Effect of tensile mechanical stress on the synthesis of metalloproteinases by rabbit coronal sutures in vitro

Timing of ectocranial suture activity in Gorilla gorilla as related to cranial volume and dental eruption

Research has shown that Pan and Homo have similar ectocranial suture synostosis patterns and a similar suture ontogeny (relative timing of suture fusion during the species ontogeny). This ontogeny includes patency during and after neurocranial expansion with a delayed bony response associated with adaptation to biomechanical forces generated by mastication. Here we investigate these relationships for Gorilla by examining the association among ectocranial suture morphology, cranial volume (as a proxy for neurocranial expansion) and dental development (as a proxy for the length of time that it has been masticating hard foods and exerting such strains on the cranial vault) in a large sample of Gorilla gorilla skulls. Two-hundred and fifty-five Gorilla gorilla skulls were examined for ectocranial suture closure status, cranial volume and dental eruption. Regression models were calculated for cranial volumes by suture activity, and Kendall's tau (a non-parametric measure of association) was calculated for dental eruption status by suture activity. Results suggest that, as reported for Pan and Homo, neurocranial expansion precedes suture synostosis activity. Here, Gorilla was shown to have a strong relationship between dental development and suture activity (synostosis). These data are suggestive of suture fusion extending further into ontogeny than brain expansion, similar to Homo and Pan. This finding allows for the possibility that masticatory forces influence ectocranial suture morphology.

The biomechanical characteristics of cranial sutures are altered by spring cranioplasty forces

BACKGROUND

The mechanical properties of the pediatric craniofacial complex allow dissipation of spring cranioplasty forces. Springs do not fully expand in situ and continue to transmit a continuous force until removal. The authors wished to investigate whether ongoing forces altered the biomechanical characteristics of cranial sutures.

METHODS

Thirty New Zealand White rabbits were divided into five groups: spring expansion for 4, 7, and 10 weeks; early spring removal at 4 weeks followed by monitoring for 3 weeks; and a control group (n = 6 each). Cranial expansion was monitored using cephalometry. The left coronal suture then underwent load-displacement testing in a dynamometer.

RESULTS

Relapse of cranial expansion was observed following early spring removal (mean, 6 percent; p = 0.017). Cranial suture thickness was significantly correlated to the length of spring insertion. Load displacement curves of sutures in all groups initially exhibited classic viscoelastic behavior. The treatment group developed intrasutural weakening before failure that was not observed in controls. The peak load before failure as a percentage of that observed in controls was 31 percent in the 4-week group (p = 0.001), 35 percent in the 7-week group (p = 0.000), and 45 percent in the 10-week group (p = 0.023).

CONCLUSIONS

Cranial suture compliance is modified in the presence of continuous spring cranioplasty forces. Thickening of the coronal sutures, which have been expanded in a shear-like manner, increases their three-dimensional surface area and may contribute to the relative lack of relapse observed after early spring removal.

Cellular response to force application at craniofacial sutures

OBJECTIVES

To provide a comprehensive review of the literature describing research done on the responses of suture cells to force application in vitro and in vivo.

DESIGN AND RESULTS

This review outlines the types of forces that can be applied, methods of applying the forces, the sutures used in experiments, and the changes in morphology, molecular biology (gene and protein expression), and cell biology (proliferation, differentiation, apoptosis) in response to these forces.

CONCLUSION

The molecular response of sutures to force needs to be further investigated as these molecules can be used to enhance the way in which craniofacial sutures respond to mechanical force during orthopedic-orthodontic treatment.

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.

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

A cranial suture consists of neural-crest derived cells and matrices between mineralized skull bones. Little is known regarding the involvement of matrix metalloproteinases (MMPs) in the degradation of extracellular matrix of cranial sutures. In the postnatal rat model, the posterior frontal suture (PFS) undergoes complete ossification between P12-P22, whereas the sagittal suture (SS) remains patent. The present study utilized reverse transcriptase-polymerase chain reaction (RT-PCR) to explore the expression of MMP-1 and MMP-2 genes in the PFS and SS in P8 and P32 rats, and also to determine whether these MMP genes are modulated by exogenous mechanical forces. RNA was isolated from P8 and P32 normal PFS and SS each by pooling sutural specimens from 14 to 20 rats. RT-PCR analysis and semi-quantitative luminosity demonstrated the expression of MMP-1 and MMP-2 genes in the patent P8 PFS, P8 SS, and P32 SS, but no apparent MMP-2 expression in the physiologically ossified P32 PFS. Exogenous cyclic forces applied to the maxilla at 1000 mN and 4 Hz elicited corresponding cyclic bone strain waveforms with peak strain of 134.14+/-38.15 muepsilon (mean+/-S.D.) for the PFS, and 28.35+/-10.86 muepsilon for the SS in P32 rats. These cyclic forces delivered for 20 min/d over 2 consecutive days induced the expression of MMP-2 gene in the physiologically fused P32 PFS that was not expressed without mechanical stresses. Taken together, these data suggest potentially important roles of MMP genes in the postnatal development of cranial sutures, and their susceptibility to mechanical stresses.

How does skin adapt to repetitive mechanical stress to become load tolerant?

Skin breakdown from mechanical stress application is a difficult health care problem for lower-limb amputees using prosthetic limbs. Post-operative treatments to encourage skin adaptation do exist, but are largely unsuccessful. Potentially, by understanding skin adaptation on a molecular level, appropriate biomolecules can be identified and then delivered to skin to encourage adaptation in at-risk patients. Based from a critical review of the literature, it is expected that adaptation occurs by forming new collagen fibrils with larger diameters as opposed to increasing diameters of existing fibrils. Small collagen fibril breakdown by stress activated metalloproteinases is expected to be followed by increased expressions of decorin, biglycan, fibromodulin, lumican, thrombospondin-2, and collagens I and III, facilitating formation of new fibrils with larger diameters. After remodeling, total collagen fibril cross-sectional area is expected to return to baseline values since increased collagen content would increase mass and be redundant towards the purpose of adaptation.

An explant model for the investigation of skin adaptation to mechanical stress

A test apparatus was developed to investigate the effects of mechanical stress application on collagen remodeling in skin. The system maintained a 4.5-cm x 5.5-cm skin explant at an air interface with controlled temperature, relative humidity (RH), and carbon dioxide concentration [CO2] while allowing controlled compressive and shear forces to be applied to the skin surface. For environmental control, a custom-designed flow system under Proportional-Integral-Derivative (PID) control was used. Evaluation tests demonstrated that the system maintained air above the explant at a temperature within 1 degrees C of the 37.5 degrees C set point, RH within 5% of the user-specified set point (range of 5% to 95%), and [CO2] within 1% of the 5% [CO2] set point. Least-squares errors in cyclic compressive and shear forces (0- to 20-Hz bandwidth) delivered to the explant were 0.9% and 2.8%, respectively, of user-specified values. Pig skin samples cyclically stressed for 1 hr/day for 3 days with either compressive force only or a combined compressive and shear force had significantly smaller collagen fibril densities compared with an unstressed control, a result consistent with in vivo test data. Collagen fibril diameters were significantly larger for stressed versus control for some of the samples, but the changes were not as substantial as from in vivo testing. This result may have been due to the shorter study duration in vitro (3 d versus 20 d in vivo). The system allows insight into the mechanisms of skin adaptation to mechanical stress to be investigated on a cellular and molecular level, potentially leading to therapies to encourage adaptation in at-risk patients.

Proteoglycans and orthodontic tooth movement

Proteoglycans represent an important and diverse family of extracellular matrix components within the connective tissues of the periodontium. This review focuses on the function and metabolism of the various proteoglycans in periodontal tissues, such as alveolar bone and periodontal ligament, and considers their potential fate in response to an orthodontic force. Such considerations provide an important background in evaluating the potential for proteoglycan metabolites, alongside other connective tissue metabolites, as biomarkers for assessing the deep-seated metabolic changes and as a diagnostic tool in monitoring orthodontic tooth movement.

Mechanical stress in vitro induces increased expression of MMPs 2 and 9 in excised Dupuytren's disease tissue

We have previously shown that the ability to mechanically extend Dupuytren's contractures in vivo by the Continuous Elongation Technique before surgery resulted in increased metalloproteinase activity. However, under these conditions it was not possible to show whether the response was proportional to the mechanical stimulus or was inflammatory cell mediated. Using an in vitro system of controlled extensions in which inflammatory involvement is absent, we have now shown that there is a clear correlation between the load applied to the tissue and the release of matrix metalloproteinase-2. The subsequent degradation of the collagen results in a loss of mechanical strength reported in the preceding paper.

Misregulation of stromelysin-1 expression in mouse mammary tumor cells accompanies acquisition of stromelysin-1-dependent invasive properties

Stromelysin-1 is a member of the metalloproteinase family of extracellular matrix-degrading enzymes that regulates tissue remodeling. We previously established a transgenic mouse model in which rat stromelysin-1 targeted to the mammary gland augmented expression of endogenous stromelysin-1, disrupted functional differentiation, and induced mammary tumors. A cell line generated from an adenocarcinoma in one of these animals and a previously described mammary tumor cell line generated in culture readily invaded both a reconstituted basement membrane and type I collagen gels, whereas a nonmalignant, functionally normal epithelial cell line did not. Invasion of Matrigel by tumor cells was largely abolished by metalloproteinase inhibitors, but not by inhibitors of other proteinase families. Inhibition experiments with antisense oligodeoxynucleotides revealed that Matrigel invasion of both cell lines was critically dependent on stromelysin-1 expression. Invasion of collagen, on the other hand, was reduced by only 40-50%. Stromelysin-1 was expressed in both malignant and nonmalignant cells grown on plastic substrata. Its expression was completely inhibited in nonmalignant cells, but up-regulated in tumor cells, in response to Matrigel. Thus misregulation of stromelysin-1 expression appears to be an important aspect of mammary tumor cell progression to an invasive phenotype.

A methodical study of shape changes in human oral cells perturbed by a simulated orthodontic strain in vitro

Cells are known to alter their shape as a response to physical and chemical changes. Mechanical loads applied to teeth produced cellular perturbations resulting in orthodontic movement. An in vitro model was developed to simulate the in vivo strain of orthodontic movement. Calibrated forces were applied to human periodontal ligament cells and buccal mucosal fibroblasts (controls). A biaxial strain-producing device was used to stretch vital cells growth on flexible polytetrafluorethylene membranes. In addition, a new cell adhesive, Cell Tak, was employed to examine the effect of an adhesive substrate on the cellular response to two known loads. The shape changes of unstrained (control) and strained cells were evaluated by time-lapse telemicroscopy, and plots of time-dependent alterations in area and shape were recorded. The fusiform cells became more rounded over a given time of up to 1400 s. The responses appeared to be independent of cell type, the strain employed, and the presence of cell adhesive. Scanning electron microscopy demonstrated, irrespective of cell type, that the surface of stressed cells produced a striking number of microvilli as compared with the relatively smooth-surfaced controls.

Guided bone regeneration in calvarial bone defects using polytetrafluoroethylene membranes

Guided bone regeneration is defined as controlled stimulation of new bone formation in a bony defect, either by osteogenesis, osteoinduction, or osteoconduction, re-establishing both structural and functional characteristics. Bony defects may be found as a result of congenital anomalies, trauma, neoplasms, or infectious conditions. Such conditions are often associated with severe functional and esthetic problems. Corrective treatment is often complicated by limitations in tissue adaptations. The aim of the investigation was to compare histologically the amount of bone formed in an experimentally created parietal bone defect protected with one or two polytetrafluoroethylene membranes with a contralateral control defect. A bony defect was created bilaterally in the parietal bone lateral to the sagittal suture in 29 6-month-old male Wistar rats. The animals were divided into two groups: (1) In the double membrane group (n = 9), the left experimental bone defect was protected by an outer polytetrafluoroethylene membrane under the periosteum and parietal muscles and an inner membrane between the dura mater and the parietal bone. (2) In the single membrane group (n = 20), only the outer membrane was placed. The right defect was not covered with any membrane and served as control. The animals were killed after 30 days. None of the control defects demonstrated complete or partial bone regeneration. In the single membrane group, the experimental site did not regenerate in 15 animals, partially in four, and completely in one. In the double membrane group, six of the experimental defects had complete closure with bone, two had partial closure, and one no closure.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of sutures in normal and abnormal craniofacial growth

The paper is a shortened version of a paper read at the symposium on craniofacial growth, in which the literature on various aspects of sutures was reviewed. Suture development, structure, growth, and closure are covered, and the response of sutures to orthopedic forces and their role in craniosynostosis exemplified. Rather than being an extensive review, references are included preferably to present diversity in results and methods within the subtitle of the symposium, 'mechanisms and study methods'.

The continuous elongation technique for severe Dupuytren's disease. A biochemical mechanism

Continuous extension of Dupuytren's contracture prior to fasciectomy results in a softening of the tissue, allowing straightening of the fingers. The observed change in cross-link profile indicates an increase in newly synthesised collagen due to increased turnover. This was confirmed by demonstration of the increases in levels of the degradative enzymes, the neutral metalloproteinases, collagenase and gelatinase and the acidic cathepsins B and L. Both types of enzyme effectively depolymerize the collagen fibres, albeit by different mechanisms, leading initially to loss of tensile strength and ultimately to solubilization. We suggest that the increase in enzyme activity is generated by tension on the fibroblasts of this metabolically active tissue produced during the continuous extension of the retracted fingers. The weakening of the fibres by degradation and the increase in newly synthesized collagen provide an explanation for the extension of the tissue without trauma.

Differential scanning calorimetric studies of superficial digital flexor tendon degeneration in the horse

Differential scanning calorimetry (DSC) of equine superficial digital flexor tendons revealed the presence of a small exothermic peak at 23 degrees C of unknown origin, and a large endothermic peak at 70 degrees C due to denaturation of cross-linked collagen fibres. In the central degenerated core of damaged tendons the denaturation temperature remained at 70 degrees C but the enthalpy decreased in relation to the extent of degeneration of the tendon. We suggest that this reduction in enthalpy is due to depolymerisation and denaturation of the collagen fibres. This contention is supported by the observed increased activity of the degradative enzyme cathepsin B secreted by the fibroblasts. DSC analysis of cultured porcine tendon fibroblasts revealed a multicomponent endotherm, denaturation beginning at 46 degrees C, a temperature capable of being achieved within the tendon during intensive exercise. DSC clearly has considerable potential in complementing morphological and biochemical studies to determine the aetiology and progress of equine tendon degeneration.

In vitro evaluation of dental materials

Biocompatibility has been described as the ability of a material to perform with an appropriate host response in a specific application. Appropriate host response means no (or a tolerable) adverse reaction of a living system to the presence of such a material. An adverse reaction may be due to the toxicity of a dental material. Therefore toxicity may be regarded as one reason for nonbiocompatibility of a dental material. The toxicity of a dental material can be evaluated by in vitro tests, animal experiments and clinical trials. There exists a variety of different in vitro test methods. The most widely used biological systems for toxicity screening of dental materials are cell cultures. Cell cultures for toxicity screening of dental materials are valuable tools for understanding their biological behavior, if the limitations of the methods are taken into consideration, especially concerning the interpretation of the results. Further research should concentrate on better simulations of the in vivo situation in cell cultures. In this review the applications of various cell culture methods to evaluate the cytotoxicity of a wide range of dental materials, e.g. metals, alloys, polymers and cements, are described.

Recent advances in understanding mechanically induced bone remodeling and their relevance to orthodontic theory and practice

This review highlights recent developments in bone cell biology, evaluates previous research, and offers future direction toward improving our understanding of events that mediate orthodontic tooth movement. The in vivo and in vitro models that have been developed to examine the responses of connective tissues and how they have contributed to our understanding of the mechanisms involved in mechanically induced bone remodeling are discussed in detail. Osteoblasts are now recognized as the cells that control both the resorptive and the formative phases of the remodeling cycle, and receptor studies have shown them to be the target cells for resorptive agents in bone. The osteoblast is perceived as a pivotal cell, controlling many of the responses of bone to stimulation with hormones and mechanical forces. It is apparent that not all the cellular responses induced by mechanically deformed tissues can be explained by the current paradigm emphasizing the importance of prostaglandin production and cAMP elevation; the mobilization of membrane phospholipids giving rise to inositol phosphates offers an alternative second messenger pathway. It is also argued from circumstantial evidence that changes in cell shape produce a range of effects mediated by membrane integral proteins (integrins) and the cytoskeleton, which may be important in transducing mechanical deformation into a meaningful biologic response.

Influence of an intermittent compressive force on matrix protein expression by ROS 17/2.8 cells, with selective stimulation of osteopontin

The purpose of this study was to determine the response of bone cells to physical stress. Intermittent compressive force (ICF) was applied to 13 kPa to subconfluent ROS 17/2.8 cells at 18 cycles/min. After 48 h of this application, the cells were labelled with [35S]-methionine or [32PO4]. Application of ICF over this time did not alter the synthesis of type I collagen, fibronectin or bone SPARC (osteonectin) compared to that of control cells. However, the activity of alkaline phosphatase was increased 1.5-fold, and the synthesis of a 32PO4-labelled, 75-kDa phosphoprotein, recognized as osteopontin by immunoprecipitation with specific antibodies, was increased 1.4-fold. Also, an increase in osteopontin mRNA starting within 12h of ICF application was observed. The selective increase in osteopontin expression associated with ICF may be important in the remodelling of bone tissues during growth and development and in response to functional forces.

Biochemical signal transduction of mechanical strain in osteoblast-like cells

The responses to mechanical loading of two types of osteoblast-like cells and skin fibroblasts were investigated using two new devices for applying defined and homogeneous strains to cells. The results indicate that only periostal (bone surface) osteoblasts are sensitive to strains within the physiological range and that a specific strain mechanism is responsible. Osteoblasts derived from the haversian system and skin fibroblasts do not respond except at higher, unphysiological strains. The mechanism is located in the cytoskeleton and activates the membrane phospholipase C within milliseconds and may react to distension of a strain sensitive protein. Activation of phospholipase C can account for only some of the observed responses of bone to mechanical loading such as stimulation of cell division, increase in collagen and collagenase production. Application of over 10,000 mu strains results in a de-differentiation of the osteoblasts and a change in cell morphology to become fibroblast-like.

A device for the application of known simulated orthodontic forces to human cells in vitro

Connective tissues are responsive to mechanical forces. In orthodontic tooth movement it appears that the periodontal ligament (PDL) is the source of a pleuropotential cell population and extracellular matrix structure which translates mechanical perturbation information into a host of cellular events. These include proliferation, repair, differentiation, and shape change. We have designed, built, and tested a simple, adaptable machine which enables us to examine molecular changes or events in the cell nucleus, cell membrane, and the cytoskeleton of any eukasytic cell that will adhere to a membrane. These responses to clinically simulated forces applied to an in vitro system can be measured.

Involvement of PGE synthesis in the effect of intermittent pressure and interleukin-1 beta on bone resorption

Human periodontal ligament (PDL) fibroblasts, cultured from extracted healthy premolars, and a cloned osteogenic cell line (MC3T3-E1) were used in this study to determine the effect of intermittent pressure on bone resorption. Cells (1 x 10(5] were incubated with BGJb medium in the presence or absence of the following factors: intermittent negative (-30 g/cm2) or positive (30 g/cm2) hydrostatic pressure and interleukin-1 beta (IL-1 beta, 1 ng/mL), for 24 h. Conditioned media (CM) generated from cultures of either cell types were used for prostaglandin E (PGE) assay, bone resorption assay, and assessment of osteoclast (OC)-like cell formation. Unstimulated PDL fibroblasts or MC3T3-E1 cells produced measurable amounts of PGE and bone-resorbing activity as measured by 45Ca released from mouse calvaria and OC-like cells. IL-1 beta-treated cells showed significantly elevated levels of PGE, bone resorption, and OC-like cell formation, as compared with unstimulated cells. Intermittent positive pressure (IPP) alone stimulated PGE production, but the resultant CM did not stimulate bone resorption or OC-like cell formation when IPP was applied to either cell type. The application of IPP, together with IL-1 beta in CM, caused a slight increase in the number of alpha-like cells, as compared with that of IL-1 beta-treated CM in both cell types. On the other hand, direct application of IPP on mouse bone-marrow cultures significantly increased the number of OC-like cells. This effect was additive in combination with either CM from unstimulated cells or exogenous addition of PGE2.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunolocalization of collagenase and tissue inhibitor of metalloproteinases (TIMP) in mechanically deformed fibrous joints

To investigate the effects of mechanical deformation on matrix degradation in fibrous joints, coronal suture explants from neonatal rabbits were stressed in vitro for 24 hours in an established tooth-movement model system. The metalloproteinase collagenase (CL) and its inhibitor, TIMP (tissue inhibitor of metalloproteinases), were immunolocalized in two ways by a two-step indirect technique: (1) extracellularly by immunoprecipitation at the site of secretion, and (2) intracellularly by incubation of the explants with the ionophore monensin. Immunoprecipitates of CL and TIMP were distributed throughout the sutural and periosteal tissues of nonstressed explants. In stressed explants, however, CL immunoprecipitates were predominantly associated with an area of rounded cells between the bone ends. In explants treated with monensin a significant increase in the number of CL-positive cells was observed in this cellular area; active enzyme was suggested by the demonstration of CL bound to collagen. Extracellular TIMP was not seen within the area of rounded cells of stressed explants, but intracellular TIMP was detectable; this suggests that insufficient TIMP was available to immunoprecipitate with anti-TIMP, probably because it had become irreversibly complexed with active CL. Since the area of rounded cells corresponds to the site of increased cell proliferation in this and other animal models of tooth movement, these data suggest that collagenase production and cell proliferation might be correlated. We speculate that matrix degradation is an essential prerequisite for cell proliferation as it creates room to accommodate an increase in cell population.

The effects of mechanical strain on osteoblasts in vitro

The effect of mechanical strain on bone is important to the field of oral and maxillofacial surgery. Oral and maxillofacial surgeons are involved on a daily basis with problems including alveolar ridge resorption, implant stability, craniofacial growth, and bone resorption related to trauma and pathology. To understand and control these effects on bone, it is important to examine the effects of mechanical strain on the osteoblasts. This series of experiments provides the changes in alkaline phosphatase, collagen synthesis, and protein synthesis in osteoblast-like cell subjected to mechanical strain.

Buccal mucosa fibroblasts and periodontal ligament cells perturbed by tensile stimuli in vitro

Human buccal mucosa fibroblasts and periodontal ligament cells grown in tissue culture were subjected to tensile forces approximating those used for orthodontic bodily tooth movement. The cells were synchronized into pre S phase and positively tested for response to nonmechanical physical stimuli. Two-dimensional gel analysis and immunohistochemical analysis of the three cytoskeletal components showed a lack of response. Similar negative results were found when the cells were perturbed in the presence of substance P. We hypothesize that perhaps these cells respond more readily to injury, a secondary effect of the forces of tooth movement, than to tensile forces.

The interactive effects of mechanical stress and interleukin-1 beta on prostaglandin E and cyclic AMP production in human periodontal ligament fibroblasts in vitro: comparison with cloned osteoblastic cells of mouse (MC3T3-E1)

Human periodontal ligament fibroblasts and a cloned osteogenic cell line (MC3T3-E1) were seeded (4 x 10(5) cells) on 60 mm Petriperm dishes, which have a flexible plastic growth surface. Cells were stretched by placing the dish on top of a spheroidal convex template, equilibrated to 37 degrees C. The amount of stretch was varied by changing the curvature of the template and calculated as percentage stretch. Both types of cell responded to mechanical stress by elevated synthesis of PGE and cAMP; the addition of interleukin-1 beta to mechanically stretched cells produced further elevation. Synergism between mechanical stress and interleukin-1 beta was found at certain lengths of incubation. The production of cAMP was secondary and dependent on the newly synthesized PGE, as shown in the presence of indomethacin. The two cell types were also different in terms of the timing of their response to mechanical stress and interleukin-1 beta. In the absence of stimuli, periodontal fibroblasts tended to produce PGE continually over time, whereas the MC3T3-E1 cells did not. However, both cell types had elevated PGE levels in response to the stimuli used in this experiment. Periodontal fibroblasts responded to mechanical stress and interleukin-1 beta with significant elevations of PGE as early as 15 min, whereas the MC3T3-E1 cells required 2 h to produce significant elevations for mechanical stress and 15 min for interleukin-1 beta. These findings indicate that the chemical and mechanical signals on these cells are mediated by surface receptors. Locally produced autocrine or paracrine factors can modify the effect of mechanical stress on periodontal and bone cells via the cAMP pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical, biomechanical, and physical changes in the skin in an experimental animal model of therapeutic tissue expansion

Biochemical, biomechanical, and physical changes occurring in the skin during tissue expansion have been studied using an animal model in which a Silastic expander was inserted into the peritoneal cavity. Forty-eight Sprague-Dawley rats were divided into four groups to be studied at 4, 8, 16 and 32 days after expansion. In the experimental animals (6 per group) the expander was inflated by a single injection of 120 ml of saline. Sixteen hours prior to sacrifice each animal received a single injection of tritiated proline. Sixteen days after expansion both the specific activity and the total content of hydroxyproline in the skin were significantly elevated in experimental animals. Intrinsic skin tension increased dramatically at the time of inflation but fell almost to control values at the end of 32 days. Skin thickness, initially decreased, returned to normal by the end of the experiment. There were no significant differences in breaking strengths between skin from experimental and control animals. Skin surface area, initially increased by stretching at the time of inflation, increased further between Days 0 and 8, possibly as a result of stress relaxation combined with enhanced remodelling of connective tissue macromolecules, and again from Days 24 to 32. We conclude that, during tissue expansion, there is a net accumulation of collagen in the skin and that this allows the local cellular environment to return to normal with respect to pressure and/or tension.

Effect of force level on synthesis of type III and type I collagen in mouse interparietal suture

Nine-week-old Swiss male white mice were divided into groups killed after time intervals of force application of six h, and one, three, five, seven, ten, 14, 21, and 28 days. Each group had 45 animals: three control, three sham-operated, and three experimental animals for each of the five force levels: 50 g, 35 g, 25 g, 15 g, and 5 g. The experimental animals had helical springs placed surgically in their calvaria for expansion of the interparietal suture. The sham-operated animals received inactive springs. Control animals were at the same age as the experimental and sham-operated animals. After death, the amount of sutural expansion was measured, and the calvaria with the implanted springs were explanted into Trowell-type organ culture dishes. [14C]-glycine was added for two h after 60 min of culture for all explants. The rate of suture expansion was directly proportional to the force value of the tensile stress, and a maximum 2.0-mm expansion was achieved for all force levels by the 28th day. Sutural collagen was solubilized by limited pepsin digestion, and radiolabeled types I and III alpha-chains were separated by SDS-PAGE, visualized fluorographically, and measured densitometrically. All the experimental and sham-operated animals responded with a rapid rise followed by an almost equally rapid fall in the proportion of newly-synthesized type III collagen before becoming stabilized for the rest of the experimental period at a level that was significantly higher than that of the control and sham-treated animals of the same age.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of sutural growth rate on collagen phenotype synthesis

We determined the ratio of newly-synthesized type III collagen to the total of type I and type III collagen in mouse interparietal sutural tissue at selected ages between birth and adulthood (36 weeks old). We incubated mouse calvaria explants in Trowell-type organ culture dishes for one h and then added [14C]-glycine for two h. We dissected the interparietal sutural tissues for collagen solubilization by limited pepsin digestion. Fluorographic visualization of separated radiolabeled collagens, after SDS-PAGE, found the ratio of collagen type III alpha-chains to the total type I and type III alpha-chains to be age-dependent. The proportion of type III alpha-chains at birth was quite high, but there was a significant drop (p less than 0.05) during the first two days of life, probably because of the sudden environmental change from in utero. The proportion of type III alpha-chains rose significantly from day 2 to day 4, reaching a maximum and then dropping significantly to about the same proportion as at birth by day 7. A further significant drop took place during the second week of life, with the proportion stabilized at around 3.5% at two weeks to ten weeks of age. A final significant drop during the eleventh week of life led to no detectable synthesis of type III collagen after 12 weeks of age. The changes in the collagen phenotype ratio did not relate to changes in body weight during growth and development, which suggests that the interparietal suture may have an independent maturing pattern.(ABSTRACT TRUNCATED AT 250 WORDS)

Degradation of basement membranes by human matrix metalloproteinase 3 (stromelysin)

Connective tissue cells synthesize and secrete a group of matrix metalloproteinases (MMPs), all of which are capable of degrading the extracellular-matrix components. One of them, MMP-3 (stromelysin) has been shown to degrade purified basement-membrane components, collagen IV and laminin [Okada, Y., Nagase, H. & Harris, E. D., Jr. (1986) J. Biol. Chem. 261, 14245-14255]. Here we report that MMP-3 degrades collagen IV and laminin in intact basement membranes from bovine glomeruli (GBM) and bovine anterior-lens capsules (LBM). Degradation products were analysed by SDS/polyacrylamide-gel electrophoresis to determine the number and sizes of polypeptide fragments. Immunoblotting techniques were used to identify the origins of the fragments, i.e. collagen IV or laminin. The fragments of collagen IV were further mapped using specific antibodies that recognize the N-terminal (7 S) domain, the C-terminal (NC-1) domain, or the major triple-helical region between the terminal domains. Degradation of collagen IV was extensive; many fragments were found, from both GBM and LBM, in the Mr range 25,000-380,000. A large fragment of laminin (Mr greater than 380,000) was found in the GBM digests without reduction, but it dissociated into 220,000-Mr chains upon reduction. The results suggest that MMP-3 plays an important role in the catabolism of basement membranes.

The precursor of a metalloendopeptidase from human rheumatoid synovial fibroblasts. Purification and mechanisms of activation by endopeptidases and 4-aminophenylmercuric acetate

Two active forms (Mr 45,000 and 28,000) of a metalloendopeptidase that digest proteoglycans and other extracellular matrix components of connective tissues have previously been purified from rheumatoid synovial cells and characterized [Okada, Nagase & Harris (1986) J. Biol. Chem. 261, 14245-14255]. To study the mechanisms of activation the precursor of this metalloendopeptidase has now been purified. The final products are homogeneous on SDS/polyacrylamide-gel electrophoresis and identified as a set of zymogens of Mr 57,000 and 59,000, in which the latter form is probably the product of post-translational glycosylation of the Mr 57,000 zymogen, as it binds to concanavalin A. The zymogen can be activated by trypsin, chymotrypsin, plasma kallikrein, plasmin and thermolysin, but not by thrombin. Although the activated metalloendopeptidase is further degraded by trypsin, plasma kallikrein and thermolysin during a prolonged incubation, it is relatively stable against plasmin and chymotrypsin. Activation with 4-aminophenylmercuric acetate is dependent on its concentration. It requires the reaction with the zymogen, possibly through thiol groups, and the continued presence of the agent. During this treatment the zymogen undergoes a sequential processing; first it becomes active without changing its apparent molecular mass, and then it is processed to low-Mr species of Mr 46,000, 45,000 (HMM) and 28,000 (LMM). The rate of conversion of the precursor into an initial intermediate of Mr 46,000 follows first-order kinetics (t1/2 2.0 h with 1.5 mM-4-amino-phenylmercuric acetate at 37 degrees C) and is independent of the initial concentration of the zymogen or the presence of up to a 676-fold molar excess of substrate, whereas the generation of HMM and LMM species is affected by these parameters. These results indicate that activation of the prometalloendopeptidase by an organomercurial compound is initiated by the molecular perturbation of the zymogen that results in conversion into the 46,000-Mr intermediate by an intramolecular action; the subsequent processing of this intermediate in HMM and LMM species is a bimolecular reaction. In vivo it is probable that the precursor of this metalloendopeptidase is activated either by direct limited proteolysis by tissue or plasma endopeptidases, or, alternatively, by factors that cause certain conformational changes in the zymogen molecule.

Sutures and forces: a review

This review gives a description of the biologic significance of craniofacial sutures with respect to growth and to growth corrections. Sutural growth and its regulation are discussed briefly. Morphogenesis of sutures, sutural morphology, both microscopic and macroscopic, the structure and function of the sutural periosteum and secondary cartilages, and the biochemical composition of sutures are described. Furthermore, in vivo and in vitro experiments, including transplantation experiments, are discussed. The relationship between extrinsic mechanical forces and the resulting tissue responses in sutures is given special attention. The present article describes the state of our knowledge on the interaction between sutures and forces, and indicates problems that need to be investigated.

The effect of mechanical stress on soft and hard tissue repair; a review

The influence of mechanical forces on intact tissue is well established. A growing body of evidence demonstrates that healing wounds also respond to the functional demands of their mechanical environment. At the present time, an understanding of the fundamental mechanism by which mechanical stress affects tissues and wounds remains elusive.

Etiology of late developing incisional hernias--the possible role of mechanical stress

The etiology of incisional hernias that develop a year or more after laparotomy is not understood. This subset of incisional hernias is not associated with the etiologic factors usually implicated in incisional hernia formation. Mechanical stress is known to have a profound influence on the structure of both normal and wounded tissue. It is hypothesised that mechanical stress plays an important role in the development of late incisional hernias.

Immunohistochemical assessment of the effect of chemical and mechanical stimuli on cAMP and prostaglandin E levels in human gingival fibroblasts in vitro

These were evaluated by: (1) a combined immunohistochemical-microphotometric procedure (IH) and (2) conventional radiometric assays. Human gingival fibroblasts were in the sixth passage, grown and maintained in Dulbecco minimal essential medium (DMEM) supplemented with 10 per cent horse serum. For chemical and hormonal stimuli, cells (2 x 10(4] were seeded on tissue-culture chamber/slides, and incubated with graded doses of either parathyroid hormone (PTH) or prostaglandin E2 (PGE2) for assessment of their adenosine-3',5'-monophosphate (cAMP) levels, and with indomethacin or colchicine for their effect on PGE levels. For mechanical stimuli, cells (1 x 10(6] were seeded on culture dishes with a flexible plastic membrane and stretched for 5, 30, 60 or 120 min by placing the membrane over a convex surface and weighting the dish cover. After freeze drying, cells were stained by an immunoperoxidase technique for either cAMP or PGE, using monoclonal antibodies. The staining intensity of fibroblasts was determined at 600 nm wavelength. Per cent light absorbance of 15 cells in each slide was measured and the results tested by analysis of variance. The gingival fibroblasts responded to the drugs and hormones in a dose- and time-related fashion. Stretching significantly increased their synthesis of PGE with concomitant increase in cAMP. The IH results were compared with the radiometric assays to confirm the validity of this technique; both assays were valid for describing the quantitative responses of these cells to the stimuli. In particular, the IH method could localize those intracellular sites which demonstrated chances in relative cAMP and PGE concentrations in response to hormonal stimuli.

The effect of mechanical stress on cultured growth cartilage cells

Mechanical stress seems to influence the growth of bone, but little is known about the transduction of mechanical forces into biochemical signals. The present study was done to resolve the important question of how a mechanical stimulus is transduced into a metabolic response by chondrocytes. Cultured growth cartilage cells isolated from rat ribs and subjected to mechanical stress (tensile force) showed a significant increase in intracellular cyclic AMP levels, but no increase in prostaglandin E2 levels. In glycosaminoglycan synthesis measured by radioactive sulfate incorporation, long-term exposure of chondrocytes to mechanical loading caused increased synthesis. Moreover, long-term exposure of chondrocytes exerted an alteration of responsiveness to parathyroid hormone and calcitonin measured as intracellular cyclic AMP content. These results suggest that mechanical stress can alter bone growth by modulating the metabolism of growth cartilage cells.

Effects of cyclic mechanical stimulation of the cellular components of the heart: in vitro

The response of the cellular components of the heart to cyclic mechanical stimulation is of particular importance because these cells are continually subjected to mechanical forces as a result of changes in blood volume and pressure. To directly investigate how mechanical tension affects these cellular components of the heart, an in vitro system that exposes the particular cell type (cardiac myocytes, endothelial cells, or fibroblasts) to a calibrated increase in cyclical linear stretch was developed. Cells were grown on silastic membranes coated with laminin and subjected to a 10% cyclical distention 10 times a minute for 72 h. Within 24 h of being exposed to the mechanical stretch, the cells became elongated and oriented perpendicular to the direction of the stretch. These results indicate that cyclical mechanical stimulation directly influences the cellular organization of the heart cells in vitro.

Effect of tensile force magnitude on release of cranial suture cells into S phase

An in vitro model was used to study the effect of tensile force magnitude and duration on cell proliferation in cranial suture tissue. Helical springs were calibrated to deliver specific magnitudes of force to rat midsagittal suture in organ culture. The explants were incubated for time periods ranging from 1 to 48 hours. The in vitro model system facilitates study of the effects of a single parameter of an applied force on suture tissue without the interference of the complicated craniofacial anatomy. Specifically, the influence of tensile force magnitude and duration on DNA synthesis was investigated. Using autoradiography, cells incorporating tritiated thymidine were counted, indicating cells released into DNA synthesis (S) phase.

A constitutive model for the mechanical behaviour of soft connective tissues

A model of the mechanical behaviour of soft connective tissue has been developed by considering the role of the collagen and glycosaminoglycan (GAG) components within the tissue in order to examine the mechanism by which a variation in the GAG components may exert a control over the mechanical properties of the tissue. It is proposed that the strain energy stored within the collagen fibrils of the loaded tissue can be transferred into a potential field created by the charged GAG components and their electrostatic interaction with the collagen fibrils. A fundamental mechanical unit is described to simulate this energy transfer and a combination of such units is used to represent the tissue. The computer implementation of the proposed tissue model shows it to reproduce many features which have been recognised in the rate dependent mechanical behaviour of soft tissues. These include the characteristic non-linearity of the force-deformation behaviour and the approximate invariance of the stress relaxation behaviour with deformation. The model is also consistent with earlier constitutive representations of tissue behaviour.

Mechanical stretching increases the number of cultured bone cells synthesizing DNA and alters their pattern of protein synthesis

A simple method was devised for applying mechanical stretching to bone cell cultures. Bone cells cultured on the flexible plastic membrane of a Petriperm dish are placed over a template with a convex surface. A lead weight is then placed on top of the dish which causes the membrane and the tightly attached cells to be stretched. Mechanical stretching, applied either intermittently or continuously for a 2-hour period resulted in a 64% increase in the number of cells synthesizing DNA. Stretching the cells also significantly increased incorporation of tritiated proline and tritiated leucine. To assay the ratio of collagenous to noncollagenous protein, medium and cell layers of cultures labeled with tritiated leucine were incubated with collagenase and the digests chromatographed on PD 10 columns. The amount of collagen synthesized by stretched and unstretched cultures did not differ; but an increased synthesis of noncollagenous proteins was observed in the stretched cultures.

Synthesis of collagenase and collagenase inhibitors by osteoblast-like cells in culture

A rat osteosarcoma cell clone (ROS 17/2), and osteoblast-enriched populations from rat calvaria cultured in the presence of concanavalin A, have been shown to produce latent collagenase and collagenase inhibitors. The enzymes and inhibitor activities from the ROS 17/2 cells were concentrated by ammonium sulphate precipitation and separated by gel filtration on AcA 54 resin. The size of the latent collagenase (Mr approximately equal to 58000) was reduced on conversion to active enzyme (Mr approximately equal to 48000) by p-aminophenylmercuric acetate. Latent and active forms of gelatinase activity, similar in size to the corresponding forms of collagenase, were also resolved. The collagenase inhibitor activity, which was sensitive to organomercurials, was recovered in two peaks (Mr approximately equal to 68000 and 30000). The active collagenase cleaved interstitial collagens (type I = III greater than II) producing typical 3/4 and 1/4 fragments. This activity was inhibited by the metal ion chelators ethylenediaminetetraacetic acid and o-phenanthroline. Additional specific cleavages of native collagen were also observed which, from the susceptibility of this activity to phenylmethylsulphonyl fluoride, leupeptin and antipain, suggested the presence of a second collagenolytic enzyme. This synthesis of collagenolytic enzymes by these osteoblast-like cells suggests that individual osteoblasts, like fibroblasts, are capable of both synthesizing and degrading their respective organic matrices in vivo.

The use of in vitro models for investigating the response of fibrous joints to tensile mechanical stress

To clarify the role of mechanical deformation in the remodeling of fibrous joints, organ culture systems have been developed to apply mechanical stress to cranial sutures under controlled experimental conditions. Tensile mechanical stress applied to cranial sutures from newborn rabbits produces a two- to threefold increase in protein synthesis and a twofold increase in collagen synthesis that can be detected within 6 hours. There is also a threefold increase in the DNA content of the sutures after 48 hours. Under normal conditions sutural fibroblasts synthesize type I collagen but respond to tensile deformation by synthesizing significant amounts of type III collagen. This suggests that the biomechanical environment of a connective tissue cell is an important determinant of the collagen type synthesized. However, the effect is likely to be an indirect one by virtue of its influence on the metabolic activity of the cells. Mechanically activated cells do not preferentially synthesize structural proteins, since mechanical stress stimulates the synthesis not only of structural macromolecules but also of the enzymes responsible for their specific hydrolysis. This is not accompanied by increased degradation, however, perhaps because the metalloproteinase inhibitor TIMP synthesized by the tissues is also increased. Confluent rabbit and mouse periosteal fibroblasts synthesize and release into the culture medium factors that can inhibit bone cell proliferation and stimulate bone resorption in vitro. It seems likely that further investigation of the interaction between fibroblasts and osteoblasts at the bone--fibrous tissue interface will require a reassessment of current thinking concerning the mechanisms regulating sutural osteogenesis.

Effect of experimental tooth movement on the mechanical strength of the periodontium in the rat mandibular first molar

A 474 microns thick latex elastic band was inserted between the mandibular first and second molars to cause tooth movement. The mechanical strength of the periodontium was measured by extracting the first molar from the socket in the dissected jaw. From the 1st to the 4th day, the ultimate extraction loads decreased markedly, while the inter-dental spaces remained about 300 microns. About 5 min after the elastic insertion, the ultimate load was not significantly changed and the inter-dental space was 61 microns. Restoration of the mechanical strength of the periodontium occurred gradually after the removal of the band. On the 4th day, recovery was complete. Restoration of the original inter-dental space was most marked in the initial 24 h. The forces exerted on the teeth by the bands were of the order of several tens of newtons initially and became reduced to a few newtons after the tooth movements. It is suggested that decreases in the ultimate extraction loads were caused by changes in constitution of the periodontal collagen, by the disorganization of the periodontium and by the loosening of the attachment of the periodontal fibres to the bone.

Rabbit cranial suture fibroblasts under tension express a different collagen phenotype

We present evidence that cells in fibrous joints respond to tensile mechanical stress by synthesizing significant quantities of type III collagen. Under normal conditions sutural fibroblasts synthesize type I collagen. Type III comprised some 20 per cent of the newly synthesized collagen in stressed joints, a level that was achieved within 24 h of the onset of tension and remained unchanged throughout a 4-day experimental period. These findings suggest that the biomechanical environment of a connective tissue cell is an important determinant of the type of collagen synthesized. However, we propose that the effect is likely to be an indirect one by virtue of its influence on the metabolic activity of the cells.