Rabbit cranial sutures in vitro: a new experimental model for studying the response of fibrous joints to mechanical stress

Bioreactor With Electrically Deformable Curved Membranes for Mechanical Stimulation of Cell Cultures

Physiologically relevant in vitro models of stretchable biological tissues, such as muscle, lung, cardiac and gastro-intestinal tissues, should mimic the mechanical cues which cells are exposed to in their dynamic microenvironment in vivo. In particular, in order to mimic the mechanical stimulation of tissues in a physiologically relevant manner, cell stretching is often desirable on surfaces with dynamically controllable curvature. Here, we present a device that can deform cell culture membranes without the current need for external pneumatic/fluidic or electrical motors, which typically make the systems bulky and difficult to operate. We describe a modular device that uses elastomeric membranes, which can intrinsically be deformed by electrical means, producing a dynamically tuneable curvature. This approach leads to compact, self-contained, lightweight and versatile bioreactors, not requiring any additional mechanical equipment. This was obtained via a special type of dielectric elastomer actuator. The structure, operation and performance of early prototypes are described, showing preliminary evidence on their ability to induce changes on the spatial arrangement of the cytoskeleton of fibroblasts dynamically stretched for 8 h.

Techniques to stimulate and interrogate cell-cell adhesion mechanics

Cell-cell adhesions maintain the mechanical integrity of multicellular tissues and have recently been found to act as mechanotransducers, translating mechanical cues into biochemical signals. Mechanotransduction studies have primarily focused on focal adhesions, sites of cell-substrate attachment. These studies leverage technical advances in devices and systems interfacing with living cells through cell-extracellular matrix adhesions. As reports of aberrant signal transduction originating from mutations in cell-cell adhesion molecules are being increasingly associated with disease states, growing attention is being paid to this intercellular signaling hub. Along with this renewed focus, new requirements arise for the interrogation and stimulation of cell-cell adhesive junctions. This review covers established experimental techniques for stimulation and interrogation of cell-cell adhesion from cell pairs to monolayers.

Force-induced craniosynostosis in the murine sagittal suture

BACKGROUND

The cause of nonsyndromic craniosynostosis remains elusive. Although compressive forces have been implicated in premature suture fusion, conclusive evidence of force-induced craniosynostosis is lacking. The purpose of this study was to determine whether cyclical loading of the murine calvaria could induce suture fusion.

METHODS

Calvarial coupons from postnatal day-21, B6CBA, wild-type mice (n = 18) were harvested and cultured. A custom appliance capable of delivering controlled, cyclical, compressive loads was applied perpendicular to the sagittal suture within the coupon in vitro. Nine coupons were subjected to 0.3 g of force for 30 minutes each day for a total of 14 days. A control group of nine coupons was clamped in the appliance without loading. Analysis of suture phenotype was performed using alkaline phosphatase and hematoxylin and eosin staining techniques and in situ hybridization analysis using bone sialoprotein.

RESULTS

Control group sagittal sutures-which normally remain patent in mice-showed their customary histologic appearance. In contradistinction, sagittal sutures subjected to cyclic loading showed histologic evidence of premature fusion (craniosynostosis). In addition, alkaline phosphatase activity and bone sialoprotein expression were observed to be increased in the experimental group when compared with matched controls.

CONCLUSIONS

An in vitro model of force-induced craniosynostosis has been devised. Premature fusion of the murine sagittal suture was induced with the application of controlled, cyclical, compressive loads. These results implicate abnormal forces in the development of nonsyndromic craniosynostosis, which supports our global hypothesis that epigenetic phenomena play a crucial role in the pathogenesis of craniosynostosis.

Development of the fetal lung

The development of the fetal lung compared to that of other organs is unusual in the degree of its dependence on extrinsic stimuli. When the space available to the growing lung is limited by space-occupying lesions or when the diaphragm is paralysed, lung growth is markedly impaired. The relationship of lung volume to growth may depend on lung distension. Lung hypoplasia associated with experimental procedures causing inhibition or blunting of fetal breathing movements suggests that the distending forces may be generated by these movements. Maturation is less dependent on distension and more dependent on the hormonal environment. Distensibility and stability of the lung in fetal sheep develops rapidly within a few days of birth and correlates strongly with the plasma cortisol concentration. Hypophysectomy retards mutation which is restored by infusing adrenocorticotropin but not cortisol into the fetus. The hormones mainly responsible for controlling the various aspects of maturation probably include cortisol, iodothyronines and catecholamines but the interrelationships of these hormones and the extent of involvement of other hormones is uncertain.

Remodeling the dentofacial skeleton: the biological basis of orthodontics and dentofacial orthopedics

Orthodontic tooth movement is dependent upon the remodeling of the periodontal ligament and alveolar bone by mechanical means. Facial sutures are also fibrous articulations, and by remodeling these joints, one can alter the positional relationships of the bones of the facial skeleton. As might be expected from the structure and mobility of the temporomandibular joint (TMJ), this articulation is more resistant to mechanical deformation, and whether functional mandibular displacement can alter the growth of the condyle remains controversial. Clinical investigations of the effects of the Andresen activator and its variants on dentofacial growth suggest that the changes are essentially dento-alveolar. However, with the popularity of active functional appliances, such as the Herbst and twin-block based on 'jumping the bite', attention has focused on how they achieve dentofacial change. Animal experimentation enables informed decisions to be made regarding the effects of orthodontic treatment on the facial skeleton at the tissue, cellular, and molecular levels. Both rat and monkey models have been widely used, and the following conclusions can be drawn from such experimentation: (1) Facial sutures readily respond to changes in their mechanical environment; (2) anterior mandibular displacement in rat models does not increase the mitotic activity of cells within the condyle to be of clinical significance, and (3) mandibular displacement in non-human primates initiates remodeling activity within the TMJ and can alter condylar growth direction. This last conclusion may have clinical utility, particularly in an actively growing child.

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.

Effects of substrate characteristics on bone cell response to the mechanical environment

The effect of substrate characteristics on primary human bone cell response to mechanical loading was investigated in this study. The substrates comprised organic and inorganic materials with a range of hydrophilic and hydrophobic features. Substrate surface topography varied from smooth to particulate to porous. It was found that hydrophilic substrates such as borosilicate glass facilitated bone cell adhesion, in contrast to hydrophobic substrates such as poly(L-lactic acid), in which clumps of cells grew unevenly across the substrate surface. All primary bone cells cultured in the various collagen-coated substrates were responsive to mechanical stimulation. The study showed that, at a low strain level of 1000 microstrain, mechanical stimulation enhanced bone cell differentiation rather than proliferation. Coating the substrates with collagen type I enhanced cell adhesion and promoted an elongated cell morphology, indicating that the presence of specific binding sites on a substrate may be more important than its hydrophilic properties, regardless of the substrate topography.

Techniques for mechanical stimulation of cells in vitro: a review

A wide variety of laboratory apparatuses have been devised for mechanical stimulation of cell and tissue cultures. This article reviews the functional attributes of several dozen systems developed for that purpose, including their major advantages and disadvantages. These devices can be categorized in terms of their primary loading modality: compression (hydrostatic pressure or direct platen contact), longitudinal stretch, bending, axisymmetric substrate bulge, in-plane substrate distention, fluid shear stress, or combined substrate distention and fluid shear.

A new experimental model for studying the response of periodontal ligament cells to hydrostatic pressure

An apparatus was developed to apply positive or negative hydrostatic pressure dynamically to periodontal ligament (PDL) cells in vitro. The objective of this investigation was to construct this apparatus and to determine its effects on PDL cells. Human PDL cells were collected from freshly extracted premolars. At the sixth passage, the cells were mechanically stimulated by this apparatus at different magnitudes of continuous positive or negative hydrostatic pressures (PHP or NHP, respectively). The application of PHP between 0.3 and 30 gm/cm2 significantly enhanced prostaglandin E (PGE) production and intracellular cyclic AMP (cAMP) of the cells. In contrast, perturbation by NHP significantly decreased PGE production and intracellular level of cAMP. Proliferation rate increased significantly at 24 and 48 hours due to stimulation of these cells with -30 gm/cm2 of NHP. Challenging these cells with +30 gm/cm2 of PHP significantly decreased the proliferation rate of these cells at 24 and 48 hours. Stimulation by PHP between +30 to +600 gm/cm2 increased cell length and width and appeared to increase surface area attachment to the bottom of the culture dishes. In contrast, NHP (between -30 and -600 gm/cm2) decreased these dimensions and appeared to reduce the surface area of attachment. These results indicate that this type of mechanical perturbation of PDL cells produces physiologic responses and is not detrimental to their vitality.

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.

The effect of mechanical deformation on the distribution of ions in fibroblasts

The extracellular and intracellular sodium, potassium and chloride concentrations were determined in fibroblast cells located in the rat calvarium. The ionic values were determined by fluorescence microscopy after incubation with the fluorescent probes, sodium-binding benzofuran isophthalate (SBFI), potassium-binding benzofuran isophthalate (PBFI) and 6-methoxy-N-(3-sulfopropyl) quinolinium (SPQ) (Dyes were supplied by CALBIOCHEM, Nottingham, England). After determination of the resting membrane potential, the calvaria were placed under tension by retraction of a micromanipulator. The fluorescence was measured again. A statistically significant difference was found in the calculated potassium ion concentration (Mann-Whitney; p < 0.05). This affected the resting cell membrane potential by an average of 5.2 mV. This effect was blocked by the addition of a potassium channel blocker, tetraethylammonium (TEA).

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'.

Sutural biology and the correlates of craniosynostosis

The purpose of this paper is to provide a new perspective on craniosynostosis by correlating what is known about sutural biology with the events of craniosynostosis per se. A number of key points emerge from this analysis: 1) Sutural initiation may take place by overlapping, which results in beveled sutures, or by end-to-end approximation, which produces nonbeveled, end-to-end sutures. All end-to-end sutures occur in the midline (e.g., sagittal and metopic) probably because embryonic biomechanical forces on either side of the initiating suture tend to be equal in magnitude. A correlate appears to be that only synostosed sutures of the midline have pronounced bony ridging. 2) Long-term histologic observations of the sutural life cycle call into question the number of layers within sutures. The structure varies not only in different sutures, but also within the same suture over time. 3) Few, if any, of the many elegant experimental research studies in the field of sutural biology have increased our understanding of craniosynostosis per se. An understanding of the pathogenesis of craniosynostosis requires a genetic animal model with primary craniosynostosis and molecular techniques to understand the gene defect. This may allow insight into pathogenetic mechanisms involved in primary craniosynostosis. It may prove to be quite heterogeneous at the basic level. 4) The relationship between suture closure, cessation of growth, and functional demands across sutures poses questions about various biological relationships. Two conclusions are provocative. First, cessation of growth does not necessarily, or always lead to fusion of sutures. Second, although patent sutures aid in the growth process, some growth can take place after suture closure. 5) In an affected suture, craniosynostosis usually begins at a single point and then spreads along the suture. This has been shown by serial sectioning and calls into question results of studies in which the affected sutures are only histologically sampled. 6) Craniosynostosis is etiologically and pathogenetically heterogeneous. Known human causes are reviewed. Is craniosynostosis simply normal suture closure commencing too early?(ABSTRACT TRUNCATED AT 400 WORDS)

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.

DNA changes in mechanically deformed osteoblasts: a new hypothesis

The available evidence strongly implicates the osteoblast as the key regulator of bone remodelling activity. Since orthodontic tooth movement is a potent inducer of such activity it is relevant to study the effect of mechanical forces on this cell population. The development of a model for mechanically deforming monolayer cultures of cells is described. The effect of mechanical forces on osteoblast-like cells was examined by a number of parameters. Changes in DNA synthesis seen in short-term experiments were at variance with previous published data. The data derived from longer term experiments was in close agreement with in vivo models; intermittent forces producing an increase in DNA synthesis when compared to static or continuous forces. These changes are discussed in relation to current perspectives of second messenger activation by mechanical strain. Prostaglandins did not appear to mediate these events. There was also no evidence that synthesis of the cytokine, interleukin-1 (IL-1), or the metalloproteinase, collagenase was altered by mechanical forces.

The effect of mechanical deformation on the distribution of potassium ions across the cell membrane of sutural cells

Ionic concentrations of potassium, sodium, and chloride were determined in osteocytes of the rat calvarium. The values were determined by fluorescent microscopy of both intra- and extracellular concentrations. Following the baseline determination, the calvaria were placed in tension by retraction of a microelectrode manipulator, and the fluorescence of the cells were measured again. A statistically significant change in the derived ion distribution was found. Thus, the tensile forces affected the distribution of ions across the cell membranes, increasing intracellular sodium and decreasing intracellular potassium. This would have an effect on the resting cell membrane potential with a change of potential of 8 mV. This has implications in the interpretation of clinical findings.

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.

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.

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)

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 effects of force magnitude on a sutural model: a quantitative approach

In an effort to quantify the biologic effects of an orthodontic tensile force, the rat interpremaxillary suture was investigated as a model for the periodontal ligament and expanded in vivo with a helical spring across the maxillary incisors. Three levels of force were used: light (50 to 75 g), medium (150 to 175 g), and heavy (250 to 300 g). Thymidine labeling and histologic studies after 12 hours and 1, 2, and 4 days of force delivery are described (n = 48 rats), as are biochemical studies after 2 and 4 days including a 6-hour organ culture (n = 32). The percentage of labeled cells increased significantly in all force groups at 1 day, followed by a rapid decline at 2 days, to a value at 4 days not significantly different from the controls. Biochemical studies showed significant increases in proline incorporation and alkaline phosphatase activity after 2 days of heavy force application. Histologic examinations showed obvious tissue changes beginning by day 1 and involving increases in suture width, vascularity, size and number of cells, amount of osteoid production, and changes in suture morphology. The experimental system was convenient, inflammation-free, and appeared to be reliable as evidenced by characteristic, synchronous tissue and autoradiographic changes in all experimental sutures through 4 days.

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.

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.

The morphologic and biochemical effects of tensile force application to the interparietal suture of the Sprague-Dawley rat

The purpose of this study was to correlate the histologic and biochemical responses of the interparietal suture to a range of tensile forces. Stainless steel spring implants, calibrated to generate expansive forces from 50 to 250 g, were placed across the interparietal suture in 85 female Sprague-Dawley rats. After experimental periods from 2 hours to 14 days, the interparietal sutures were evaluated by radiography, histology, and biochemistry. An in vivo/in vitro system was used for the biochemical analysis; total protein, proline incorporated, percent collagen, and alkaline phosphatase activity were measured. The radiographs and histology showed that in vivo suture expansion was achievable with 50 to 70 g of force, but the heavier forces showed greater sutural opening, more cellular proliferation, and more bone formation. This increased biologic response by the heavier forces was substantiated by an increase in sutural protein and alkaline phosphatase activity but not in percent collagen. It was concluded that changes in the total protein content of the suture were not primarily caused by proliferation of osteogenic cells and fibroblasts but due to an influx of transudate. In contrast, the increase in incorporation of 3H-proline and alkaline phosphatase activity correlated with the observance of bone formation. This study indicated a positive correlation between the magnitude of tensile forces and osteogenic response.

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.

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.

A radioautographic study of the effect of age on the protein-synthetic and bone-deposition activity in interparietal sutures of male white mice

Groups of male white mice were killed at 4, 5, 6, 7, 8, 9 and 10 weeks of age 2 h after intraperitoneal injection with [3H]-proline. Radioautographic analysis of sections of the interparietal suture demonstrated significantly greater protein-synthetic activity in the para-osseous zones relative to the middle zone (p less than 0.01) and a plateau of lower protein-synthetic activity by 7-8 weeks of age (p less than 0.05). Groups of mice were selected at 4, 6, 8 and 10 weeks of age. Each mouse was injected intraperitoneally with [3H]-proline three times at one-week intervals. Sutural growth rate was determined from incremental lines revealed by radioautographs prepared from serial paraffin sections of the interparietal suture and demonstrated a stabilization of growth by 8 weeks of age. This, together with the grain counting data, suggested that a mouse of 7-8 weeks would provide a suitable model for experimental studies in sutural remodelling response without masking effects by normal growth.

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.

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

The application of a continuous tensile mechanical stress (30 g) to explants of coronal sutures from newborn rabbits (1-2 days) produced increases in enzyme activity of 33.7% for collagenase, 95.2% for gelatinase, and 35.9% for NMP III over a 4-day culture period. All three activities were in latent form and required activation with either 4-APMA or trypsin. The increases in enzyme activities were not accompanied by an alteration in the degradation of structural proteins. This was due to the ability of the cells to synthesize an inhibitor (mol wt 29,000 daltons) which complexed the increased quantities of enzyme. This necessitated a substantial stimulation of inhibitor production because there was still a residue of free inhibitory activity in the media of stressed cultures after 4 days. We previously showed using the same model system that coronal sutures respond to tensile mechanical stress by a two-fold increase in collagen synthesis. The present data suggest that when the priority of the cell population is the synthesis of structural proteins, the inhibitor, in addition to preventing the hydrolysis of newly synthesized peptides, also maintains matrix degradation at normal turnover levels.