optimum concentration. The selected 0.01% (w/v) RA-loaded PCL meshes were further analysed using five different seeding cultures to investigate and extensively distinguish the effects of RA release with or without cell contact to the PCL electrospun meshes for cell morphology, proliferation and extracellular matrix (ECM) protein secretion of collagen and elastin. Upon exposure to RA-loaded PCL scaffolds, an increase of human dermal fibroblast (HDF) proliferation was observed. In contrast, human mesenchymal stromal cell (hMSC) cultures showed a decrease in cell proliferation. For both hMSC and HDF cultures, exposure to RA-loaded PCL scaffolds provided a significant increase in elastin production per cell. For collagen expression, a slight increase was measured and was outperformed by the 3D geometry stimulation from PCL scaffolds. In contrast to hMSCs, HDFs showed enhanced stress actin fibres in cultures with RA-loaded PCL scaffolds. Both cell types exhibited more vinculin expression when seeded to RA-loaded PCL scaffolds. Hence, electrospun scaffolds releasing RA in a controlled manner were able to regulate cell proliferation, morphology and ECM secretion, and present an attractive approach for optimizing tissue regeneration.

Impact of flow shear stress on morphology of osteoblast-like IDG-SW3 cells

This study constructed an in situ cell culture, real-time observation system based originally on a microfluidic channel, and reported the morphological changes of late osteoblast-like IDG-SW3 cells in response to flow shear stress (FSS). The effects of high (1.2 Pa) and low (0.3 Pa) magnitudes of unidirectional FSS and three concentrations of extracellular Type I collagen (0.1, 0.5, and 1 mg/mL) coating on cell morphology were investigated. IDG-SW3 cells were cultured in polydimethylsiloxane microfluidic channels. Cell images were recorded real-time under microscope at intervals of 1 min. Cell morphology was characterized by five parameters: cellular area, cell elongation index, cellular alignment, cellular process length, and number of cellular process per cell. Immunofluorescence assay was used to detect stress fiber distribution and vinculin expression. The results showed that 1.2 Pa, but not 0.3 Pa of FSS induced a significant morphological change in late osteoblast-like IDG-SW3 cells, which may be caused by the alteration of cellular adhesion with matrix in response to FSS. Moreover, the amount of collagen matrix, alignment of fiber stress and expression of vinculin were closely correlated with the morphological changes of IDG-SW3 cells. This study suggests that osteoblasts are very responsive to the magnitudes of FSS, and extracellular collagen matrix and focal adhesion are directly involved in the morphological changes adaptive to FSS.

A Precisely Flow-Controlled Microfluidic System for Enhanced Pre-Osteoblastic Cell Response for Bone Tissue Engineering

Bone tissue engineering provides advanced solutions to overcome the limitations of currently used therapies for bone reconstruction. Dynamic culturing of cell-biomaterial constructs positively affects the cell proliferation and differentiation. In this study, we present a precisely flow-controlled microfluidic system employed for the investigation of bone-forming cell responses cultured on fibrous collagen matrices by applying two flow rates, 30 and 50 μL/min. We characterized the collagen substrates morphologically by means of scanning electron microscopy, investigated their viscoelastic properties, and evaluated the orientation, proliferation and osteogenic differentiation capacity of pre-osteoblastic cells cultured on them. The cells are oriented along the direction of the flow at both rates, in contrast to a random orientation observed under static culture conditions. The proliferation of cells after 7 days in culture was increased at both flow rates, with the flow rate of 50 μL/min indicating a significant increase compared to the static culture. The alkaline phosphatase activity after 7 days increased at both flow rates, with the rate of 30 μL/min indicating a significant enhancement compared to static conditions. Our results demonstrate that precisely flow-controlled microfluidic cell culture provides tunable control of the cell microenvironment that directs cellular activities involved in bone regeneration.

Muscle-bone interactions: From experimental models to the clinic? A critical update

Bone is a biomechanical tissue shaped by forces from muscles and gravitation. Simultaneous bone and muscle decay and dysfunction (osteosarcopenia or sarco-osteoporosis) is seen in ageing, numerous clinical situations including after stroke or paralysis, in neuromuscular dystrophies, glucocorticoid excess, or in association with vitamin D, growth hormone/insulin like growth factor or sex steroid deficiency, as well as in spaceflight. Physical exercise may be beneficial in these situations, but further work is still needed to translate acceptable and effective biomechanical interventions like vibration therapy from animal models to humans. Novel antiresorptive and anabolic therapies are emerging for osteoporosis as well as drugs for sarcopenia, cancer cachexia or muscle wasting disorders, including antibodies against myostatin or activin receptor type IIA and IIB (e.g. bimagrumab). Ideally, increasing muscle mass would increase muscle strength and restore bone loss from disuse. However, the classical view that muscle is unidirectionally dominant over bone via mechanical loading is overly simplistic. Indeed, recent studies indicate a role for neuronal regulation of not only muscle but also bone metabolism, bone signaling pathways like receptor activator of nuclear factor kappa-B ligand (RANKL) implicated in muscle biology, myokines affecting bone and possible bone-to-muscle communication. Moreover, pharmacological strategies inducing isolated myocyte hypertrophy may not translate into increased muscle power because tendons, connective tissue, neurons and energy metabolism need to adapt as well. We aim here to critically review key musculoskeletal molecular pathways involved in mechanoregulation and their effect on the bone-muscle unit as a whole, as well as preclinical and emerging clinical evidence regarding the effects of sarcopenia therapies on osteoporosis and vice versa.

Preclinical models for in vitro mechanical loading of bone-derived cells

It is well established that bone responds to mechanical stimuli whereby physical forces are translated into chemical signals between cells, via mechanotransduction. It is difficult however to study the precise cellular and molecular responses using in vivo systems. In vitro loading models, which aim to replicate forces found within the bone microenvironment, make the underlying processes of mechanotransduction accessible to the researcher. Direct measurements in vivo and predictive modeling have been used to define these forces in normal physiological and pathological states. The types of mechanical stimuli present in the bone include vibration, fluid shear, substrate deformation and compressive loading, which can all be applied in vitro to monolayer and three-dimensional (3D) cultures. In monolayer, vibration can be readily applied to cultures via a low-magnitude, high-frequency loading rig. Fluid shear can be applied to cultures in multiwell plates via a simple rocking platform to engender gravitational fluid movement or via a pump to cells attached to a slide within a parallel-plate flow chamber, which may be micropatterned for use with osteocytes. Substrate strain can be applied via the vacuum-driven FlexCell system or via a four-point loading jig. 3D cultures better replicate the bone microenvironment and can also be subjected to the same forms of mechanical stimuli as monolayer, including vibration, fluid shear via perfusion flow, strain or compression. 3D cocultures that more closely replicate the bone microenvironment can be used to study the collective response of several cell types to loading. This technical review summarizes the methods for applying mechanical stimuli to bone cells in vitro.

Biomechanical forces in the skeleton and their relevance to bone metastasis: biology and engineering considerations

Bone metastasis represents the leading cause of breast cancer related-deaths. However, the effect of skeleton-associated biomechanical signals on the initiation, progression, and therapy response of breast cancer bone metastasis is largely unknown. This review seeks to highlight possible functional connections between skeletal mechanical signals and breast cancer bone metastasis and their contribution to clinical outcome. It provides an introduction to the physical and biological signals underlying bone functional adaptation and discusses the modulatory roles of mechanical loading and breast cancer metastasis in this process. Following a definition of biophysical design criteria, in vitro and in vivo approaches from the fields of bone biomechanics and tissue engineering that may be suitable to investigate breast cancer bone metastasis as a function of varied mechano-signaling will be reviewed. Finally, an outlook of future opportunities and challenges associated with this newly emerging field will be provided.

A new method to investigate how mechanical loading of osteocytes controls osteoblasts

Mechanical loading, a potent stimulator of bone formation, is governed by osteocyte regulation of osteoblasts. We developed a three-dimensional (3D) in vitro co-culture system to investigate the effect of loading on osteocyte-osteoblast interactions. MLO-Y4 cells were embedded in type I collagen gels and MC3T3-E1(14) or MG63 cells layered on top. Ethidium homodimer staining of 3D co-cultures showed 100% osteoblasts and 86% osteocytes were viable after 7 days. Microscopy revealed osteoblasts and osteocytes maintain their respective ovoid/pyriform and dendritic morphologies in 3D co-cultures. Reverse-transcriptase quantitative polymerase chain reaction (RT-qPCR) of messenger ribonucleic acid (mRNA) extracted separately from osteoblasts and osteocytes, showed that podoplanin (E11), osteocalcin, and runt-related transcription factor 2 mRNAs were expressed in both cell types. Type I collagen (Col1a1) mRNA expression was higher in osteoblasts (P < 0.001), whereas, alkaline phosphatase mRNA was higher in osteocytes (P = 0.001). Immunohistochemistry revealed osteoblasts and osteocytes express E11, type I pro-collagen, and connexin 43 proteins. In preliminary experiments to assess osteogenic responses, co-cultures were treated with human recombinant bone morphogenetic protein 2 (BMP-2) or mechanical loading using a custom built loading device. BMP-2 treatment significantly increased osteoblast Col1a1 mRNA synthesis (P = 0.031) in MLO-Y4/MG63 co-cultures after 5 days treatment. A 16-well silicone plate, loaded (5 min, 10 Hz, 2.5 N) to induce 4000-4500 με cyclic compression within gels increased prostaglandin E2 (PGE2) release 0.5 h post-load in MLO-Y4 cells pre-cultured in 3D collagen gels for 48, 72 h, or 7 days. Mechanical loading of 3D co-cultures increased type I pro-collagen release 1 and 5 days later. These methods reveal a new osteocyte-osteoblast co-culture model that may be useful for investigating mechanically induced osteocyte control of osteoblast bone formation.

Constitutive nuclear expression of dentin matrix protein 1 fails to rescue the Dmp1-null phenotype

Dentin matrix protein 1 (DMP1) plays multiple roles in bone, tooth, phosphate homeostasis, kidney, salivary gland, reproductive cycles, and the development of cancer. In vitro studies have indicated two different biological mechanisms: 1) as a matrix protein, DMP1 interacts with αvβ3 integrin and activates MAP kinase signaling; and 2) DMP1 serves as a transcription co-factor. In vivo studies have demonstrated its key role in osteocytes. This study attempted to determine whether DMP1 functions as a transcription co-factor and regulates osteoblast functions. For gene expression comparisons using adenovirus constructs, we targeted the expression of DMP1 either to the nucleus only by replacing the endogenous signal peptide with a nuclear localization signal (NLS) sequence (referred to as (NLS)DMP1) or to the extracellular matrix as the WT type (referred to as (SP)DMP1) in MC3T3 osteoblasts. High levels of DMP1 in either form greatly increased osteogenic gene expression in an identical manner. However, the targeted (NLS)DMP1 transgene driven by a 3.6-kb rat Col 1α1 promoter in the nucleus of osteoblasts and osteocytes failed to rescue the phenotyope of Dmp1-null mice, whereas the (SP)DMP1 transgene rescued the rickets defect. These studies support the notion that DMP1 functions as an extracellular matrix protein, rather than as a transcription co-factor in vivo. We also show that DMP1 continues its expression in osteoblasts during postnatal development and that the deletion of Dmp1 leads to an increase in osteoblast proliferation. However, poor mineralization in the metaphysis indicates a critical role for DMP1 in both osteoblasts and osteocytes.

Fluid flow modulates the expression of genes involved in the Wnt signaling pathway in osteoblasts in 3D culture conditions

The balance between osteoclastic bone resorption and osteoblastic bone formation maintains bone mass, while mechanical loads stimulate bone formation and suppress resorption. The molecular mechanisms responsible for this process have not yet been fully elucidated. In the present study, we assessed whether mechanical stimulation by pulsating fluid flow (PFF) leads to functional Wnt production and affects the function of osteoblasts. ROS17/2.8 osteoblasts were submitted to 1-4 h PFF (0.8 Pa) by three-dimensional (3D) cell culture system with fluid flow. PFF upregulated the gene expression levels of adenomatous polyposis coli, alkaline phosphatase, low density lipoprotein receptor-related protein 5 (LRP5), Wnt3a and β-catenin [catenin beta 1 (CTNNB1)] in all the groups of osteoblasts. Our results suggest that mechanical stimulation by PFF induces the differentiation of osteoblasts and the activation of the Wnt/β-catenin signaling pathway in a 3D cell culture system. Furthermore, mechanical stress plays an important role in the Wnt/β-catenin signaling pathway and is involved in bone formation.

Macro and microfluidic flows for skeletal regenerative medicine

Fluid flow has a great potential as a cell stimulatory tool for skeletal regenerative medicine, because fluid flow-induced bone cell mechanotransduction in vivo plays a critical role in maintaining healthy bone homeostasis. Applications of fluid flow for skeletal regenerative medicine are reviewed at macro and microscale. Macroflow in two dimensions (2D), in which flow velocity varies along the normal direction to the flow, has explored molecular mechanisms of bone forming cell mechanotransduction responsible for flow-regulated differentiation, mineralized matrix deposition, and stem cell osteogenesis. Though 2D flow set-ups are useful for mechanistic studies due to easiness in in situ and post-flow assays, engineering skeletal tissue constructs should involve three dimensional (3D) flows, e.g., flow through porous scaffolds. Skeletal tissue engineering using 3D flows has produced promising outcomes, but 3D flow conditions (e.g., shear stress vs. chemotransport) and scaffold characteristics should further be tailored. Ideally, data gained from 2D flows may be utilized to engineer improved 3D bone tissue constructs. Recent microfluidics approaches suggest a strong potential to mimic in vivo microscale interstitial flows in bone. Though there have been few microfluidics studies on bone cells, it was demonstrated that microfluidic platform can be used to conduct high throughput screening of bone cell mechanotransduction behavior under biomimicking flow conditions.

Mechanical stretching for tissue engineering: two-dimensional and three-dimensional constructs

Mechanical cell stretching may be an attractive strategy for the tissue engineering of mechanically functional tissues. It has been demonstrated that cell growth and differentiation can be guided by cell stretch with minimal help from soluble factors and engineered tissues that are mechanically stretched in bioreactors may have superior organization, functionality, and strength compared with unstretched counterparts. This review explores recent studies on cell stretching in both two-dimensional (2D) and three-dimensional (3D) setups focusing on the applications of stretch stimulation as a tool for controlling cell orientation, growth, gene expression, lineage commitment, and differentiation and for achieving successful tissue engineering of mechanically functional tissues, including cardiac, muscle, vasculature, ligament, tendon, bone, and so on. Custom stretching devices and lab-specific mechanical bioreactors are described with a discussion on capabilities and limitations. While stretch mechanotransduction pathways have been examined using 2D stretch, studying such pathways in physiologically relevant 3D environments may be required to understand how cells direct tissue development under stretch. Cell stretch study using 3D milieus may also help to develop tissue-specific stretch regimens optimized with biochemical feedback, which once developed will provide optimal tissue engineering protocols.

Effect of pulse frequency on the osteogenic differentiation of mesenchymal stem cells in a pulsatile perfusion bioreactor

Perfusion bioreactors are a promising in vitro strategy to engineer bone tissue because they supply needed oxygen and nutrients and apply an osteoinductive mechanical stimulus to osteoblasts within large porous three-dimensional scaffolds. Model two-dimensional studies have shown that dynamic flow conditions (e.g., pulsatile oscillatory waveforms) elicit an enhanced mechanotransductive response and elevated expression of osteoblastic proteins relative to steady flow. However, dynamic perfusion of three-dimensional scaffolds has been primarily examined in short term cultures to probe for early markers of mechanotransduction. Therefore, the objective of this study was to investigate the effect of extended dynamic perfusion culture on osteoblastic differentiation of primary mesenchymal stem cells (MSCs). To accomplish this, rat bone marrow-derived MSCs were seeded into porous foam scaffolds and cultured for 15 days in osteogenic medium under pulsatile regimens of 0.083, 0.050, and 0.017 Hz. Concurrently, MSCs seeded in scaffolds were also maintained under static conditions or cultured under steady perfusion. Analysis of the cells after 15 days of culture indicated that alkaline phosphatase (ALP) activity, mRNA expression of osteopontin (OPN), and accumulation of OPN and prostaglandin E(2) were enhanced for all four perfusion conditions relative to static culture. ALP activity, OPN and OC mRNA, and OPN protein accumulation were slightly higher for the intermediate frequency (0.05 Hz) as compared with the other flow conditions, but the differences were not statistically significant. Nevertheless, these results demonstrate that dynamic perfusion of MSCs may be a useful strategy for stimulating osteoblastic differentiation in vitro.

Biological characterization of woven fabric using two- and three-dimensional cell cultures

The integration and long-term functional retention of tissue implants are both strongly linked to the implant material characteristics. As a first approach, the cytocompatibility and bioactivity of such materials are evaluated using in vitro-based cell culture models. Typically, in vitro bioactivity is assessed by seeding single cells onto the test material to evaluate certain parameters such as cell adhesion, survival, proliferation, and functional differentiation. Probably, due to the reduction from three dimensional (3D) toward the two dimensional (2D) situation the data obtained from 2D culture models falls short of predicting the in vivo behavior of the biomaterial in question. In this study, a three dimensional (3D) in vitro cell culture model was applied to evaluate the bioactivity of well characterized fiber-based scaffolds using scaffold colonization as a bioactivity indicator. Cell behavior in this culture model was evaluated against a classical comparable, 2D cell culture system using polyethylene terephthalat and polyamide 6.6 fabrics. By using the 3D culture model, however, differences in cell population performance as a function of fiber diameter and mesh angle were evident. The use of 3D cell culture model clearly outperformed typical cell culture setup as means to evaluate cell population-scaffold interaction.

Mechanical regulation of vascular growth and tissue regeneration in vivo

New vascular network formation is a critical step in the wound healing process and a primary limiting factor in functional tissue regeneration. Like many tissues, neovascular networks have been shown in vitro to be highly sensitive to mechanical conditions; however, the effects of matrix deformations on neovascular network formation and remodeling in engineered tissue regeneration in vivo have not been evaluated. We quantified the effects of early and delayed functional loading on neovascular growth in a rat model of large bone defect regeneration using compliant fixation plates that were unlocked to allow transfer of ambulatory loads to the defect either at the time of implantation (early), or after 4 wk of stiff fixation (delayed). Neovascular growth and bone regeneration were quantitatively evaluated 3 wk after the onset of loading by contrast-enhanced microcomputed tomography and histology. The initial vascular response to bone injury featured robust angiogenesis and collateral vessel formation, increasing parameters such as vascular volume and connectivity while decreasing degree of anisotropy. Application of early mechanical loading significantly inhibited vascular invasion into the defect by 66% and reduced bone formation by 75% in comparison to stiff plate controls. In contrast, delaying the onset of loading by 4 wk significantly enhanced bone formation by 20% and stimulated vascular remodeling by increasing the number of large vessels and decreasing the number of small vessels. Together, these data demonstrate the mechanosensitivity of neovascular networks and highlight the capacity of biomechanical stimulation to modulate postnatal vascular growth and remodeling.

Role of NF-κB in the skeleton

Since the discovery that deletion of the NF-κB subunits p50 and p52 causes osteopetrosis in mice, there has been considerable interest in the role of NF-κB signaling in bone. NF-κB controls the differentiation or activity of the major skeletal cell types - osteoclasts, osteoblasts, osteocytes and chondrocytes. However, with five NF-κB subunits and two distinct activation pathways, not all NF-κB signals lead to the same physiologic responses. In this review, we will describe the roles of various NF-κB proteins in basal bone homeostasis and disease states, and explore how NF-κB inhibition might be utilized therapeutically.