Expression of matrix metalloproteinase genes in the rat intramembranous bone during postnatal growth and upon mechanical stresses

FACEts of mechanical regulation in the morphogenesis of craniofacial structures

During embryonic development, organs undergo distinct and programmed morphological changes as they develop into their functional forms. While genetics and biochemical signals are well recognized regulators of morphogenesis, mechanical forces and the physical properties of tissues are now emerging as integral parts of this process as well. These physical factors drive coordinated cell movements and reorganizations, shape and size changes, proliferation and differentiation, as well as gene expression changes, and ultimately sculpt any developing structure by guiding correct cellular architectures and compositions. In this review we focus on several craniofacial structures, including the tooth, the mandible, the palate, and the cranium. We discuss the spatiotemporal regulation of different mechanical cues at both the cellular and tissue scales during craniofacial development and examine how tissue mechanics control various aspects of cell biology and signaling to shape a developing craniofacial organ.

Nanomechanical Assessment of Bone Surrounding Implants Loaded for 3 Years in a Canine Experimental Model

PURPOSE

This work evaluated the nanomechanical properties of bone surrounding submerged and immediately loaded implants after 3 years in vivo. It was hypothesized that the nanomechanical properties of bone would markedly increase in immediately and functionally loaded implants compared with submerged implants.

MATERIALS AND METHODS

The second, third, and fourth right premolars and the first molar of 10 adult Doberman dogs were extracted. After 6 months, 4 implants were placed in 1 side of the mandible. The mesial implant received a cover screw and remained unloaded. The remaining 3 implants received fixed dental prostheses within 48 hours after surgery that remained in occlusal function for 3 years. After sacrifice, the bone was prepared for histologic and nanoindentation analysis. Nanoindentation was carried out under wet conditions on bone areas within the plateaus. Indentations (n = 30 per histologic section) were performed with a maximum load of 300 μN (loading rate, 60 μN per second) followed by a holding and unloading time of 10 and 2 seconds, respectively. Elastic modulus (E) and hardness (H) were computed in giga-pascals. The amount of bone-to-implant contact (BIC) also was evaluated.

RESULTS

The E and H values for cortical bone regions were higher than those for trabecular bone regardless of load condition, but this difference was not statistically significant (P > .05). The E and H values were higher for loaded implants than for submerged implants (P < .05) for cortical and trabecular bone. For the same load condition, the E and H values for cortical and trabecular bone were not statistically different (P > .05). The loaded and submerged implants presented BIC values (mean ± standard deviation) of 57.4 ± 12.1% and 62 ± 7.5%, respectively (P > .05).

CONCLUSION

The E and H values of bone surrounding dental implants, measured by nanoindentation, were higher for immediately loaded than for submerged implants.

Expression of MMP-2 and TIMP-1 during rapid maxillary expansion in rats

OBJECTIVE

The aim of this study was to investigate the expression of MMP-2 and TIMP-1 during midpalatal suture expansion in rats.

DESIGN

72 male Wistar rats were randomly divided into 2 groups: the experimental group and the control group. In the experimental group, opening loops were applied across the midpalatal suture with an initial force of 50g, whereas in the control group, rats were subjected to sham installation of opening loops without activation. On day 1, 4, 7 and 14, nine rats from each group were sacrificed, and the maxillae were dissected and prepared for Immunohistochemistry (IHC) and RT- PCR examination of MMP-2 and TIMP-1 expression.

RESULTS

The results of IHC and Real Time PCR revealed that both protein and mRNA expression of MMP-2 and TIMP-1 were significantly increased after midpalatal expansion, and the ratio of MMP-2/TIMP-1 was also significantly enhanced.

CONCLUSIONS

The data suggested that MMP-2 and TIMP-1 might play an important role during the mid-palatal suture remodeling process of maxillary expansion.

Effects of Blocking αvβ₃ integrin by a recombinant RGD disintegrin on remodeling of wound healing after induction of incisional hernia in rats

PURPOSE

Incisional hernia (IH) is characterized by defective wound healing process. Disba-01, a αvb₃ integrin blocker has shown to control the rate of wound repair and therefore it could be a target for new wound healing therapies.The objective of the study was to determine the changes induced by Disba-01 on repair of wound healing after induced IH in rats.

METHODS

Thirty two male albino rats were submitted to IH and divided into 4 experimental groups: G1, placebo control; G2, DisBa-01-treated; G3, anti-αvβ₃ antibodies-treated and G4, anti-α₂ antibodies-treated. Histological. biochemical and extracellular matrix remodeling analysis of abdominal wall were evaluated.

RESULTS

After 14 days, 100% of the G2 did not present hernia, and the hernia ring was closed by a thin membrane. In contrast, all groups maintained incisional hernia. DisBa-01 also increased the number macrophages and fibroblasts and induced the formation of new vessels. Additionally, MMP-2 was strongly activated only in G2 (P<0.05). Anti- αvβ₃-integrin antibodies produced similar results than Disba-01 but not anti-α₂ integrin blocking antibodies.

CONCLUSION

These results strongly indicate that Disba-01 has an important role in the control of wound healing and the blocking of this integrin may be an interesting therapeutical strategy in IH.

Simulated remodeling of loaded collagen networks via strain-dependent enzymatic degradation and constant-rate fiber growth

Recent work has demonstrated that enzymatic degradation of collagen fibers exhibits strain-dependent kinetics. Conceptualizing how the strain dependence affects remodeling of collagenous tissues is vital to our understanding of collagen management in native and bioengineered tissues. As a first step towards this goal, the current study puts forward a multiscale model for enzymatic degradation and remodeling of collagen networks for two sample geometries we routinely use in experiments as model tissues. The multiscale model, driven by microstructural data from an enzymatic decay experiment, includes an exponential strain-dependent kinetic relation for degradation and constant growth. For a dogbone sample under uniaxial load, the model predicted that the distribution of fiber diameters would spread over the course of degradation because of variation in individual fiber load. In a cross-shaped sample, the central region, which experiences smaller, more isotropic loads, showed more decay and less spread in fiber diameter compared to the arms. There was also a slight shift in average orientation in different regions of the cruciform.

Involvement of Wnt activation in the micromechanical vibration-enhanced osteogenic response of osteoblasts

BACKGROUND

Low-magnitude vibration has been widely used as a tool for rehabilitation, enhancing physical performance, and stimulating bone development. Although mechanical stimulation generated by vibrations is regarded as important factor in bone remodeling, the underlying cellular and molecular regulatory mechanisms of this response, which may be important in the development of new mechanobiological strategies, currently remain unclear.

METHODS

In this study, to investigate the mechanobiological mechanisms of vibration-enhanced osteogenic responses in osteoblasts, MC3T3-E1 cells were subjected to vibrations of different amplitude (0.06, 0.14, 0.32, 0.49, 0.66, and 0.8 × g) at 40 Hz for 30 min/day over 3 days. The osteogenesis-related transcription factors Wnt10B, Sclerostin, OPG, and RANKL were analyzed for mRNA and protein expression.

RESULTS

The results revealed that protein expression of Wnt10B and OPG was increased in a magnitude-dependent manner by mechanical vibrations at amplitudes of 0.06, 0.14, 0.32, and 0.49 × g; the maximum increases were 2.4-fold (p < 0.001) and 7.9-fold (p < 0.001), respectively, at 0.49 × g. Sclerostin and RANKL levels were reduced at all amplitudes. On the basis of mRNA levels, the reduced expression of RANKL was further downregulated (p < 0.05) whereas OPG expression was further increased (p < 0.01) when the MC3T3-E1 cells were treated with LiCl compared with the effects of vibration alone.

CONCLUSIONS

The findings may indicate that Wnt signaling is involved in mechanotransduction at low-magnitude vibration; this may provide a cellular basis, and impetus for further development of, biomechanically based intervention for enhancing bone strength and accelerating implant osseointegration.

Upregulation of MMP-13 and TIMP-1 expression in response to mechanical strain in MC3T3-E1 osteoblastic cells

BACKGROUND

Mechanical strain plays a significant role in the regulation of bone matrix turnover, which is mediated in part by matrix metalloproteinase (MMP)-13 and tissue inhibitors of matrix metalloproteinase (TIMP)-1. However, little is known about the correlation between mechanical strain and osteoblastic cell activities, including extracellular matrix (ECM) metabolism. Herein, we determined the effect of different magnitudes of cyclic tensile strain (0%, 6%, 12%, and 18%) on MMP-13 and TIMP-1 mRNA and protein expression in MC3T3-E1 osteoblasts. Furthermore, we employed specific inhibitors to examine the role of distinct signal transduction pathways known to mediate cellular responses to mechanical strain.

RESULTS

We identified a magnitude-dependent increase in MMP-13 and TIMP-1 mRNA and protein levels in response to mechanical strains corresponding to 6%, 12%, and 18% elongation. The strain-induced increases in MMP-13 and TIMP-1 mRNA expression were inhibited by PD098059 and cycloheximide, respectively.

CONCLUSIONS

Our results suggest a mechanism for the regulation of bone matrix metabolism mediated by the differential expression of MMP-13 and TIMP-1 in response to increasing magnitudes of mechanical strain.

Microarray analysis of the role of regional dura mater in cranial suture fate

BACKGROUND

Craniosynostosis, the premature fusion of cranial sutures, results in serious neurologic and morphologic abnormalities when left untreated. Surgical excision of the fused sutures and remodeling of the skull remains the standard therapy. Development of novel, minimally invasive therapies for craniosynostosis will undoubtedly be dependent on a more thorough understanding of the molecular mechanisms underlying this abnormality. Significant evidence suggests the influence of regional dura mater on the behavior of the overlying suture complex. The mouse model has been instrumental in investigating this observation because of the natural juxtaposition of the posterior frontal suture, which fuses early in life, with the other cranial sutures, which remain patent.

METHODS

The authors used microarray analysis to compare genomic changes in the dura mater underlying the posterior frontal and sagittal sutures of mice. Suture-associated dura mater was harvested from mice before (postnatal day 5), during (postnatal day 10), and after (postnatal day 20) posterior frontal suture fusion (n = 20 mice for each of the three time points).

RESULTS

Microarray results confirmed differential regulation of genes involved in paracrine signaling, extracellular matrix, and bone remodeling between the dura mater underlying the fusing posterior frontal suture and the patent sagittal suture.

CONCLUSIONS

These data confirm global differences in gene expression between regional dura mater underlying fusing and patent sutures. These results provide further insight into potential molecular mechanisms that may play a role in cranial suture biology.

Mechanism of skull suture maintenance and interdigitation

Skull sutures serve as growth centers whose function involves multiple molecular pathways. During periods of brain growth the sutures remain thin and straight, later developing complex fractal interdigitations that provide interlocking strength. The nature of the relationship between the molecular interactions and suture pattern formation is not understood. Here we show that by classifying the molecules involved into two groups, stabilizing factors and substrate molecules, complex molecular networks can be modeled by a simple two-species reaction-diffusion model that recapitulates all the known behavior of suture pattern formation. This model reproduces the maintenance of thin sutural tissue at early stages, the later modification of the straight suture to form osseous interdigitations, and the formation of fractal structures. Predictions from the model are in good agreement with experimental observations, indicating that the model captures the essential nature of the interdigitation process.

Differential regulation of blood vessel formation between standard and delayed bone healing

Blood vessel formation is a prerequisite for bone healing. In this study, we tested the hypothesis that a delay in bone healing is associated with an altered regulation of blood vessel formation. A tibial osteotomy was performed in two groups of sheep and stabilized with either a rigid external fixator leading to standard healing or with a highly rotationally unstable one leading to delayed healing. At days 4, 7, 9, 11, 14, 21, and 42 after surgery, total RNA was extracted from the callus. Gene expressions of vWF, an endothelial cell marker, and of several molecules related to blood vessel formation were studied by qPCR. Furthermore, histology was performed on fracture hematoma and callus sections. Histologically, the first blood vessels were detected at day 7 in both groups. mRNA expression levels of vWF, Ang1, Ang2, VEGF, CYR61, FGF2, MMP2, and TIMP1 were distinctly lower in the delayed compared to the standard healing group at several time points. Based on differential expression patterns, days 7 and 21 postoperatively were revealed to be essential time points for vascularization of the ovine fracture callus. This work demonstrates for the first time a differential regulation of blood vessel formation between standard and mechanically induced delayed healing in a sheep osteotomy model.

Synergistic actions of hematopoietic and mesenchymal stem/progenitor cells in vascularizing bioengineered tissues

Poor angiogenesis is a major road block for tissue repair. The regeneration of virtually all tissues is limited by angiogenesis, given the diffusion of nutrients, oxygen, and waste products is limited to a few hundred micrometers. We postulated that co-transplantation of hematopoietic and mesenchymal stem/progenitor cells improves angiogenesis of tissue repair and hence the outcome of regeneration. In this study, we tested this hypothesis by using bone as a model whose regeneration is impaired unless it is vascularized. Hematopoietic stem/progenitor cells (HSCs) and mesenchymal stem/progenitor cells (MSCs) were isolated from each of three healthy human bone marrow samples and reconstituted in a porous scaffold. MSCs were seeded in micropores of 3D calcium phosphate (CP) scaffolds, followed by infusion of gel-suspended CD34(+) hematopoietic cells. Co-transplantation of CD34(+) HSCs and CD34(-) MSCs in microporous CP scaffolds subcutaneously in the dorsum of immunocompromised mice yielded vascularized tissue. The average vascular number of co-transplanted CD34(+) and MSC scaffolds was substantially greater than MSC transplantation alone. Human osteocalcin was expressed in the micropores of CP scaffolds and was significantly increased upon co-transplantation of MSCs and CD34(+) cells. Human nuclear staining revealed the engraftment of transplanted human cells in vascular endothelium upon co-transplantation of MSCs and CD34(+) cells. Based on additional in vitro results of endothelial differentiation of CD34(+) cells by vascular endothelial growth factor (VEGF), we adsorbed VEGF with co-transplanted CD34(+) and MSCs in the microporous CP scaffolds in vivo, and discovered that vascular number and diameter further increased, likely owing to the promotion of endothelial differentiation of CD34(+) cells by VEGF. Together, co-transplantation of hematopoietic and mesenchymal stem/progenitor cells may improve the regeneration of vascular dependent tissues such as bone, adipose, muscle and dermal grafts, and may have implications in the regeneration of internal organs.

Mechanical influences on suture development and patency

In addition to their role in skull growth, sutures are sites of flexibility between the more rigid bones. Depending on the suture, predominant loading during life may be either tensile or compressive. Loads are transmitted across sutures via collagenous fibers and a fluid-rich extracellular matrix and can be quasi-static (growth of neighboring tissues) or intermittent (mastication). The mechanical properties of sutures, while always viscoelastic, are therefore quite different for tensile versus compressive loading. The morphology of individual sutures reflects the nature of local loading, evidently by a process of developmental adaptation. In vivo or ex vivo, sutural cells respond to tensile or cyclic loading by expressing markers of proliferation and differentiation, whereas compressive loading appears to favor osteogenesis. Braincase and facial sutures exhibit similar mechanical behavior and reactions despite their different natural environments.

Autologous stem cell regeneration in craniosynostosis

Craniosynostosis occurs in one of 2500 live human births and may manifest as craniofacial disfiguration, seizure, and blindness. Craniotomy is performed to reshape skull bones and resect synostosed cranial sutures. We demonstrate for the first time that autologous mesenchymal stem cells (MSCs) and controlled-released TGFbeta3 reduced surgical trauma to localized osteotomy and minimized osteogenesis in a rat craniosynostosis model. Approximately 0.5 mL tibial marrow content was aspirated to isolate mononucleated and adherent cells that were characterized as MSCs. Upon resecting the synostosed suture, autologous MSCs in collagen carriers with microencapsulated TGFbeta3 (1 ng/mL) generated cranial suture analogs characterized as bone-soft tissue-bone interface by quantitative histomorphometric and microCT analyses. Thus, surgical trauma in craniosynostosis can be minimized by a biologically viable implant. We speculate that proportionally larger amounts of human marrow aspirates participate in the healing of craniosynostosis defects in patients. The engineered soft tissue-bone interface may have implications in the repair of tendons, ligaments, periosteum and periodontal ligament.

Inhibition of osteogenic differentiation of human mesenchymal stem cells

Mesenchymal stem cells (hMSCs) have been shown to differentiate into osteoblasts that, in turn, are capable of forming tissues analogous to bone. The present study was designed to investigate the inhibition of osteogenesis by hMSCs. Bone marrow-derived hMSCs were treated with transforming growth factor beta-3 (TGFbeta3) at various doses during or after their differentiation into osteogenic cells. TGFbeta3 was encapsulated in poly(DL-lactic-co-glycolic acid) (PLGA) microspheres and released via controlled delivery in the osteogenic culture of hMSCs and hMSC-derived osteoblasts for up to 28 days. Controlled release of TGFbeta3 inhibited the osteogenic differentiation of hMSCs, as evidenced by significantly reduced alkaline phosphatase activity and staining, as well as decreased mineral deposition. After hMSCs had been differentiated into osteoblasts, controlled release of TGFbeta3 further inhibited not only alkaline phosphatase and mineral deposition but also osteocalcin expression. These findings demonstrate the potential for sustained modulation of the behavior of stem cells and/or stem cell-derived lineage-specific cells via controlled release of growth factor(s). The attenuation of osteogenic differentiation of MSCs may facilitate understanding not only the regulation and patterning of osteogenesis in development but also several pathological models such as osteopetrosis, craniosynostosis, and heart valve calcification.

A crucial role for matrix metalloproteinase 2 in osteocytic canalicular formation and bone metabolism

Extracellular matrix production and degradation by bone cells are critical steps in bone metabolism. Mutations of the gene encoding MMP-2, an extracellular matrix-degrading enzyme, are associated with a human genetic disorder characterized by subcutaneous nodules, arthropathy, and focal osteolysis. It is not known how the loss of MMP-2 function results in the pathology. Here, we show that Mmp2(-/-) mice exhibited opposing bone phenotypes caused by an impaired osteocytic canalicular network. Mmp2(-/-) mice showed decreased bone mineral density in the limb and trunk bones but increased bone volume in the calvariae. In the long bones, there was moderate disruption of the osteocytic networks and reduced bone density throughout life, whereas osteoblast and osteoclast function was normal. In contrast, aged but not young Mmp2(-/-) mice had calvarial sclerosis with osteocyte death. Severe disruption of the osteocytic networks preceded osteocyte loss in Mmp2(-/-) calvariae. Successful transplantation of wild-type periosteum restored the osteocytic canalicular networks in the Mmp2(-/-) calvariae, suggesting local roles of MMP-2 in determining bone phenotypes. Our results indicate that MMP-2 plays a crucial role in forming and maintaining the osteocytic canalicular network, and we propose that osteocytic network formation is a determinant of bone remodeling and mineralization.

Effect of compressive force on the expression of MMPs, PAs, and their inhibitors in osteoblastic Saos-2 cells

Bone matrix turnover is regulated by matrix metalloproteinases (MMPs), tissue inhibitors of matrix metalloproteinases (TIMPs), and the plasminogen activation system, including tissue-type plasminogen activator (tPA), urokinase-type plasminogen activator (uPA), and plasminogen activator inhibitor type-1 (PAI-1). We previously demonstrated that 1.0g/cm(2) of compressive force was an optimal condition for inducing bone formation by osteoblastic Saos-2 cells. Here, we examined the effect of mechanical stress on the expression of MMPs, TIMPs, tPA, uPA, and PAI-1 in Saos-2 cells. The cells were cultured in Dulbecco's modified Eagle's medium containing 10% fetal bovine serum and with or without continuously compressive force (0.5-3.0g/cm(2)) for up to 24h. The levels of MMPs, TIMPs, uPA, tPA, and PAI-1 gene expression were estimated by determining the mRNA levels using real-time PCR, and the protein levels were determined using ELISA. The expression levels of MMP-1, MMP-2, MMP-14, and TIMP-1 markedly exceeded the control levels at 1.0g/cm(2) of compressive force, whereas the expression levels of MMP-3, MMP-13, TIMP-2, TIMP-3, TIMP-4, tPA, uPA, and PAI-1 markedly exceeded the control levels at 3.0g/cm(2). These results suggest that mechanical stress stimulates bone matrix turnover by increasing these proteinases and inhibitors, and that the mechanism for the proteolytic degradation of bone matrix proteins differs with the strength of the mechanical stress.

Real-time reverse transcriptase polymerase chain reaction: an improvement in detecting mRNA levels in mouse cranial tissue

BACKGROUND

Quantitation of messenger RNA levels has traditionally been carried out by Northern blot analysis. While this is regarded as the standard method, it is time-consuming and requires large quantities of RNA. Reverse-transcriptase polymerase chain reaction is a semiquantitative method that has been used as a more rapid and sensitive alternative to Northern blotting. Real-time reverse-transcriptase polymerase chain reaction is a quantitative technique that is gaining widespread acceptance as a rapid and reliable way of quantifying mRNA. Since both techniques are currently being used to evaluate gene expression in the murine cranial suture model, the present study was performed to compare the sensitivity and variability of real-time to conventional reverse-transcriptase polymerase chain reaction in this model.

METHODS

Mouse brain RNA was isolated and amplified using real-time and conventional methods. For the real-time method, a serial 10-fold dilution of RNA, ranging from 1 fg to 100 ng, was performed. For the conventional method, the minimum amount of RNA needed for consistent polymerase chain reaction amplification was determined. Transforming growth factor beta-1 and beta-actin RNA transcripts were measured using both techniques.

RESULTS

One femtogram of RNA could be detected by the real-time method, although 10 fg were required to reliably detect differences; 500 ng of RNA was required for consistent polymerase chain reaction amplification using the conventional method. The variability of real-time reverse-transcriptase polymerase chain reaction when expressed as a coefficient of variation (SD as a percentage of the mean) ranged from 0.23 to 2.6 percent for all genes tested, as compared with 9 to 70 percent for conventional reverse-transcriptase polymerase chain reaction.

CONCLUSIONS

Real-time reverse-transcriptase polymerase chain reaction was used successfully to detect mRNA from different mouse genes. The real-time method is much more sensitive in detecting small amounts of mRNA than both Northern blot analysis and conventional polymerase chain reaction. The variability of the real-time method is more than 10-fold lower compared with the conventional method performed in the authors' laboratory for all genes tested.

The extracellular matrix environment in suture morphogenesis and growth

Sutures are the major bone growth sites of the craniofacial skeleton and form in response to developmental approximation of and interaction between two opposing osteogenic fronts. Premature obliteration of these craniofacial bone growth sites or craniosynostosis results in compensatory growth at other bone growth sites, with concomitant craniofacial dysmorphology. While much is now known about the growth and transcriptional factor regulation of suture formation and maintenance, little about the nature of the extracellular environment within sutures and their surrounding bones has been described. This review elucidates the nature of the sutural extracellular matrix and its role in mediating suture maintenance and growth through the regulation of cellular and biomechanical signaling.

Matrix and gene expression in the rat cranial base growth plate

Recent data have shown that the proliferation and differentiation of the cranial base growth plate (CBGP) chondrocytes are modulated by mechanical stresses. However, little is known about the expression of genes and matrix molecules in the CBGP during development or under mechanical stresses. The objective of the present study was to determine whether several cartilage- and bone-related molecules are expressed in the CBGP and whether their expression is modulated by cyclic loading. The CBGP of normal 8-day-old rats (n=8) were isolated immediately after death, followed by extraction of total RNA and reverse transcription/polymerase chain reaction (RT-PCR) analysis. All studied genes, including type II and X collagens, biglycan, versican, osteocalcin, osteopontin, and fetal liver kinase 1, were expressed in the CBGP with a reproducible absence of decorin mRNA. In age- and sex-matched rats (n=10), exogenous cyclic forces were applied to the maxilla at 500 mN and 4 Hz for 20 min/day over 2 days, followed by RNA isolation and RT-PCR analysis. This exogenous cyclic loading consistently induced the expression of the decorin gene, which was non-detectable, by the current RT-PCR approach, in control neonatal CBGPs without loading. Immunolocalization of several of the above-studied gene products demonstrated their remarkable site-specific expression. Decorin proteoglycan was primarily expressed in the perichondrium instead of various cartilage growth zones, especially upon mechanical loading. These findings serve as baseline data for the expression of several genes and gene products in the neonatal CBGP. Mechanical modulation of decorin expression is consistent with recent reports of its susceptibility to mechanical loading in several connective tissues.

Geometry and cell density of rat craniofacial sutures during early postnatal development and upon in vivo cyclic loading

Cranial sutures are unique to skull bones and consist of multiple connective tissue cell lineages such as mesenchymal cells, fibroblast-like cells, and osteogenic cells, in addition to osteoclasts. Mechanical modulation of intramembranous bone growth in the craniofacial suture is not well understood, especially during postnatal development. This study investigated whether in vivo mechanical forces regulate sutural growth responses in postnatal rats. Cyclic compressive forces with a peak-to-peak magnitude of 300 mN and 4 Hz were applied to the maxilla in each of 17-, 23-and 32-day-old rats for 20 min/day over 5 consecutive days. Computerized histomorphometric analysis revealed that cyclic loading significantly increased the average geometric widths of the premaxillomaxillary suture (PMS) to 86 +/- 7 microm, 99 +/- 12 microm, and 149 +/- 30 microm, representing 32%, 50%, and 39% increases for P17, P23, and P32 in comparison with age-matched sham controls. For the nasofrontal suture (NFS), cyclic loading significantly increased the average sutural widths to 88 +/- 15 microm, 92 +/- 10 microm, and 100 +/- 14 microm, representing 33%, 24%, and 32% increases for P17, P23, and P32 relative to age-matched controls. The average PMS cell density upon cyclic loading was 10182 +/- 132 cells/mm(2), 9752 +/- 661 cells/mm(2), and 9521 +/- 628 cells/mm(2), representing 62%, 35%, and 30% increases for P17, P23, and P32 in comparison with age-matched controls. For the NFS, cyclic loading increased the average cell density to 9884 +/- 893 cells/mm(2), 9818 +/- 1091 cells/mm(2), 9355 +/- 661 cells/mm(2), representing 44%, 46% and 40% increases at P17, P23, and P32 respectively. Osteoblast-occupied sutural bone surface was significantly greater in cyclically loaded sutures for P17, P23, and P32 than corresponding controls for both the PMS and NFS. On the other hand, cyclic loading elicited significantly higher sutural bone surface populated by osteoclast-like cells by P17 and P23 days, but not P32 days, for the PMS. For the NFS, sutural osteoclast surface was significantly higher upon cyclic loading for P23 and P32 days, but not P17. The present data demonstrate that cyclic forces are potent stimuli for modulating postnatal sutural development, potentially by stimulating both osteogenesis and osteoclastogenesis. Cyclic loading may have clinical implications as novel mechanical stimuli for modulating craniofacial growth in patients suffering from craniofacial anomalies and dentofacial deformities.

Functional influence on sutural bone apposition in the growing rat

INTRODUCTION

The aim of this study was to quantify the influence of reduced masticatory muscle function on sutural bone apposition in the growing rat.

METHODS

Twenty-six growing male albino rats were randomly divided into 2 equal groups; 1 (hard-diet group) received the ordinary diet of hard pellets, and the other (soft-diet group) received the powdered form of the ordinary diet mixed with water. The experimental period started when the rats were 4 weeks old and lasted 42 days. At days 0, 14, and 28, calcein was injected into all animals. At the end of the experiment, the animals were killed, and the heads were taken for preparation of undecalcified frontal sections, 120 microm thick. Three representative homologous sections for each animal in both groups were selected and studied under a fluorescence microscope. The level of bone apposition at the time of calcein injection was marked with separate fluorescing lines. Because the lines from the first injection could not be seen in all areas, bone apposition in the internasal, naso-premaxillary, and inter-premaxillary sutures was quantified from day 14 to the end of the experimental period, by using an image analysis software.

RESULTS

In both groups, greater bone apposition was found between days 14 and 28 than between days 28 and 42 of the experimental period. Less bone apposition was found in the soft-diet group than in the hard-diet group in all sutures studied.

CONCLUSIONS

The findings suggest that bone apposition in the studied facial sutures in the anterior facial skeleton of the growing rat is significantly affected by reduced masticatory function.

Expression and mechanical modulation of matrix metalloproteinase-1 and -2 genes in facial and cranial sutures

Craniofacial sutures create a soft tissue interface between various calvarial and facial bones. Facial and cranial sutures show differences in their surrounding anatomical structures and local mechanical strain environments. Despite previous attempts to identify the expression of matrix metalloproteinase genes (MMPs) in cranial sutures, little is known regarding whether facial and cranial sutures differ in MMP expression. We have investigated the expression of MMP-1 and MMP-2 in the pre-maxillomaxillary suture (PMS; facial suture) and the frontoparietal suture (FPS; cranial suture) in 32-day-old rats with or without the application of cyclic loading. Expression of MMP-1 and MMP-2 was detected by the reverse transcription/polymerase chain reaction technique. At 32 days of postnatal development (n=6), both MMP-1 and MMP-2 were reproducibly expressed in the facial PMS, in comparison with negligible MMP-1 and MMP-2 expression in the cranial FPS. In six age- and sex-matched control rats, cyclic loading at 4 Hz and 1000 mN was applied to the maxilla for two 20-min episodes within a 12-h interval. In some (but not all) cases, cyclic loading induced marked expression of MMP-1 and MMP-2 in the PMS and FPS in comparison with corresponding non-loaded controls. These data confirm our previous finding that short doses of cyclic loading upregulate MMP-2 expression in craniofacial sutures and suggest the possibility that facial and cranial sutures differ in matrix degradation rates during postnatal development.

Modulation of bone ingrowth of rabbit femur titanium implants by in vivo axial micromechanical loading

Titanium implants commonly used in orthopedics and dentistry integrate into host bone by a complex and coordinated process. Despite increasingly well illustrated molecular healing processes, mechanical modulation of implant bone ingrowth is poorly understood. The objective of the present study was to determine whether micromechanical forces applied axially to titanium implants modulate bone ingrowth surrounding intraosseous titanium implants. We hypothesized that small doses of micromechanical forces delivered daily to the bone-implant interface enhance implant bone ingrowth. Small titanium implants were placed transcortically in the lateral aspect of the proximal femur in 15 New Zealand White rabbits under general anesthesia and allowed to integrate with the surrounding bone for 6 wk. Micromechanical forces at 200 mN and 1 Hz were delivered axially to the right femur implants for 10 min/day over 12 consecutive days, whereas the left femur implants served as controls. The average bone volume 1 mm from mechanically loaded implants (n = 15) was 73 +/- 12%, which was significantly greater than the average bone volume (52 +/- 21%) of the contralateral controls (n = 15) (P < 0.01). The average number of osteoblast-like cells per endocortical bone surface was 55 +/- 8 cells/mm(2) for mechanically loaded implants, which was significantly greater than the contralateral controls (35 +/- 6 cells/mm(2)) (P < 0.01). Dynamic histomorphometry showed a significant increase in mineral apposition rate and bone-formation rate of mechanically stressed implants (3.8 +/- 1.2 microm/day and 2.4 +/- 1.0 microm(3).microm(-2).day(-1), respectively) than contralateral controls (2.2 +/- 0.92 microm/day and 1.2 +/- 0.60 microm(3).microm(-2).day(-1), respectively; P < 0.01). Collectively, these data suggest that micromechanical forces delivered axially on intraosseous titanium implants may have anabolic effects on implant bone ingrowth.