Mechanical loading stimulates the release of transforming growth factor-beta activity by cultured mouse calvariae and periosteal cells

A periosteum-derived cell line to study the role of BMP/TGFβ signaling in periosteal cell behavior and function

The periosteum is a thin tissue surrounding each skeletal element that contains stem and progenitor cells involved in bone development, postnatal appositional bone growth, load-induced bone formation, and fracture repair. BMP and TGFβ signaling are important for periosteal activity and periosteal cell behavior, but thorough examination of the influence of these pathways on specific cell populations resident in the periosteum is lacking due to limitations associated with primary periosteal cell isolations and in vitro experiments. Here we describe the generation of a novel periosteum-derived clonal cell (PDC) line from postnatal day 14 mice and use it to examine periosteal cell behavior in vitro. PDCs exhibit key characteristics of periosteal cells observed during skeletal development, maintenance, and bone repair. Specifically, PDCs express established periosteal markers, can be expanded in culture, demonstrate the ability to differentiate into chondrocytes, osteoblasts, and adipocytes, and exhibit an osteogenic response to physical stimulation. PDCs also engage in BMP and/or TGFβ signaling when treated with the activating ligands BMP2 and TGFβ-1, and in response to mechanical stimulation via fluid shear. We believe that this PDC line will be useful for large-scale, long-term experiments that were not feasible when using primary periosteal cells. Anticipated future uses include advancing our understanding of the signaling interactions that occur during appositional bone growth and fracture repair and developing drug screening platforms to discover novel growth and fracture healing factors.

A Bioreactor for 3D In Vitro Modeling of the Mechanical Stimulation of Osteocytes

The bone is a mechanosensitive organ that is also a common metastatic site for prostate cancer. However, the mechanism by which the tumor interacts with the bone microenvironment to further promote disease progression remains to be fully understood. This is largely due to a lack of physiological yet user-friendly models that limit our ability to perform in-depth mechanistic studies. Here, we report a tunable bioreactor which facilitates the 3D culture of the osteocyte cell line, MLO-Y4, in a hydroxyapatite/tricalcium phosphate (HA/TCP) scaffold under constant fluidic shear stress and tunable hydrostatic pressure within physiological parameters. Increasing hydrostatic pressure was sufficient to induce a change in the expression of several bone remodeling genes such as Dmp1, Rankl, and Runx2. Furthermore, increased hydrostatic pressure induced the osteocytes to promote the differentiation of the murine macrophage cell line RAW264.7 toward osteoclast-like cells. These results demonstrate that the bioreactor recapitulates the mechanotransduction response of osteocytes to pressure including the measurement of their functional ability in a 3D environment. In conclusion, the bioreactor would be useful for exploring the mechanisms of osteocytes in bone health and disease.

Differential behaviour and gene expression in 3D cultures of femoral- and calvarial-derived human osteoblasts under a cyclic compressive mechanical load

The aim of the study was to compare the response of calvarial and femoral osteoblasts cultured in a 3D hydrogel environment to cyclic compressive mechanical loading. Human foetal femoral and calvarial osteoblasts were encapsulated in a semi-synthetic thiol-modified hyaluronan gelatin polyethylene glycol diacrylate (PEGDA) cross-linked HyStemC hydrogel. Constructs were subjected to a cyclic compressive strain of 33.4 kPa force every second for 5 s every hour for 6 h per day using FlexCell BioPress culture plates and compared to non-compressed constructs. Cell viability, mineralisation, and morphological changes were observed over 21 days. BMP2, ALP, COL1A1, COL2A1, and OCN gene expression levels were quantified. Encapsulated osteoblast numbers increased and formed hydroxyapatite over a 21-day period. Cell viability decreased under a cyclical strain when compared to cells under no strain. Femoral osteoblasts under strain expressed increased levels of BMP2 (53.9-fold) and COL1A1 (5.1-fold) mRNA compared to no strain constructs. Surprisingly, no BMP2 mRNA was detected in calvarial osteoblasts. Osteoblasts derived from endochondral (femoral) and intra-membranous (calvarial) processes behaved differently in 3D-constructs. We therefore recommend that site-specific osteoblasts be used for future bone engineering and bone replacement materials and further research undertaken to elucidate how site-specific osteoblasts respond to cyclic compressive loads.

Shear Stress Modulates Osteoblast Cell and Nucleus Morphology and Volume

Mechanical loading preserves bone mass and function—yet, little is known about the cell biological basis behind this preservation. For example, cell and nucleus morphology are critically important for cell function, but how these morphological characteristics are affected by the physiological mechanical loading of bone cells is under-investigated. This study aims to determine the effects of fluid shear stress on cell and nucleus morphology and volume of osteoblasts, and how these effects relate to changes in actin cytoskeleton and focal adhesion formation. Mouse calvaria 3T3-E1 (MC3T3-E1) osteoblasts were treated with or without 1 h pulsating fluid flow (PFF). Live-cell imaging was performed every 10 min during PFF and immediately after PFF. Cytoskeletal organization and focal adhesions were visualized, and gene and protein expression quantified. Two-dimensional (2D) and three-dimensional (3D) morphometric analyses were made using MeasureStack and medical imaging interaction toolkit (MITK) software. 2D-images revealed that 1 h PFF changed cell morphology from polygonal to triangular, and nucleus morphology from round to ellipsoid. PFF also reduced cell surface area (0.3-fold), cell volume (0.3-fold), and nucleus volume (0.2-fold). During PFF, the live-cell volume gradually decreased from 6000 to 3000 µm³. After PFF, α-tubulin orientation was more disorganized, but F-actin fluorescence intensity was enhanced, particularly around the nucleus. 3D-images obtained from Z-stacks indicated that PFF increased F-actin fluorescence signal distribution around the nucleus in the XZ and YZ direction (2.3-fold). PFF increased protein expression of phospho-paxillin (2.0-fold) and integrin-α5 (2.8-fold), but did not increase mRNA expression of paxillin-a (PXNA), paxillin-b (PXNB), integrin-α5 (ITGA51), or α-tubulin protein expression. In conclusion, PFF induced substantial changes in osteoblast cytoskeleton, as well as cell and nucleus morphology and volume, which was accompanied by elevated gene and protein expression of adhesion and structural proteins. More insights into the mechanisms whereby mechanical cues drive morphological changes in bone cells, and thereby, possibly in bone cell behavior, will aid the guidance of clinical treatment, particularly in the field of orthodontics, (oral) implantology, and orthopedics.

FGF and TGFβ signaling link form and function during jaw development and evolution

How does form arise during development and change during evolution? How does form relate to function, and what enables embryonic structures to presage their later use in adults? To address these questions, we leverage the distinct functional morphology of the jaw in duck, chick, and quail. In connection with their specialized mode of feeding, duck develop a secondary cartilage at the tendon insertion of their jaw adductor muscle on the mandible. An equivalent cartilage is absent in chick and quail. We hypothesize that species-specific jaw architecture and mechanical forces promote secondary cartilage in duck through the differential regulation of FGF and TGFβ signaling. First, we perform transplants between chick and duck embryos and demonstrate that the ability of neural crest mesenchyme (NCM) to direct the species-specific insertion of muscle and the formation of secondary cartilage depends upon the amount and spatial distribution of NCM-derived connective tissues. Second, we quantify motility and build finite element models of the jaw complex in duck and quail, which reveals a link between species-specific jaw architecture and the predicted mechanical force environment. Third, we investigate the extent to which mechanical load mediates FGF and TGFβ signaling in the duck jaw adductor insertion, and discover that both pathways are mechano-responsive and required for secondary cartilage formation. Additionally, we find that FGF and TGFβ signaling can also induce secondary cartilage in the absence of mechanical force or in the adductor insertion of quail embryos. Thus, our results provide novel insights on molecular, cellular, and biomechanical mechanisms that couple musculoskeletal form and function during development and evolution.

Periosteal progenitors contribute to load-induced bone formation in adult mice and require primary cilia to sense mechanical stimulation

BACKGROUND

The fully developed adult skeleton adapts to mechanical forces by generating more bone, usually at the periosteal surface. Progenitor cells in the periosteum are believed to differentiate into bone-forming osteoblasts that contribute to load-induced adult bone formation, but in vivo evidence does not yet exist. Furthermore, the mechanism by which periosteal progenitors might sense physical loading and trigger differentiation is unknown. We propose that periosteal osteochondroprogenitors (OCPs) directly sense mechanical load and differentiate into bone-forming osteoblasts via their primary cilia, mechanosensory organelles known to be involved in osteogenic differentiation.

METHODS

We generated a diphtheria toxin ablation mouse model and performed ulnar loading and dynamic histomorphometry to quantify the contribution of periosteal OCPs in adult bone formation in vivo. We also generated a primary cilium knockout model and isolated periosteal cells to study the role of the cilium in periosteal OCP mechanosensing in vitro. Experimental groups were compared using one-way analysis of variance or student's t test, and sample size was determined to achieve a minimum power of 80%.

RESULTS

Mice without periosteal OCPs had severely attenuated mechanically induced bone formation and lacked the mineralization necessary for daily skeletal maintenance. Our in vitro results demonstrate that OCPs in the periosteum uniquely sense fluid shear and exhibit changes in osteogenic markers consistent with osteoblast differentiation; however, this response is essentially lost when the primary cilium is absent.

CONCLUSIONS

Combined, our data show that periosteal progenitors are a mechanosensitive cell source that significantly contribute to adult skeletal maintenance. More importantly, an OCP population persists in the adult skeleton and these cells, as well as their cilia, are promising targets for bone regeneration strategies.

Shock Wave Therapy as an Innovative Technology in Skeletal Disorders: Study on Transmembrane Current in Stimulated Osteoblast-Like Cells

Extracorporeal shock wave treatment (ESWT) is successfully used in various musculoskeletal disorders and pathologies. Despite the increasing use of this kind of therapy, some aspects of its mechanism of action are still unclear. In vitro bone cell behavior under ESWT were previously investigated by the present author and MG63 osteoblast-like cells showed an enhancement in proliferation and in the osteoblast differentiation after therapy with a low-energy flux density. The aim of the present study was to evaluate the effect of ESWT on the permeabilization of cell membrane. We characterized physiological changes in the MG63 associated with ESWT generated by an ESW device and patch clamp recording was performed to study ion channels. Experiments were carried out using the whole-cell recording configuration of the patch-clamp technique and the ionic current measurements were performed on cell samples of ESW treated and control groups. The patch-clamp technique showed the effect of ESWT on the amplitude of transmembrane currents. The treatment with ESW enhanced the transmembrane current as well the voltage dependence of Ca-activated and K channels that mediate these currents: the differences between treated cells and control at 80mV were over 1000 pA (P<0.05). These modifications of ion channels activity positively influence cell proliferation (MTT test, P<0.0001) without interfering with the normal synthesis activity of stimulated osteoblasts.

Three-Dimensional Mechanical Loading Modulates the Osteogenic Response of Mesenchymal Stem Cells to Tumor-Derived Soluble Signals

Dynamic mechanical loading is a strong anabolic signal in the skeleton, increasing osteogenic differentiation of bone marrow-derived mesenchymal stem cells (BM-MSCs) and increasing the bone-forming activity of osteoblasts, but its role in bone metastatic cancer is relatively unknown. In this study, we integrated a hydroxyapatite-containing three-dimensional (3D) scaffold platform with controlled mechanical stimulation to investigate the effects of cyclic compression on the interplay between breast cancer cells and BM-MSCs as it pertains to bone metastasis. BM-MSCs cultured within mineral-containing 3D poly(lactide-co-glycolide) (PLG) scaffolds differentiated into mature osteoblasts, and exposure to tumor-derived soluble factors promoted this process. When BM-MSCs undergoing osteogenic differentiation were exposed to conditioned media collected from mechanically loaded breast cancer cells, their gene expression of osteopontin was increased. This was further enhanced when mechanical compression was simultaneously applied to BM-MSCs, leading to more uniformly deposited osteopontin within scaffold pores. These results suggest that mechanical loading of 3D scaffold-based culture models may be utilized to evaluate the role of physiologically relevant physical cues on bone metastatic breast cancer. Furthermore, our data imply that cyclic mechanical stimuli within the bone microenvironment modulate interactions between tumor cells and BM-MSCs that are relevant to bone metastasis.

The adipokine lipocalin-2 in the context of the osteoarthritic osteochondral junction

Obesity and osteoarthritis (OA) form a vicious circle in which obesity contributes to cartilage destruction in OA, and OA-associated sedentary behaviour promotes weight gain. Lipocalin-2 (LCN2), a novel adipokine with catabolic activities in OA joints, contributes to the obesity and OA pathologies and is associated with other OA risk factors. LCN2 is highly induced in osteoblasts in the absence of mechanical loading, but its role in osteoblast metabolism is unclear. Therefore, because osteochondral junctions play a major role in OA development, we investigated the expression and role of LCN2 in osteoblasts and chondrocytes in the OA osteochondral junction environment. Our results showed that LCN2 expression in human osteoblasts and chondrocytes decreased throughout osteoblast differentiation and was induced by catabolic and inflammatory factors; however, TGF-β1 and IGF-1 reversed this induction. LCN2 reduced osteoblast viability in the presence of iron and enhanced the activity of MMP-9 released by osteoblasts. Moreover, pre-stimulated human osteoblasts induced LCN2 expression in human chondrocytes, but the inverse was not observed. Thus, LCN2 is an important catabolic adipokine in osteoblast and chondrocyte metabolism that is regulated by differentiation, inflammation and catabolic and anabolic stimuli, and LCN2 expression in chondrocytes is regulated in a paracrine manner after osteoblast stimulation.

Combined exposure to big endothelin-1 and mechanical loading in bovine sternal cores promotes osteogenesis

Increased bone formation resulting from mechanical loading is well documented; however, the interactions of the mechanotransduction pathways are less well understood. Endothelin-1, a ubiquitous autocrine/paracrine signaling molecule promotes osteogenesis in metastatic disease. In the present study, it was hypothesized that exposure to big endothelin-1 (big ET1) and/or mechanical loading would promote osteogenesis in ex vivo trabecular bone cores. In a 2×2 factorial trial of daily mechanical loading (-2000με, 120cycles daily, "jump" waveform) and big ET1 (25ng/mL), 48 bovine sternal trabecular bone cores were maintained in bioreactor chambers for 23days. The bone cores' response to the treatment stimuli was assessed with percent change in core apparent elastic modulus (ΔEapp), static and dynamic histomorphometry, and prostaglandin E2 (PGE2) secretion. Two-way ANOVA with a post hoc Fisher's LSD test found no significant treatment effects on ΔEapp (p=0.25 and 0.51 for load and big ET1, respectively). The ΔEapp in the "no load + big ET1" (CE, 13±12.2%, p=0.56), "load + no big ET1" (LC, 17±3.9%, p=0.14) and "load + big ET1" (LE, 19±4.2%, p=0.13) treatment groups were not statistically different than the control group (CC, 3.3%±8.6%). Mineralizing surface (MS/BS), mineral apposition (MAR) and bone formation rates (BFR/BS) were significantly greater in LE than CC (p=0.037, 0.0040 and 0.019, respectively). While the histological bone formation markers in LC trended to be greater than CC (p=0.055, 0.11 and 0.074, respectively) there was no difference between CE and CC (p=0.61, 0.50 and 0.72, respectively). Cores in LE and LC had more than 50% greater MS/BS (p=0.037, p=0.055 respectively) and MAR (p=0.0040, p=0.11 respectively) than CC. The BFR/BS was more than two times greater in LE (p=0.019) and LC (p=0.074) than CC. The PGE2 levels were elevated at 8days post-osteotomy in all groups and the treatment groups remained elevated compared to the CC group on days 15, 19 and 23. The data suggest that combined exposure to big ET1 and mechanical loading results in increased osteogenesis as measured in biomechanical, histomorphometric and biochemical responses.

Gene-environment interaction between body mass index and transforming growth factor beta 1 (TGFβ1) gene in knee and hip osteoarthritis

Introduction

The objective was to investigate potential gene-environment interaction between body mass index (BMI) and each of eight TGFβ1 polymorphisms in knee and hip osteoarthritis (OA).

Methods

We conducted a case-control study of Caucasian men and women aged 45 to 86 years from Nottingham, United Kingdom (Genetics of OA and Lifestyle (GOAL) study). Cases had clinically severe symptoms and radiographic knee or hip OA; controls had no symptoms and no radiographic knee/hip OA. We used logistic regression to investigate the association of TGFβ1 polymorphisms and OA when stratifying by BMI. Knee and hip OA were analyzed separately with adjustment for potential confounders. Additive and multiplicative interactions were examined.

Results

2,048 cases (1,042 knee OA, 1,006 hip OA) and 967 controls were studied. For hip OA, the highest risk was in overweight (BMI ≥25 kg/m²) individuals with the variant allele of single-nucleotide polymorphism (SNP) rs1800468 (odds ratio (OR) 2.21, 95% confidence interval (CI) 1.55, 3.15). Evaluation of gene-environment interaction indicated significant synergetic interaction (relative excess risk due to interaction (RERI) = 0.93, synergy index (SI) = 4.33) with an attributable proportion due to interaction (AP) of 42% (AP = 0.42; 95% CI 0.16, 0.68). Multiplicative interaction was also significant (OR for interaction (ORINT) = 2.27, P = 0.015). For knee OA, the highest risk was in overweight individuals with homozygous genotype 11 of SNP rs2278422 (OR = 6.95, P <0.001). In contrast, the variant allele indicated slightly lower risks (OR = 4.72, P <0.001), a significant antagonistic interaction (RERI = -2.66, SI = 0.59), AP = -0.56 (95%CI -0.94, -0.17) and a significant multiplicative interaction (ORINT = 0.47, P = 0.013).

Conclusion

TGFβ1 gene polymorphisms interact with being overweight to influence the risk of large joint OA.

Mechanical stimulation of bone cells using fluid flow

This chapter describes several methods suitable for mechanically stimulating monolayers of bone cells by fluid shear stress (FSS) in vitro. Fluid flow is generated by pumping culture medium through two parallel plates, one of which contains a monolayer of cells. Methods for measuring nitric oxide production by bone cells in response to FSS are also described.

Development of a micromanipulator-based loading device for mechanoregulation study of human mesenchymal stem cells in three-dimensional collagen constructs

Mechanical signal is important for regulating cellular activities, including proliferation, metabolism, matrix production, and orientation. Bioreactors with loading functions can be used to precondition cells in three-dimensional (3D) constructs so as to study the cellular responses to mechanical stimulation. However, full-scale bioreactor is not always an affordable option considering the high cost of equipment and the liter-sized medium with serum and growth factor supplements. In this study, a custom-built loading system was developed by coupling a conventional camera-equipped inverted research microscope with two micromanipulators. The system was programmed to deliver either cyclic compressive loading with different frequencies or static compressive loading for 1 week to investigate the cellular responses of human mesenchymal stem cells (hMSCs) entrapped in a 3D construct consists of reconstituted collagen fibers. Cellular properties, including their alignment, cytoskeleton, and cell metabolism, and properties of matrix molecules, such as collagen fiber alignment and glycosaminoglycan deposition, were evaluated. Using a MatLab-based image analysis program, reorientation of the entrapped cells from a random distribution to a preferred alignment along the loading direction in constructs with both static and cyclic compression has been demonstrated, but no such alignment was found in the free-floating controls. Fluorescent staining on filamentous actin cytoskeleton also confirmed the finding. Nevertheless, the collagen fiber meshwork entrapping the hMSCs remained randomly distributed, and no change in cellular metabolism and glycosaminoglycans production was noted. The current study provides a simple and affordable option toward setting up a mechanoregulation facility based on existing laboratory equipment and sheds new insights on the effect of mechanical loading on the alignment of hMSCs in 3D collagen constructs.

Effects of hydrostatic pressure on leporine meniscus cell-seeded PLLA scaffolds

Hydrostatic pressure (HP) is an important component of the loading environment of the knee joint. Studies with articular chondrocytes and TMJ disc fibrochondrocytes have identified certain benefits of HP for tissue engineering purposes. However, similar studies with meniscus cells are lacking. Thus, in this experiment, the effects of applying 10 MPa of HP at three different frequencies (0, 0.1, and 1 Hz) to leporine meniscus cell-seeded PLLA scaffolds were examined. HP was applied once every 3 days for 1 h for a period of 28 days. Constructs were analyzed for cellular, biochemical, and biomechanical properties. At t = 4 weeks, total collagen/scaffold was found to be significantly higher in the 10 MPa, 0 Hz group when compared with other groups. This despite the fact that the cell numbers/scaffold were found to be lower in all HP groups when compared with the culture control. Additionally, the total GAG/scaffold, instantaneous modulus, and relaxation modulus were significantly increased in the 10 MPa, 0 Hz group when compared with the culture control. In summary, this experiment provides evidence for the benefit of a 10 MPa, 0 Hz stimulus, on both biochemical and biomechanical aspects, for the purposes of meniscus tissue engineering using PLLA scaffolds.

The Influence of Bioreactor Geometry and the Mechanical Environment on Engineered Tissues

A three phase model for the growth of a tissue construct within a perfusion bioreactor is examined. The cell population (and attendant extracellular matrix), culture medium, and porous scaffold are treated as distinct phases. The bioreactor system is represented by a two-dimensional channel containing a cell-seeded rigid porous scaffold (tissue construct), which is perfused with a culture medium. Through the prescription of appropriate functional forms for cell proliferation and extracellular matrix deposition rates, the model is used to compare the influence of cell density-, pressure-, and culture medium shear stress-regulated growth on the composition of the engineered tissue. The governing equations are derived in O’Dea et al. “A Three Phase Model for Tissue Construct Growth in a Perfusion Bioreactor,” Math. Med. Biol., in which the long-wavelength limit was exploited to aid analysis; here, finite element methods are used to construct two-dimensional solutions to the governing equations and to investigate thoroughly their behavior. Comparison of the total tissue yield and averaged pressures, velocities, and shear stress demonstrates that quantitative agreement between the two-dimensional and long-wavelength approximation solutions is obtained for channel aspect ratios of order 10−2 and that much of the qualitative behavior of the model is captured in the long-wavelength limit, even for relatively large channel aspect ratios. However, we demonstrate that in order to capture accurately the effect of mechanotransduction mechanisms on tissue construct growth, spatial effects in at least two dimensions must be included due to the inherent spatial variation of mechanical stimuli relevant to perfusion bioreactors, most notably, fluid shear stress, a feature not captured in the long-wavelength limit.

Mechanical load modulates chondrogenesis of human mesenchymal stem cells through the TGF-beta pathway

This study investigated the effect of mechanical load on human mesenchymal stem cell (hMSC) differentiation under different exogenous transforming growth factor-β1 (TGF-β₁) concentrations (0, 1 or 10 ng/ml).The role of the TGF-β signalling pathway in this process was also studied. Human MSCs were seeded into fibrin-biodegradable polyurethane scaffolds at a cell density of 5 × 10⁶ cells per scaffold and stimulated using our bioreactor. One hour of surface motion superimposed on cyclic compression was applied once a day over seven consecutive days. Scaffolds were analysed for gene expression, DNA content and glycosaminoglycan amount. Addition of TGF-β₁ in the culture medium was sufficient to induce chondrogenesis of hMSCs. Depending on the TGF-β₁ concentration of the culture medium, mechanical load stimulated chondrogenesis of hMSCs compared to the unloaded scaffolds, with a much stronger effect on gene expression at lower TGF-β₁ concentrations. With TGF-β₁ absent in the culture medium, mechanical load stimulated gene transcripts and protein synthesis of TGF-β₁ and TGF-β₃. TGF-β type I receptor inhibitor LY364947 blocked the up-regulation on TGF-β₁ and TGF-β₃ production stimulated by mechanical load, and also blocked the chondrogenesis of hMSCs. Taken together, these findings suggest that mechanical load promotes chondrogenesis of hMSCs through TGF-β pathway by up-regulating TGF-β gene expression and protein synthesis.

Cyclic Hydraulic Pressure and Fluid Flow Differentially Modulate Cytoskeleton Re-Organization in MC3T3 Osteoblasts

Mechanical loads are essential towards maintaining bone mass and skeletal integrity. Such loads generate various stimuli at the cellular level, including cyclic hydraulic pressure (CHP) and fluid shear stress (FSS). To gain insight into the anabolic responses of osteoblasts to CHP and FSS, we subjected MC3T3-E1 preosteoblasts to either FSS (12 dynes/cm(2)) or CHP varying from 0 to 68 kPa at 0.5 Hz. As with FSS, CHP produced a significant increase in ATP release over static controls within 5 min of onset. Cell stiffness examined by atomic force microscopy increased after 15 min of either CHP or FSS stimulation, which was attenuated when extracellular ATP was hydrolyzed with apyrase. As previously shown FSS induced polymerization of actins into stress fibers. However, the microtubule network was completely disrupted under FSS. In contrast, CHP appeared to maintain strong microtubule and f-actin networks. The purinergic signaling was found to be involved in the remodeling of f-actin, but not microtubule. Both CHP and FSS applied for 1 hour increased expression of COX-2. These data indicate that, while CHP and FSS produce similar anabolic responses, these stimuli have very different effects on the cytoskeleton remodeling and could contribute to loss of mechanosensitivity with extended loading.

MMP-3 response to compressive forces in vitro and in vivo

During orthodontic tooth movement, bone resorption occurs at the compression site. However, the mechanism underlying resorption remains unclear. Applying compressive force to human osteoblast-like cells grown in a 3D collagen gel, we examined gene induction by using microarray and RT-PCR analysis. Among 43 genes exhibiting significant changes, cyclo-oxygenase-2, ornithine decarboxylase, and matrix metalloproteinase-3 (MMP-3) were up-regulated, whereas membrane-bound interleukin-1 receptor accessory protein was down-regulated. The MMP-3 protein increases were further confirmed by Western blot. To ascertain whether MMP-3 is up-regulated in vivo by orthodontic force, we examined human bone samples at the compressive site by realigning the angulated molars. Immunohistochemical staining revealed MMP-3 distributed along the compressive site of the bony region within 3 days of compression. Since MMP-3 participates in degradation of a wide range of extracellular matrix molecules, we propose that MMP-3 plays an important role in bone resorption during orthodontic tooth movement.

Cellular strategies for enhancement of fracture repair

Tissue engineering seeks to translate scientific knowledge into tangible products to advance the repair, replacement, or regeneration of organs and tissues. Current tissue engineering strategies have progressed recently from a historical approach that is based primarily on biomaterials to a cell and tissue-based approach that includes understanding of cell-sourcing and bioactive stimuli. New options include methods for harvest and transplantation of tissue-forming cells, bioactive matrix materials that act as tissue scaffolds, and delivery of bioactive molecules within scaffolds. These strategies are already benefiting patients, and they place increasing demands on orthopaedic surgeons to have a solid foundation in the contemporary concepts and principles of cell-based tissue engineering. Essentially all orthopaedic tissue engineering strategies can be distilled to a strategy or combination of strategies that seek to increase the number or relative performance of bone-forming cells. The global term connective tissue progenitors has been used to define the heterogeneous populations of stem and progenitor cells that are found in native tissue and that are capable of differentiating into one or more connective tissue phenotypes. These stem or progenitor populations are found in various tissue sources, with varying degrees of ability to differentiate along connective tissue lineages. Available cell-based strategies include targeting local cells with use of scaffolds or bioactive factors, or transplantation of autogenous connective tissue progenitor cells derived from bone marrow or other tissues, with or without processing to change their concentration or prevalence. The future may include means of homing circulating connective tissue progenitor cells with use of intrinsic chemokine systems, or modifying the biological performance of connective tissue progenitor cells by means of genetic modifications.

Association of TIMP-2 with extracellular matrix exposed to mechanical stress and its co-distribution with periostin during mouse mandible development

Matrix remodeling is regulated by matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs). Periostin, originally identified in a mouse osteoblastic library, plays a role in cell adhesion and migration and in mechanical stress-induced matrix remodeling. In this study, we analyzed and compared the distribution patterns of TIMP-2 and periostin during mouse mandible development. Immunohistochemical staining for TIMP-2 and periostin was carried out on serial cryosections obtained from mice at embryonic days 13-16, postnatal day 2 (P2), P35, and 12 weeks of age. TIMP-2 and periostin exhibited a strikingly similar protein distribution during mandible development. From bud to early bell stages of molars, TIMP-2 and periostin were highly expressed on the lingual and anterior sides of the basement membrane and on the adjacent jaw mesenchyme. In pre- and postnatal incisors, the basement membrane of the apical loop and dental follicle was immunostained for TIMP-2 and periostin. At postnatal stages, TIMP-2 and periostin were prominently confined to the extracellular matrix (ECM) of gingival tissues, periodontal ligaments, and tendons (all recipients of mechanical strain). However, periostin was solely detected in the lower portion of the inner root sheath of hair follicles. Gingiva of P2 cultured in anti-TIMP-2 antibody-conditioned medium showed markedly reduced staining of periostin. We suggest that TIMP-2 and periostin are co-distributed on ECM exposed to mechanical forces and coordinately function as ECM modulators.

1[alpha],25-dihydroxyvitamin D(3) enhances annexin II dependent proliferation of osteoblasts

Cells experience a variety of physiological and non-physiological stresses and consequently have appropriate mechanisms to deal with such deviations from homeostasis. Particularly subject to mechanical stress and shear forces are the cells that make up the bones. Osteoblastic cells can interpret this stress as a stimulus for proliferation; however, the molecular mechanisms underlying this phenomenon are poorly understood. We have identified annexin II as being specifically upregulated in mechanically stressed osteoblasts and found that increased levels of this protein are necessary for 1[alpha],25-dihydroxyvitamin D(3) mediated augmentation of the proliferative response of osteoblasts after mechanical stress. Our data demonstrate a novel interaction between 1[alpha],25-dihydroxyvitamin D(3) and annexin II in the proliferative response of osteoblasts as well as a novel function for annexin II in the stress response. These findings may offer new therapeutic opportunities for conditions that require regenerative osteoblastic activity such as osteoporosis.

Effect of transforming growth factor beta 2 on marrow-infused foam poly(propylene fumarate) tissue-engineered constructs for the repair of critical-size cranial defects in rabbits

This study investigates the osseointegration of poly(propylene fumarate) (PPF) with beta-tricalcium phosphate (beta-TCP) scaffolds in a critical-size (diameter, 1.6 cm), cranial defect in 4-month-old rabbits (n = 51), killed at 6 or 12 weeks. Two molecular weights of PPF were used to produce bilayer scaffolds with 0.5-mm solid external and 2.0-mm porous internal layers. The porous layer was infused with bone marrow aspirate, with half the animals receiving 0.8 microg of transforming growth factor beta2 (TGF-beta2). No foreign body or inflammatory response was observed externally or on histological examination of explants. Statistical analysis of histological areal and linear measures of new bone formation found significantly more bone at the later sacrifice time, followed by implants receiving TGF-beta2, followed by low molecular weight PPF implants. Approximately 40% of the explants were tested for incorporation strength with a one-point "push-in" test. Because no permanent fixation was used, implant strength (28.37-129.03 N; range, 6.4 to 29.0 lb of resistance) was due entirely to new bone formation. The strongest bone was seen in implants receiving TGF-beta2-infused marrow in animals killed at 12 weeks. These results support the use of PPF as an osteogenic substrate and future research into preoperative fabrication of critical size and supercritical-size cranial prosthetic implants.

Accelerated closure of biopsy-type wounds by mechanical stimulation

OBJECTIVE

To determine whether a device designed to provide low-intensity, low-frequency mechanical stimulation improves healing time of acute wounds.

DESIGN

Repeated measures using mechanical stimulation on one side of a rat and sham stimulation on the contralateral side.

SETTING

Academic animal facility.

PARTICIPANTS

Six male Sprague-Dawley rats, approximately 400 g.

INTERVENTION

Mechanical stimulation of 4-mm biopsy wounds in rats was produced through the use of permanent magnets cyclically attracted and repelled by activation of an electromagnet by a square wave generator at a frequency of 1 Hz and a force equivalent to 64 mm Hg pressure.

MAIN OUTCOME MEASURE

Days to complete closure of 4-mm biopsy punch wounds.

MAIN RESULTS

This form of stimulation reduced time to close the biopsy wounds by nearly 50%. Mechanically stimulated wounds closed in 3.8 +/- 1.6 days (mean +/- SD) compared with 6.8 +/- 1.9 days for sham-stimulated wounds (P = .0002).

CONCLUSION

Production of a mechanical stimulation device with a miniaturized controller and power source and trials on humans are needed to determine the efficacy and potential cost savings of such a device in the management of wounds.

Up-regulation of the Wnt, estrogen receptor, insulin-like growth factor-I, and bone morphogenetic protein pathways in C57BL/6J osteoblasts as opposed to C3H/HeJ osteoblasts in part contributes to the differential anabolic response to fluid shear

C57BL/6J (B6), but not C3H/HeJ (C3H), mice responded to mechanical loading with an increase in bone formation. A 30-min steady fluid shear of 20 dynes/cm(2) increased [(3)H]thymidine incorporation and alkaline phosphatase activity and up-regulated the expression of early mechanoresponsive genes (integrin beta1 (Igtb1) and cyclooxygenase-2 (Cox-2)) in B6 but not C3H osteoblasts, indicating that the differential mechanosensitivity was intrinsic to osteoblasts. In-house microarray analysis with 5,500 gene fragments revealed that the expression of 669 genes in B6 osteoblasts and 474 genes in C3H osteoblasts was altered 4 h after the fluid shear. Several genes associated with the insulin-like growth factor (IGF)-I, the estrogen receptor (ER), the bone morphogenetic protein (BMP)/transforming growth factor-beta, and Wnt pathways were differentially up-regulated in B6 osteoblasts. In vitro mechanical loading also led to up-regulation of these genes in the bones of B6 but not C3H mice. Pretreatment of B6 osteoblasts with inhibitors of the Wnt pathway (endostatin), the BMP pathway (Noggin), or the ER pathway (ICI182780) blocked the fluid shear-induced proliferation. Inhibition of integrin and Cox-2 activation by echistatin and indomethacin, respectively, each blocked the fluid shear-induced up-regulation of genes associated with these four pathways. In summary, up-regulation of the IGF-I, ER, BMP, and Wnt pathways is involved in mechanotransduction. These four pathways are downstream to the early mechanoresponsive genes, i.e. Igtb1 and Cox-2. In conclusion, differential up-regulation of these anabolic pathways may in part contribute to the good and poor response, respectively, in the B6 and C3H mice to mechanical loading.

Effects of cyclic compressive loading on chondrogenesis of rabbit bone-marrow derived mesenchymal stem cells

The objective of this study was to examine the effects of cyclic compressive loading on chondrogenic differentiation of rabbit bone-marrow mesenchymal stem cells (BM-MSCs) in agarose cultures. Rabbit BM-MSCs were obtained from the tibias and femurs of New Zealand white rabbits. After the chondrogenic potential of BM-MSCs was verified by pellet cultures, cell-agarose constructs were made by suspending BM-MSCs in 2% agarose (10(7) cells/ml) for a cyclic, unconfined compression test performed in a custom-made bioreactor. Specimens were divided into four groups: control; transforming growth factor (TGF-beta) (with TGF-beta1 treatment); loading (with stimulation of cyclic, unconfined compressive loading); and TGF-beta loading (with TGF-beta1 treatment and loading stimulation) groups. In the loading experiment, specimens were subjected to sinusoidal loading with a 10% strain magnitude at a frequency of 1 Hz for 4 hours a day. Experiments were conducted for 3, 7, and 14 consecutive days. While the experimental groups (TGF-beta, loading, and TGF-beta loading) exhibited significantly higher levels of expressions of chondrogenic markers (collagen II and aggrecan) at three time periods, there were no differences among the experimental groups after an extra 5-day culture. This suggests that compressive loading alone induces chondrogenic differentiation of rabbit BM-MSCs as effectively as TGF-beta or TGF-beta plus loading treatment. Moreover, both the compressive loading and the TGF-beta1 treatment were found to promote the TGF-beta1 gene expression of rabbit BM-MSCs. These findings suggest that cyclic compressive loading can promote the chondrogenesis of rabbit BM-MSCs by inducing the synthesis of TGF-beta1, which can stimulate the BM-MSCs to differentiate into chondrocytes.

Initial stress-kick is required for fluid shear stress-induced rate dependent activation of bone cells

The shear stress induced by the loading-mediated flow of interstitial fluid through the lacuno-canalicular network is a likely stimulus for bone cell adaptive responses. Furthermore, the magnitude of the cellular response is related to the rate of mechanical loading rather than its magnitude. Thus, bone cells might be very sensitive to sudden stress-kicks, as occuring e.g., during impact loading. There is evidence that cells change stiffness under stress, which might make them more sensitive to subsequent loading. We studied the influence of a stress-kick on the mechanosensitivity of MC3T3-E1 osteoblast-like cells under different peak shear rate conditions, as measured by nitric oxide production. MC3T3-E1 bone cells were treated with steady or pulsating fluid shear stress (PFSS) for 5 min with different peak rates (9.70, 17.5, and 22.0 Pa Hz) using varying frequencies (5 and 9 Hz), and amplitudes (0.70 and 0.31 Pa). PFSS treatment was done with or without fluid flow pretreatment phase, which removed the initial stress-kick by first applying a slow fluid flow increase. Nitric oxide production in response to fluid shear stress was rate dependent, but necessitated an initial stress-kick to occur. This suggests that high-rate stimuli condition bone cells to be more sensitive for high-frequency, low-amplitude loads.

Strain-Dependent Control of Transforming Growth Factor-?? Function in Osteoblasts in an In Vitro Model: Biochemical Events Associated with Distraction Osteogenesis

BACKGROUND

Distraction osteogenesis is an important clinical method for increasing bone mass, but its effects on bone-forming cells are not well understood. In this study, the authors asked how the mechanical forces that occur during this procedure alter specific osteoblast activities such as matrix synthesis, the rate of cell replication, and enzyme activities. The authors further asked whether these changes relate to differences in the biochemical response of osteoblasts to transforming growth factor-beta (TGF-beta), a potent regulator of bone formation.

METHODS

Osteoblasts were plated on flexible, collagen-coated membranes. One group was unstrained, a second group experienced a single maximum strain load once every 6 hours to simulate intermittent force associated with a distraction protocol of four screw turns per day, and a third group was strained continuously for 24 hours. In the third group, some cell cultures were allowed to recover from strain before analysis. Subsequently, each group was treated with vehicle or TGF-beta at 12 pM (0.3 ng/ml) or 120 pM (3 ng/ml). Data were collected from a minimum of 15 replicate cell culture wells obtained from at least three separate primary culture preparations. Results were assessed with statistical software. Differences were considered significant with values of p < 0.05.

RESULTS

Both strain protocols increased basal osteoblast DNA synthesis but suppressed the relative stimulatory effect of TGF-beta on this event. However, neither intermittent nor continuous strain significantly altered collagen or noncollagen protein synthesis or the relative effect of TGF-beta on these processes in osteoblasts. Basal alkaline phosphatase activity, an intermediate marker of osteoblast differentiation and an early marker of matrix mineralization, decreased significantly in response to continuous strain or to TGF-beta treatment, and even more so in response to both conditions. In addition, TGF-beta binding to the type III TGF-beta receptor was increased in proportion to strain intensity.

CONCLUSIONS

This study shows that cyclic strain can alter osteoblast activity in multiple ways and predicts that TGF-beta has different effects during the distraction process on osteoblasts and therefore on their ability to effect bone formation. They further indicate that mechanical load permits early aspects of osteoblast activation but delays in part later biochemical parameters associated with mineralization to allow new bone growth before consolidation.

Experimental model for observation of micromotion in cell culture

It is known that the micromotion between implant and bone inhibits direct bone growth either on or into implant surfaces in vivo. Nevertheless, biocompatibility tests in vitro of biomaterials for bone/implant interfaces are mainly performed under static conditions. This work describes a dynamic, in vitro experimental simulation of the effect of mutual, small-scale implant surface-tissue displacement on adhered cells. Disks of simulated tissue (PVP hydrogel) were subjected to cyclic micromotion ranging from 0 at the center to 1000 microm at the periphery at approximately 13 Hz, relative to biomaterial surfaces or tissue culture polystyrene controls populated with human osteoblasts in standard tissue culture plate wells. The effect of the interfacial micromotion on the number of cells remaining attached was quantitated by XTT assay. The activity level of the remaining cells was determined by an alkaline phosphatase assay, and cell stress was evaluated by nitrogen assay. Significantly more cells (ANOVA) became detached from similarly prepared surfaces of titanium, hydroxyapatite, and alumina compared to the polystyrene control, and detachment from alumina was greater than for the other two materials. The activity of the remaining attached cells was lower as compared to the static (no micromotion) control but not significantly different among the biomaterials. All nitrogen assays were negative, suggesting minimal cell stress occurred. The method is proposed as a useful and discriminating in vitro tool for biocompatibility studies focused on cell adhesion to biomaterials under conditions related to those which exist at the implant/bone interface in vivo, and it allows subsequent studies of the still-viable cells by other methods.

Mechanobiology of bone tissue

In order to obtain bones that combine a proper resistance against mechanical failure with a minimum use of material, bone mass and its architecture are continuously being adapted to the prevailing mechanical loads. It is currently believed that mechanical adaptation is governed by the osteocytes, which respond to a loading-induced flow of interstitial fluid through the lacuno-canalicular network by producing signaling molecules. An optimal bone architecture and density may thus not only be determined by the intensity and spatial distribution of mechanical stimuli, but also by the mechanoresponsiveness of osteocytes. Bone cells are highly responsive to mechanical stimuli, but the critical components in the load profile are still unclear. Whether different components such as fluid shear, tension or compression may affect cells differently is also not known. Although both tissue strain and fluid shear stress cause cell deformation, these stimuli might excite different signaling pathways related to bone growth and remodeling. In order to define new approaches for bone tissue engineering in which bioartificial organs capable of functional load bearing are created, it is important to use cells responding to the local forces within the tissue, whereby biophysical stimuli need to be optimized to ensure rapid tissue regeneration and strong tissue repair.

Engineering principles of clinical cell-based tissue engineering

Tissue engineering is a rapidly evolving discipline that seeks to repair, replace, or regenerate specific tissues or organs by translating fundamental knowledge in physics, chemistry, and biology into practical and effective materials, devices, systems, and clinical strategies. Stem cells and progenitors that are capable of forming new tissue with one or more connective tissue phenotypes are available from many adult tissues and are defined as connective tissue progenitors. There are four major cell-based tissue-engineering strategies: (1) targeting local connective tissue progenitors where new tissue is desired, (2) transplanting autogenous connective tissue progenitors, (3) transplanting culture-expanded or modified connective tissue progenitors, and (4) transplanting fully formed tissue generated in vitro or in vivo. Stem cell function is controlled by changes in stem cell activation and self-renewal or by changes in the proliferation, migration, differentiation, or survival of the progeny of stem cell activation, the downstream progenitor cells. Three-dimensional porous scaffolds promote new tissue formation by providing a surface and void volume that promotes the attachment, migration, proliferation, and desired differentiation of connective tissue progenitors throughout the region where new tissue is needed. Critical variables in scaffold design and function include the bulk material or materials from which it is made, the three-dimensional architecture, the surface chemistry, the mechanical properties, the initial environment in the area of the scaffold, and the late scaffold environment, which is often determined by degradation characteristics. Local presentation or delivery of bioactive molecules can change the function of connective tissue progenitors (activation, proliferation, migration, differentiation, or survival) in a manner that results in new or enhanced local tissue formation. All cells require access to substrate molecules (oxygen, glucose, and amino acids). A balance between consumption and local delivery of these substrates is needed if cells are to survive. Transplanted cells are particularly vulnerable. Theoretical calculations can be used to explore the relationships among cell density, diffusion distance, and cell viability within a graft and to design improved strategies for transplantation of connective tissue progenitors. Rational strategies for tissue engineering seek to optimize new tissue formation through the logical selection of conditions that modulate the performance of connective tissue progenitors in a graft site to produce a desired tissue. This increasingly involves strategies that combine cells, matrices, inductive stimuli, and techniques that enhance the survival and performance of local or transplanted connective tissue progenitors.

Pathogenesis of Rheumatoid Arthritis

Although widely diverse mechanisms have been held responsible for tissue damage in rheumatoid arthritis (RA), it is likely that immune complexes are the underlying cause. Self-aggregating complexes of 7s rheumatoid factors in synovial fluid are a distinguishing feature of RA, whilst circulating complexes of 19s rheumatoid factor directed against the hinge region of 7s immunoglobulins are perhaps less specific. Other autoimmune complexes, such as those containing antibodies directed against citrullinated peptides, have been identified and may be more specific for RA, although the antigens against which these antibodies are directed have not been fully characterized. Together with phagocytic cells such as neutrophils, immune complexes are critical to the pathogenesis of RA; their effects are mediated by a complex cascade involving complement activation and stimulation of phagocytes via C5a and Fc receptors. These mechanisms result in a release of mediators of inflammation and joint destruction: cytokines, metalloproteinases, and reactive oxygen intermediates. This article will review recent, and some not too recent, progress made towards working out the pathogenesis of RA.

Role of extracellular matrix in adaptation of tendon and skeletal muscle to mechanical loading

The extracellular matrix (ECM), and especially the connective tissue with its collagen, links tissues of the body together and plays an important role in the force transmission and tissue structure maintenance especially in tendons, ligaments, bone, and muscle. The ECM turnover is influenced by physical activity, and both collagen synthesis and degrading metalloprotease enzymes increase with mechanical loading. Both transcription and posttranslational modifications, as well as local and systemic release of growth factors, are enhanced following exercise. For tendons, metabolic activity, circulatory responses, and collagen turnover are demonstrated to be more pronounced in humans than hitherto thought. Conversely, inactivity markedly decreases collagen turnover in both tendon and muscle. Chronic loading in the form of physical training leads both to increased collagen turnover as well as, dependent on the type of collagen in question, some degree of net collagen synthesis. These changes will modify the mechanical properties and the viscoelastic characteristics of the tissue, decrease its stress, and likely make it more load resistant. Cross-linking in connective tissue involves an intimate, enzymatical interplay between collagen synthesis and ECM proteoglycan components during growth and maturation and influences the collagen-derived functional properties of the tissue. With aging, glycation contributes to additional cross-linking which modifies tissue stiffness. Physiological signaling pathways from mechanical loading to changes in ECM most likely involve feedback signaling that results in rapid alterations in the mechanical properties of the ECM. In developing skeletal muscle, an important interplay between muscle cells and the ECM is present, and some evidence from adult human muscle suggests common signaling pathways to stimulate contractile and ECM components. Unaccostumed overloading responses suggest an important role of ECM in the adaptation of myofibrillar structures in adult muscle. Development of overuse injury in tendons involve morphological and biochemical changes including altered collagen typing and fibril size, hypervascularization zones, accumulation of nociceptive substances, and impaired collagen degradation activity. Counteracting these phenomena requires adjusted loading rather than absence of loading in the form of immobilization. Full understanding of these physiological processes will provide the physiological basis for understanding of tissue overloading and injury seen in both tendons and muscle with repetitive work and leisure time physical activity.

Nitric oxide production by bone cells is fluid shear stress rate dependent

Shear stress due to mechanical loading-induced flow of interstitial fluid through the lacuno-canalicular network is a likely signal for bone cell adaptive responses. Moreover, the rate (determined by frequency and magnitude) of mechanical loading determines the amount of bone formation. Whether the bone cells' response to fluid shear stress is rate dependent is unknown. Here we investigated whether bone cell activation by fluid shear stress is rate dependent. MC3T3-E1 osteoblastic cells were subjected for 15 min to fluid shear stress of varying frequencies and amplitudes, resulting in peak fluid shear stress rates ranging from 0 to 39.6 Pa-Hz. Nitric oxide production, a parameter for bone cell activation, was found to be linearly dependent on the fluid shear stress rate; the slope was steepest at 5 min (0.11 Pa-Hz(-1)) and decreased to 0.03 Pa-Hz(-1) at 15 min. We conclude that the fluid shear stress rate is an important parameter for bone cell activation.

New frontiers in bone tissue engineering

No single scientific field can generate the ideal method of engineering bone. However, through collaboration and expansion of programs in bone tissue engineering, the right combination of materials, cells, growth factors, and methodology will come together for each clinical situation such that harvesting bone grafts will become obsolete. This article reviews the need for engineered bone and provides a historical perspective of bone engineering research, current research efforts, and the future direction of this work.

Systemic regulation of distraction osteogenesis: a cascade of biochemical factors

This study investigates the systemic biochemical regulation of fracture healing in distraction osteogenesis compared with rigid osteotomy in a prospective in vivo study in humans. To further clarify the influence of mechanical strain on the regulation of bone formation, bone growth factors (insulin-like growth factor [IGF] I, IGF binding protein [IGFBP] 3, transforming growth factor [TGF] beta1, and basic FGF [bFGF]), bone matrix degrading enzymes (matrix-metalloproteinases [MMPs] 1, 2, and 3), human growth hormone (hGH), and bone formation markers (ALP, bone-specific ALP [BAP], and osteocalcin [OC]) have been analyzed in serum samples from 10 patients in each group pre- and postoperatively. In the distraction group, a significant postoperative increase in MMP-1, bFGF, ALP, and BAP could be observed during the lengthening and the consolidation period when compared with the baseline levels. Osteotomy fracture healing without the traction stimulus failed to induce a corresponding increase in these factors. In addition, comparison of both groups revealed a significantly higher increase in TGF-beta1, IGF-I, IGFBP-3, and hGH in the lengthening group during the distraction period, indicating key regulatory functions in mechanotransduction. The time courses of changes in MMP-1, bone growth factors (TGF-beta1 and bFGF), and hGH, respectively, correlated significantly during the lengthening phase, indicating common regulatory pathways for these factors in distraction osteogenesis. Significant correlation between the osteoblastic marker BAP, TGF-beta1, and bFGF suggests strain-activated osteoblastic cells as a major source of systemically increased bone growth factors during callus distraction. The systemic increase in bFGF and MMP-1 might reflect an increased local stimulation of angiogenesis during distraction osteogenesis.

Craniosynostosis and altered patterns of fetal TGF-beta expression induced by intrauterine constraint

Recent work has demonstrated that fusion of the calvarial sutures is mediated by locally elaborated soluble growth factors, including the transforming growth factor-betas (TGF-betas), leading some to speculate that external biomechanical forces play little role in suture development. Clinical evidence has long suggested, however, that fetal head constraint may play a critical role in the pathogenesis of many cases of nonsyndromic craniosynostosis. The purpose of these experiments was to test the hypothesis that intrauterine constraint leads to an alteration in normal patterns of TGF-beta expression and that these alterations are associated with craniosynostosis. Fetal constraint was induced by allowing C57Bl/6 murine fetuses to grow for 2.5 days beyond the normal 20-day gestation by performing uterine cerclage on the eighteenth day. Cranial suture morphology was examined in hematoxylin and eosin-stained sections and in cleared whole-mount specimens, double stained with alizarin red S and Alcian blue. Expression patterns of TGF-beta1 and TGF-beta3 were examined by immunohistochemical techniques. Gross and microscopic examination of the cranial sutures of 17 constrained fetuses revealed changes that ranged from narrowing to complete osseous obliteration of the coronal and squamosal sutures. All sutures of 14 nonconstrained control pups remained patent. Fetal head constraint was associated with increased TGF-beta1 immunoreactivity within the new bone and the underlying dura when compared with nonconstrained age-matched controls. TGF-beta3 immunoreactivity was associated with the dura underlying patent, nonconstrained sutures, whereas constraint-induced synostosis was characterized by down-regulation of dural TGF-beta3 expression. These experiments confirm the ability of intrauterine constraint to induce premature fusion of the cranial sutures and provide evidence that intrauterine head constraint induces the expression of osteogenic growth factors in fetal calvarial bone and the underlying dura.

Numerical experiments on long-time orthodontic tooth movement

In orthodontic treatment, teeth are moved by the use of specific force systems. The force system used depends on the patient's orthodontic situation characterized by the geometry of the tooth and the surrounding alveolar bone, which defines the position of the center of resistance. Therefore, the simulation of bone remodeling could be helpful for the treatment strategy. In this study, the optimal force system for bodily movement of a single-root tooth, with an orthodontic bracket attached, was determined. This was achieved by the use of the numerical finite element method, including a distinct mechanical bone-remodeling algorithm. This algorithm works with equilibrium iterations separated in 2 calculation steps. Furthermore, a parametric 3-dimensional finite element model, which allows modifications in the root length and its diameter, is described. For different geometries, the ideal moment-by-force ratios that induce a bodily movement were determined. The knowledge of root geometry is important in defining an optimal force system.

Soft-tissue osteochondroma in the calcaneal pad: a case report

Soft-tissue osteochondromas of the foot are infrequently occurring lesions. A 65-year-old woman presented with soft-tissue osteochondroma in the soft tissues superficial to the plantar aspect of the calcaneus. The mass was densely ossified without cortical destruction or other alteration in the adjacent calcaneal bone on x-rays and computed tomography images. A 99m Tc bone scan showed a focal area of an increased uptake at the site of the mass below the calcaneus. Magnetic resonance imaging demonstrated high signal mass on T1-weighted images, isointense to fatty marrow, adjacent to the thickened plantar aponeurosis. The mass was completely excised. Histologically, mature lamellar bone was seen centrally, and the periphery was capped with hyaline cartilage. Since the tumor was located in the plantar aponeurosis and comprised of a single nodule of well-developed bone with well-organized endochondral ossification, the pathogenesis of this lesion might be related to metaplasia.

Toward an understanding of nonsyndromic craniosynostosis: altered patterns of TGF-beta receptor and FGF receptor expression induced by intrauterine head constraint

Although the etiology of nonsyndromic forms of craniosynostosis remains uncertain, recent experiments from our laboratory have demonstrated that fetal head constraint induces cranial suture fusion in mice through a process associated with altered patterns of transforming growth factor beta (TGF-beta) isoform expression. Other recent studies have highlighted the role of secreted signaling molecules, including members of the TGF-beta superfamily and the fibroblast growth factors (FGFs), as well as their receptors, in regulating suture development and fusion. The purpose of these experiments was to examine the potential role of TGF-beta receptors and FGF receptor 2 (FGFR2) in nonsyndromic craniosynostosis by determining their temporospatial patterns of expression during development complicated by intrauterine head constraint. This study consisted of two groups of C57BI/6J mice: an experimental group subjected to intrauterine constraint and a control unconstrained group. Fetal head constraint was induced by performing uterine cerclage on day 17.5 of gestation and allowing intrauterine fetal growth to continue 24 and 48 hours beyond the normal gestational period. Control animals underwent hysterotomy on day 17.5 and the nonconstrained pups were allowed to continue intra-abdominal fetal growth 48 hours beyond normal gestation. Expression of TGF-beta receptor types I and II, and FGFR2 in the calvarial tissue was determined by immunohistochemical analysis. In the unconstrained control animals, there was minimal immunoreactivity for both TGF-beta receptors and FGFR2 within the coronal suture. After 24 hours of constraint, however, there was a marked increase in immunoreactivity of TGF-beta receptors and FGFR2 in the osteoblasts along the osteogenic fronts and in the dural cells. After 48 hours, there was continued expression of both type I and type II receptors and FGFR2 within the midsutural mesenchyme of the coronal suture, in the osteoblasts, and in the dura. The authors demonstrated substantial upregulation of TGF-beta receptor types I and II and FGFR2 in coronal sutures subjected to in utero constraint. These results suggest an important role for TGF-beta/TGF-beta receptor, and FGF/FGFR signaling in the pathogenesis of constraint-induced craniosynostosis.

Different responsiveness of cells from adult and neonatal mouse bone to mechanical and biochemical challenge

Neonatal rodent calvarial bone cell cultures are often used to study bone cell responsiveness to biochemical and mechanical signals. However, mechanical strains in the skull are low compared to the axial and appendicular skeleton, while neonatal, rapidly growing bone has a more immature cell composition than adult bone. In the present study, we tested the hypothesis that bone cell cultures from neonatal and adult mouse calvariae, as well as adult mouse long bones, respond similarly to treatment with mechanical stress or 1,25-dihydroxyvitamin D3 (1,25(OH)2 D3). Treatment with pulsating fluid shear stress (0.6 +/- 0.3 Pa, 5 Hz) caused a rapid (within 5 min) 2-4-fold increase in NO production in all cases, without significant differences between the three cell preparations. However, basal NO release was significantly higher in neonatal calvarial cells than adult calvarial and long bone cells. The response to 1,25(OH)2 D3), measured as increased alkaline phosphatase activity, was about three times higher in the neonatal cells than the adult cell cultures. We conclude that all three types of primary bone cell cultures responded similarly to fluid shear stress, by rapid production of NO. However, the neonatal cell cultures were different in basal metabolism and vitamin D3 responsiveness, suggesting that cell cultures from adult bone are best used for in vitro studies on bone cell biology.

Epigenetic mechanisms of character origination

The close mapping between genotype and morphological phenotype in many contemporary metazoans has led to the general notion that the evolution of organismal form is a direct consequence of evolving genetic programs. In contrast to this view, we propose that the present relationship between genes and form is a highly derived condition, a product of evolution rather than its precondition. Prior to the biochemical canalization of developmental pathways, and the stabilization of phenotypes, interaction of multicellular organisms with their physicochemical environments dictated a many-to-many mapping between genomes and forms. These forms would have been generated by epigenetic mechanisms: initially physical processes characteristic of condensed, chemically active materials, and later conditional, inductive interactions among the organism's constituent tissues. This concept, that epigenetic mechanisms are the generative agents of morphological character origination, helps to explain findings that are difficult to reconcile with the standard neo-Darwinian model, e.g., the burst of body plans in the early Cambrian, the origins of morphological innovation, homology, and rapid change of form. Our concept entails a new interpretation of the relationship between genes and biological form.

Dynamic cell stretching increases human osteoblast proliferation and CICP synthesis but decreases osteocalcin synthesis and alkaline phosphatase activity

The cell activity of human-bone-derived cell cultures was studied after mechanical stimulation by cyclic strain at a magnitude occurring in physiologically loaded bone tissue. Monolayers of subconfluently grown human-bone-derived cells were stretched in rectangular silicone dishes with cyclic predominantly uniaxial movement along their longitudinal axes. Strain was applied over two days for 30 min per day with a frequency of 1 Hz and a strain magnitude of 1000 microstrain. Cyclic stretching of the cells resulted in an increased proliferation (10-48%) and carboxyterminal collagen type I propeptide release (7-49%) of human-cancellous bone-derived osteoblasts while alkaline phosphatase activity and osteocalcin release were significantly reduced by 9-25 and 5-32%, respectively. These results demonstrate that cyclic strain at physiologic magnitude leads to an increase of osteoblast activities related to matrix production while those activities which are characteristic for the differentiated osteoblast and relevant for matrix mineralization are decreased.