In vitro studies of the effect of intermittent compressive forces on cartilage cell proliferation

A CLOSED LOOP PERFUSION BIOREACTOR FOR DYNAMIC HYDROSTATIC PRESSURE LOADING AND CARTILAGE TISSUE ENGINEERING

In the present study, a novel bioreactor for dynamic hydrostatic pressure loading that simultaneously permits medium perfusion was established. This bioreactor enables continuous cultivation without manual attendance. Additional emphasis was placed on a simple bioreactor design which was achieved by pressurizing the medium directly and by applying pressure loading and perfusion through the same piping. Straight forward pressure control and at the same time maintaining sterility were achieved by using a peristaltic pump including inlet and outlet magnetic pinch valves connected with a real-time control. Cell tests using chondrocytes were performed and similar cell proliferation rates in the bioreactor and in the incubator were found. We conclude that the novel bioreactor introduced here, has the potential to be easily applied for cartilage tissue engineering on a larger scale.

Evidence for JNK-dependent up-regulation of proteoglycan synthesis and for activation of JNK1 following cyclical mechanical stimulation in a human chondrocyte culture model

OBJECTIVE

To examine the expression of mitogen-activated protein kinases (MAPKs) in human chondrocytes, to investigate whether selective activation of MAPKs is involved in up-regulation of proteoglycan (PG) synthesis following cyclical mechanical stimulation (MS), and to examine whether MS is associated with integrin-dependent or independent activation of MAPKs.

METHODS

The C-28/I2 and C-20/A4 human chondrocyte cell lines were mechanically stimulated in monolayer cell culture. PG synthesis was assessed by [(35)S]-sulphate incorporation in the presence and absence of the p38 inhibitor SB203580, and the extracellular-regulated kinase (ERK1/2) inhibitor PD98059. Kinase expression and activation were assessed by Western blotting using phosphorylation status-dependent and independent antibodies, and by kinase assays. The Jun N-terminal kinase (JNK) inhibitor SP600125 and the anti-beta(1) integrin (CD29) function-blocking antibody were used to assess JNK activation and integrin dependence, respectively.

RESULTS

Increased PG synthesis following 3 h of cyclic MS was abolished by pretreatment with 10 microM SB203580, but was not affected by 50 microM PD98059. The kinases p38, ERK1/ERK2 and JNKs were expressed in both stimulated and unstimulated cells. Phosphorylated p38 was detected at various time points following 0.5, 1, 2 and 3 h MS in C-28/I2, but not detected in C-20/A4 cell lines. Phosphorylation of ERK1 and ERK2 was not significantly affected by MS. Phosphorylation of the 54 and 46 kDa JNKs increased following 0.5, 1, 2 and 3 h of MS, and following CO(2) deprivation. MS-induced JNK phosphorylation was inhibited by SB203580 at concentrations > or =5 microM and activation of JNK1 following MS was blocked by SP600125 and partially inhibited by anti-CD29.

CONCLUSIONS

The data suggest JNK, rather than p38 or ERK dependent increases in PG synthesis, and selective, partially integrin-dependent, activation of JNK kinases in human chondrocyte cell lines following cyclical MS. JNK activation is also very sensitive to changes in CO(2)/pH in this chondrocyte culture model.

Redifferentiation of chondrocytes and cartilage formation under intermittent hydrostatic pressure

Since articular cartilage is subjected to varying loads in vivo and undergoes cyclic hydrostatic pressure during periods of loading, it is hypothesized that mimicking these in vivo conditions can enhance synthesis of important matrix components during cultivation in vitro. Thus, the influence of intermittent loading during redifferentiation of chondrocytes in alginate beads, and during cartilage formation was investigated. A statistically significant increased synthesis of glycosaminoglycan and collagen type II during redifferentiation of chondrocytes embedded in alginate beads, as well as an increase in glycosaminoglycan content of tissue-engineered cartilage, was found compared to control without load. Immunohistological staining indicated qualitatively a high expression of collagen type II for both cases.

Effect of hyaluronic acid (MW 500-730 kDa) on proteoglycan and nitric oxide production in human osteoarthritic chondrocyte cultures exposed to hydrostatic pressure

OBJECTIVE

This study investigated the in vitro effects of hyaluronic acid (HA) of molecular weight (MW) 500-730 kDa on human articular chondrocytes cultivated for 48 h in the presence of interleukin-1beta (IL-1beta) with and without hydrostatic cyclical pressure.

DESIGN

The effects of 10 and 100 microg/ml HA with and without IL-1beta were assessed in the culture medium of cells exposed to pressurization cycles in the form of sinusoidal waves (minimum pressure 1MPa, maximum pressure 5MPa) at a frequency of 0.25Hz for 3h, by the immunoenzymatic method on microplates for the quantitative measurement of human proteoglycans (PG) and by the Griess method for nitrites (NO). Morphological analyses were performed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM).

RESULTS

The presence of IL-1beta determines a significant decrease in PG and a significant increase in NO concentrations measured in the culture medium. When the cells are cultured in the presence of IL-1beta and HA at the two concentrations, a statistically significant restoration of PG and a decrease in NO levels are observed. Under pressurization conditions, we observed that the PG concentration in the medium of cells presented a very significant increase in all the conditions used in the study, except for IL-1beta alone. NO production decreased very significantly in the presence of IL-1beta+HA 10 and IL-1beta+HA 100. The results of metabolic evaluation are confirmed by morphological findings obtained by TEM and SEM.

CONCLUSIONS

These in vitro studies confirm both the protective role of HA (MW 500-730 kDa), which counteracts the IL-1beta-induced effects, and the importance of pressure on chondrocyte metabolism and morphology.

A voltage-dependent K+ current contributes to membrane potential of acutely isolated canine articular chondrocytes

The electrophysiological properties of acutely isolated canine articular chondrocytes have been characterized using patch-clamp methods. The 'steady-state' current-voltage relationship (I-V) of single chondrocytes over the range of potentials from -100 to +40 mV was highly non-linear, showing strong outward rectification positive to the zero-current potential. Currents activated at membrane potentials negative to -50 mV were time independent, and the I-V from -100 to -60 mV was linear, corresponding to an apparent input resistance of 9.3 +/- 1.4 G Omega (n= 23). The outwardly rectifying current was sensitive to the K(+) channel blocking ion tetraethylammonium (TEA), which had a 50% blocking concentration of 0.66 mM (at +50 mV). The 'TEA-sensitive' component of the outwardly rectifying current had time- and membrane potential-dependent properties, activated near -45 mV and was half-activated at -25 mV. The reversal potential of the 'TEA-sensitive' current with external K(+) concentration of 5 mm and internal concentration of 145 mM, was -84 mV, indicating that the current was primarily carried by K(+) ions. The resting membrane potential of isolated chondrocytes (-38.1 +/- 1.4 mV; n= 19) was depolarized by 14.8 +/- 0.9 mV by 25 mM TEA, which completely blocked the K(+) current of these cells. These data suggest that this voltage-sensitive K(+) channel has an important role in regulating the membrane potential of canine articular chondrocytes.

Periosteal cells in bone tissue engineering

In 1742, H.L. Duhamel published a report in which the osteogenic function of periosteum was described. In 1932 H.B. Fell was the first to successfully culture periosteum; Fell concluded that this tissue might have the capability to form mineralized tissue in vitro. In the 1990s the research group of A.L. Caplan pioneered work exploring the osteogenic potential of periosteal cells in the field of bone engineering. On the basis of these studies a number of research groups have developed hard tissue generation concepts that aim to repeat the clinical success of bone autografts by culturing cells from periosteum and seeding a sufficient quantity of those cells into scaffolds made of biomaterials of natural and synthetic origin. The highly porous matrices support the induction of bone regeneration by creating and maintaining a space that facilitates progenitor cell migration, proliferation, and differentiation as well as graft revascularization. In this way, a host tissue-scaffold cell interphase might be created that allows reproduction of the intrinsic properties of autogenous bone, including the ability to be incorporated into the surrounding host bone and to continue normal bone-remodeling processes. This review discusses the history and state of the art of bone tissue engineering from a periosteum and periosteal cell source point of view and attempts to indicate future research directions.

Hydrostatic pressure induces apoptosis in human chondrocytes from osteoarthritic cartilage through up-regulation of tumor necrosis factor-alpha, inducible nitric oxide synthase, p53, c-myc, and bax-alpha, and suppression of bcl-2

Hydrostatic pressure (HP) is thought to increase within cartilage extracellular matrix as a consequence of fluid flow inhibition. The biosynthetic response of human articular chondrocytes to HP in vitro varies with the load magnitude, load frequency, as well as duration of loading. We found that continuous cyclic HP (5 MegaPascals (MPa) for 4 h; 1 Hz frequency) induced apoptosis in human chondrocytes derived from osteoarthritic cartilage in vitro as evidenced by reduced chondrocyte viability which was independent of initial cell densities ranging from 8.1 x 10(4) to 1.3 x 10(6) cells ml(-1). HP resulted in internucleosomal DNA fragmentation, activation of caspase-3, and cleavage of poly-ADP-ribose polymerase (PARP). At the molecular level, induction of apoptosis by HP was characterized by up-regulation of p53, c-myc, and bax-alpha after 4 h with concomitant down-regulation of bcl-2 after 2 h at 5 MPa as measured by RT-PCR. In contrast, beta-actin expression was unchanged. Real-time quantitative RT-PCR confirmed a HP-induced (5 MPa) 1.3-2.6 log-fold decrease in bcl-2 mRNA copy number after 2 and 4 h, respectively, and a significant increase (1.9-2.5 log-fold) in tumor necrosis factor-alpha (TNF-alpha) and inducible nitric oxide synthase (iNOS) mRNA copy number after 2 and 4 h, respectively. The up-regulation of p53 and c-myc, and the down-regulation of bcl-2 caused by HP were confirmed at the protein level by Western blotting. These results indicated that HP is a strong inducer of apoptosis in osteoarthritic human chondrocytes in vitro.

Activation of Integrin-RACK1/PKCalpha signalling in human articular chondrocyte mechanotransduction

OBJECTIVE

The objective of this study was to examine PKC isozyme expression in human articular chondrocytes and assess roles for RACK1, a receptor for activated C kinase in the mechanotransduction process.

METHODS

Primary cultures of human articular chondrocytes and a human chondrocyte cell line were studied for expression of PKC isozymes and RACK1 by western blotting. Following mechanical stimulation of chondrocytes in vitro in the absence or presence of anti-integrin antibodies and RGD containing oligopeptides, subcellular localization of PKCalpha and association of RACK1 with PKCalpha and beta1 integrin was assessed.

RESULTS

Human articular chondrocytes express PKC isozymes alpha, gamma, delta, iota, and lambda. Following mechanical stimulation at 0.33Hz chondrocytes show a rapid, beta1 integrin dependent, translocation of PKCalpha to the cell membrane and increased association of RACK1 with PKCalpha and beta1 integrin.

CONCLUSIONS

RACK1 mediated translocation of activated PKCalpha to the cell membrane and modulation of integrin-associated signaling are likely to be important in regulation of downstream signaling cascades controlling chondrocyte responses to mechanical stimuli.

Integrin-interleukin-4 mechanotransduction pathways in human chondrocytes

Mechanical stimuli are known to have major influences on chondrocyte function. The molecular events that regulate chondrocyte responses to mechanical stimulation are beginning to be understood. In vitro analyses have allowed identification of mechanotransduction pathways that control molecular and biochemical responses of human articular chondrocytes to cyclical mechanical stimulation. These studies have shown that human articular chondrocytes use alpha5beta1 integrin as a mechanoreceptor. After stimulation of this integrin by mechanical stimulation, there is activation of a signal cascade, involving stretch-activated ion channels, the actin cytoskeleton and tyrosine phosphorylation of the focal adhesion complex molecules pp125 focal adhesion kinase and paxillin, and beta-catenin. Subsequently, there is secretion of interleukin-4, which acts in an autocrine manner via Type II receptors, to induce membrane hyperpolarization, increase levels of aggrecan messenger ribonucleic acid, and decrease levels of matrix metalloproteinase 3 messenger ribonucleic acid. Chondrocytes from osteoarthritic cartilage also use alpha5beta1 integrin as a mechanoreceptor, but downstream signaling cascades and cell responses including changes in aggrecan messenger ribonucleic acid are different. Abnormalities of chondroprotective mechanotransduction pathways in osteoarthritis may contribute to disease progression.

Combination of reduced oxygen tension and intermittent hydrostatic pressure: a useful tool in articular cartilage tissue engineering

Cartilage cells are normally studied under atmospheric pressure conditions and without loading. However, since cartilage exists in a condition of reduced oxygen and intermittent hydrostatic pressure we hypothesized lower partial oxygen pressures (PO2) and different intermittent hydrostatic pressures (IHP) would increase articular chondrocyte proliferation and matrix production and to stabilize chondrocyte phenotype in vitro. Monolayers of adult bovine articular chondrocytes were cultured under 5% or 21% PO2 in combination with IHP (0.2 MPa amplitude, frequencies 5/5s = 0.1 Hz, 30/2 or 2/30 min on/off loading). We measured proliferation (3H-thymidine incorporation) and collagen secretion (protein-binding assay, collagen type II-ELISA and immunocytochemical staining of pericellular collagen types I, II and IX). Reduced PO2 stimulated proliferation and collagen type II and IX secretion of chondrocytes in comparison to 21% PO2. Additionally, collagen type I expression was delayed by low PO2, indicating a stabilization of the cell phenotype. IHP 5/5s and 30/2 min inhibited proliferation but increased collagen secretion (pericellular collagen type IX was decreased). IHP 30/2 min delayed first expression of collagen type I. In contrast, IHP 2/30 min increased proliferation, but lowered collagen expression. All stimulating or inhibiting effects of PO2 and IHP were additive and vice versa. Reduced PO2 and different settings of IHP increased proliferation, collagen secretion, and phenotype stability of chondrocytes. The oxygen- and IHP-induced effects were additive, suggesting that a combination of these parameters might be a useful tool in cartilage tissue engineering.

The chondrogenic potential of periosteum decreases with age

Periosteum contains undifferentiated mesenchymal stem cells that possess the potential for chondrogenesis during cartilage repair and in fracture healing. With aging, the chondrogenic potential of periosteum declines significantly. An organ-culture model was used to investigate the relationship between the chondrogenic potential of periosteum and aging. A total of 736 periosteal explants from the proximal medial tibiae of 82 rabbits, aged 2 weeks to 2 years, were cultured in agarose suspension conditions conductive for chondrogenesis. and analyzed using histomorphometry, collagen typing, wet weight measurement, 3H-thymidine and 35S-sulfate uptake, autoradiography, and PCNA immunostaining. The rabbits were skeletally mature by 6 months and stopped increasing in weight by 12 months. Chondrogenesis declined significantly with age (P < 0.0001) and was maximal in the 1.5-2 month-old rabbits. Explants from the 6 month-old rabbits formed 50% less cartilage. and by 12 months chondrogenesis reached a steady state minimal level. In parallel with this decrease in chondrogenic potential similar decreases were measured in 3H-thymidine uptake (P < 0.0001). 35S-sulfate uptake (P = 0.0117), as well as the thickness (P < 0.0001) and the total number of cells in the cambium layer of the periosteum (P < 0.0001). Autoradiography with 3H-thymidine and PCNA immunostaining confirmed the measured decrease in proliferative activity in the cambium layer where the chondrocyte precursors reside, although the percentage of proliferating cells did not change significantly with age. The most dramatic change was the marked decrease (87%) in the thickness and total cell number in the cambium layer of the perisoteum between the 2 and 12 month-old rabbits (P < 0.05). These data confirm a decline in the chondrogenic potential of periosteum with aging. Thus, one possibility for improving cartilage formation by periosteal transplantation after skeletal maturity would be to stimulate an increase in the total number of cells in the chondrocyte precursor pool early during chondrogenesis.

Human bone cell hyperpolarization response to cyclical mechanical strain is mediated by an interleukin-1beta autocrine/paracrine loop

Mechanical stimuli imparted by stretch, pressure, tension, fluid flow, and shear stress result in a variety of biochemical responses important in bone (re)modeling. The molecules involved in the recognition and transduction of mechanical stimuli that lead to modulation of bone cell function are not yet fully characterized. Cyclical pressure-induced strain (PIS) induces a rapid change in membrane potential of human bone cells (HBC) because of opening of membrane ion channels. This response is mediated via integrins and requires tyrosine kinase activity and an intact actin cytoskeleton. We have used this electrophysiological response to further study the signaling events occurring early after mechanical stimulation of HBC. Stimulation of HBC at 0.33 Hz PIS, but not 0.104 Hz PIS, results in the production of a transferable factor that induces membrane hyperpolarization of unstimulated HBC. The production of this factor is inhibited by antibodies to beta1-integrin. Interleukin-1beta (IL-1beta and prostaglandin E2 (PGE2) were identified as candidate molecules for the transferable factor as both were shown to induce HBC hyperpolarization by opening of small conductance calcium-activated potassium channels, the means by which 0.33 Hz PIS causes HBC hyperpolarization. Antibodies to IL-1beta, but not other cytokines studied, inhibit the hyperpolarization response of HBC to 0.33 Hz PIS. Comparison of the signaling pathways required for 0.33 Hz PIS and IL-1beta-induced membrane hyperpolarization shows that both involve the phospholipase C/inositol triphosphate pathway, protein kinase C (PKC), and prostaglandin synthesis. Unlike 0.33 Hz PIS-induced membrane hyperpolarization, IL-1beta-induced hyperpolarization does not require tyrosine kinase activity or an intact actin cytoskeleton. These studies suggest that 0.33 Hz PIS of HBC induces a rapid, integrin-mediated, release of IL-1beta with a subsequent autocrine/paracrine loop resulting in membrane hyperpolarization. IL-1beta production in response to mechanical stimuli is potentially of importance in regulation of bone (re)modeling.

Integrin and mechanosensitive ion channel-dependent tyrosine phosphorylation of focal adhesion proteins and beta-catenin in human articular chondrocytes after mechanical stimulation

Mechanical forces influence chondrocyte metabolism and function. We have previously shown that 0.33 Hz cyclical pressure-induced strain (PIS) results in membrane hyperpolarization of normal human articular chondrocytes (HAC) by activation of Ca(2+)-dependent K+ small conductance potassium activated calcium (SK) channels. The mechanotransduction pathway involves alpha 5 beta 1-integrin, stretch-activated ion channels (SAC) actin cytoskeleton and tyrosine protein kinases, with subsequent release of the chondroprotective cytokine interleukin-4 (IL-4). The objective of this study was to examine in detail tyrosine phosphorylation events in the mechanotransduction pathway. The results show tyrosine phosphorylation of three major proteins, p125, p90, and p70 within 1 minute of onset of mechanical stimulation. Immunoblotting and immunoprecipitation show these to be focal adhesion kinase (pp125FAK), beta-catenin, and paxillin, respectively. Tyrosine phosphorylation of all three proteins is inhibited by RGD containing oligopeptides and gadolinium, which is known to block SAC. beta-catenin coimmunoprecipitates with FAK and is colocalized with alpha 5-integrin and pp125FAK. These results indicate a previously unrecognized role for an integrin-beta-catenin signaling pathway in human articular chondrocyte (HAC) responses to mechanical stimulation.

Effects of chondroitin sulfate and interleukin-1beta on human chondrocyte cultures exposed to pressurization: a biochemical and morphological study

Objective This study investigated the in vitro effects of chondroitin sulfate (CS) on human articular chondrocytes cultivated in the presence or in the absence of interleukin-1beta (IL-1beta) during 10 days of culture with and without pressurization cycles. Design The effects of CS (10 and 100 microg/ml) with and without IL-1beta were assessed in the culture medium of cells exposed to pressurization cycles in the form of synusoidal waves (minimum pressure 1 Mpa, maximum pressure 5 Mpa) and a frequency of 0.25 Hz for 3 h by immunoenzymatic method on microplates for the quantitative measurement of human proteoglycans (PG). On the 4th and 10th day of culture the cells were used for morphological analysis by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Results The presence of IL-1beta determines a significant decrease in PG concentration measured in the culture medium. When the cells are cultured in the presence of IL-1beta and CS, a statistically significant restoration of PG levels is observed. Under pressurization conditions, we observed that PG concentration in the medium of cells presents a significant increase at baseline conditions, in the presence of IL-1beta+CS10 and IL-1beta+CS100, but not with IL-1beta alone. The results concerning metabolic evaluation are confirmed by the morphologic findings obtained by TEM and SEM. Conclusions These in vitro studies confirm the protective role of CS, which counteracts the IL-1beta induced effects and they confirm the importance of pressure on chondrocyte metabolism and morphology.

Novel strategies for the treatment of osteoarthritis

Osteoarthritis is a worldwide heterogeneous group of conditions that leads to joint symptoms, which are associated with defective integrity of articular cartilage, in addition to related changes in the underlying bone at the joint margins. The prevalence of the disease after the age of 65 years, is about 60% in men and 70% in women. The aetiology of osteoarthritis is multifactorial, with the end result being mechanical joint failure and varying degrees of loss of joint function. The pathophysiological events associated with osteoarthritis are beginning to be understood. Essential inflammatory cytokines, such as IL-1beta and TNF-alpha, are involved initiating a vicious cycle of catabolic and degradative events in cartilage, mediated by metalloproteinases, which degrade cartilage extracellular matrix. The role of inflammation in the pathophysiology and progression of early osteoarthritis is supported further by the observation that C-reactive protein levels are raised in women with early knee osteoarthritis and higher levels predict those whose disease will progress. The synovium from osteoarthritis joints stains for IL-1beta and TNF-alpha. Nitric oxide, which exerts pro-inflammatory effects, is released during inflammation. Cartilage from patients with rheumatoid arthritis and osteoarthritis spontaneously produces nitric oxide in vitro. In experimental osteoarthritis, nitric oxide induces chondrocyte apoptosis, thus contributing to cartilage degradation. Hence unregulated nitric oxide production in humans plays a part in the pathophysiology of the disease. These recent observations suggest that therapy can now be targeted at specific sites of pathophysiological pathways involved in the pathogenesis of osteoarthritis. The novel strategies under consideration for the treatment of osteoarthritis can be divided into five main areas. These are COX-2 inhibitors, nitric oxide synthesis inhibitors and anti-oxidants, chondrocyte and bone growth promoters, metalloproteinase and cytokine inhibitors and gene therapy.

Responses of different cell lines from ocular tissues to elevated hydrostatic pressure

BACKGROUND/AIMS

Mechanical forces are thought to induce cellular responses through activation of signalling pathways. Cells within the intraocular environment are exposed to constant changes in the levels of intraocular pressure. In this study, an attempt was made to determine the acute effects of elevated hydrostatic pressure on different intraocular cells grown in culture.

METHODS

Different cell lines derived from ocular tissues including non-pigmented and pigmented ciliary epithelium, trabecular meshwork, retina, and lamina cribrosa were incubated in a pressurised chamber at 50 mm Hg in a culture incubator at 37 degrees C for up to 6 hours. Control cells were incubated at atmospheric pressure. The viability of the cells was examined using their intracellular esterase activity. The morphology and cytoskeleton of the cells were investigated using microscopy and phalloidin staining. Adenylyl cyclase activity was assessed by measuring the conversion of [(3)H]-cAMP from [(3)H]-ATP in response to elevated hydrostatic pressure for 1-6 hours. In addition, at the end of incubation period under elevated hydrostatic pressure the recovery of adenylyl cyclase activity to control levels was examined.

RESULTS

Cell viability did not change following exposure to elevated hydrostatic pressure for 6 hours. Cells subjected to elevated hydrostatic pressure demonstrated morphological differences characterised by a more rounded shape and a redistribution of actin stress fibres that was most prominent in lamina cribrosa astrocytes. A time dependent increase in basal adenylyl cyclase activity, and a decrease in maximum forskolin stimulated activity were observed in all cell lines following exposure to elevated hydrostatic pressure.

CONCLUSION

These observations demonstrate that cell lines from different ocular tissues are sensitive to changes in external pressure in vitro. They exhibit morphological and cytoskeletal changes as well as significant alterations of intracellular adenylyl cyclase activity following exposure to acute and sustained levels of elevated hydrostatic pressure of up to 6 hours' duration.

Periosteum responds to dynamic fluid pressure by proliferating in vitro

Periosteum provides a source of undifferentiated chondrocyte precursor cells for fracture healing that can also be used for cartilage repair. The quantity of cartilage that can be produced, which is a determining factor in fracture healing and cartilage repair, is related to the number of available stem cells in the cambium layer. Cartilage formation during both of these processes is enhanced by motion of the fracture or joint in which periosteum has been transplanted. The effect of dynamic fluid pressure on cell proliferation in periosteal tissue cultures was determined in 452 explants from 60 immature (2-month-old) New Zealand White rabbits. The explants were cultured in agarose suspension for 1-14 days. One group was subjected to cyclic hydrostatic pressure, which is referred to as dynamic fluid pressure, at 13 kPa and a frequency of 0.3 Hz. Control explants were cultured in similar chambers without application of pressure. DNA synthesis ([3H]thymidine uptake) and total DNA were measured. The temporal pattern and distribution of cell proliferation in periosteum were evaluated with autoradiography and immunostaining with proliferating cell nuclear antigen. Dynamic fluid pressure increased proliferation of periosteal cells significantly, as indicated by a significant increase in [3H]thymidine uptake at all time points and a higher amount of total DNA compared with control values. On day 3, when DNA synthesis reached a peak in periosteal explants, [3H]thymidine uptake was 97,000+/-5,700 dpm/microg DNA in the group exposed to dynamic fluid pressure and 46,000+/-6,000 dpm/microg in the controls (p < 0.001). Aphidicolin, which blocks DNA polymerase alpha, inhibited [3H]thymidine uptake in a dose-dependent manner in the group subjected to dynamic fluid pressure as well as in the positive control (treated with 10 ng/ml of transforming growth factor-beta1) and negative control (no added growth factors) groups, confirming that [3H]thymidine measurements represent proliferation and dynamic fluid pressure stimulates DNA synthesis. Total DNA was also significantly higher in the group exposed to dynamic fluid pressure (5,700+/-720 ng/mg wet weight) than in the controls (3,700+/-630) on day 3 (p < 0.01). Autoradiographs with [3H]thymidine revealed that one or two cell cycles of proliferation took place in the fibrous layer prior to proliferation in the cambium layer (where chondrocyte precursors reside). Proliferating cell nuclear antigen immunophotomicrographs confirmed the increased proliferative activity due to dynamic fluid pressure. These findings suggest either a paracrine signaling mechanism between the cells in these two layers of the periosteum or recruitment/migration of proliferating cells from the fibrous to the cambium layer. On the basis of the data presented in this study, we postulate that cells in the fibrous layer respond initially to mechanical stimulation by releasing growth factors that induce undifferentiated cells in the cambium layer to divide and differentiate into chondrocytes. These data indicate that cell proliferation in the early stages of chondrogenesis is stimulated by mechanical factors. These findings are important because they provide a possible explanation for the increase in cartilage repair tissue seen in joints subjected to continuous passive motion. The model of in vitro periosteal chondrogenesis under dynamic fluid pressure is valuable for studying the mechanisms by which mechanical factors might be involved in the formation of cartilage in the early fracture callus and during cartilage repair.

Integrin-regulated secretion of interleukin 4: A novel pathway of mechanotransduction in human articular chondrocytes

Chondrocyte function is regulated partly by mechanical stimulation. Optimal mechanical stimulation maintains articular cartilage integrity, whereas abnormal mechanical stimulation results in development and progression of osteoarthritis (OA). The responses of signal transduction pathways in human articular chondrocytes (HAC) to mechanical stimuli remain unclear. Previous work has shown the involvement of integrins and integrin-associated signaling pathways in activation of plasma membrane apamin-sensitive Ca2+-activated K+ channels that results in membrane hyperpolarization of HAC after 0. 33 Hz cyclical mechanical stimulation. To further investigate mechanotransduction pathways in HAC and show that the hyperpolarization response to mechanical stimulation is a result of an integrin-dependent release of a transferable secreted factor, we used this response. Neutralizing antibodies to interleukin 4 (IL-4) and IL-4 receptor alpha inhibit mechanically induced membrane hyperpolarization and anti-IL-4 antibodies neutralize the hyperpolarizing activity of medium from mechanically stimulated cells. Antibodies to interleukin 1beta (IL-1beta) and cytokine receptors, interleukin 1 receptor type I and the common gamma chain/CD132 (gamma) have no effect on me- chanically induced membrane hyperpolarization. Chondrocytes from IL-4 knockout mice fail to show a membrane hyperpolarization response to cyclical mechanical stimulation. Mechanically induced release of the chondroprotective cytokine IL-4 from HAC with subsequent autocrine/paracrine activity is likely to be an important regulatory pathway in the maintenance of articular cartilage structure and function. Finally, dysfunction of this pathway may be implicated in OA.

The transduction of very small hydrostatic pressures

This paper reviews experiments in which cells, subjected to hydrostatic pressures of 20 kPa or less, (micro-pressures), demonstrate a perturbation in growth and or metabolism. Similarly, the behavioural responses of aquatic animals (lacking an obvious compressible gas phase) to comparable pressures are reviewed. It may be shown that in both cases the effect of such very low hydrostatic pressures cannot be mediated through the thermodynamic mechanisms which are invoked for the effects of high hydrostatic pressure. The general conclusion is that cells probably respond to micro-pressures through a mechanical process. Differential compression of cellular structures is likely to cause shear and strain, leading to changes in enzyme and/or ion channel activity. If this conclusion is true then it raises a novel question about the involvement of 'micro-mechanical' effects in cells subjected to high hydrostatic pressure. The responses of aquatic animals to micro-pressures may be accounted for, using the model case of the crab, by the mechanical, bulk, compression of hair cells in the statocysts, the organ of balance. If this is true, it raises the interesting question of why the putative cellular mechanisms of micro-pressure transduction appear to have been superseded by the statocyst.

Role of epidermal growth factor and its receptor in mechanical stress-induced differentiation of human periodontal ligament cells in vitro

The periodontal ligament (PDL) contains precursor cells for osteoblasts and cementoblasts. It has been shown that epidermal growth factor (EGF) inhibits dexamethasone-induced differentiation and up-regulates EGF-receptor (EGF-R) expression, whereas EGF-R is down-regulated in the course of differentiation. Thus it was suggested that EGF and its receptors act as a negative regulator of osteoblastic differentiation in PDL cells. In order to investigate further this hypothesis, human PDL cells were now used to elucidate the role of EGF and EGF-R in their proliferation and differentiation under mechanical stress-loaded conditions in vitro, as the PDL regularly receives mechanical stress from occlusal forces. As a model of mechanical stress, a cyclic stretch of 9 or 18% elongation was applied to the cells with a Flexercell cell-strain unit system. Alkaline phosphatase activity and osteocalcin mRNA expression were significantly induced by loading cyclic stretch for more than 4 days, whereas stretch slightly inhibited cell proliferation. Visualization of the actin stress fibres of the cells by rhodamine phalloidin revealed that approx. 10% of the total number of cells had become aligned perpendicularly to the direction of the stretch. The effects of stretch on alkaline phosphatase activity and cell proliferation were totally abolished by the presence of 10 ng/ml EGF. Western blotting of EGF-R protein demonstrated that stretch-induced differentiation accompanied the decreased expression of EGF-R protein in the cells. However, the amount of tyrosine-phosphorylated EGF-R upon EGF stimulation was restored to the control level in stretched cells. These results suggest that the EGF/EGF-R system acts as a negative regulator of differentiation of PDL cells regardless of the type of differentiation stimuli. Also, interaction between mechanical stress and the EGF/EGF-R system may participate in the osteoblastic differentiation of PDL cells and thereby regulate the source of cementoblasts and osteoblasts.

Pathomechanics and classification of cartilage lesions, facilitation of repair

Joint forces have a high potential to promote degenerative changes in articular cartilage. Researchers have not yet developed a material that simulates natural articular cartilage, and replacement procedures have finite lives. In all patients, regardless of diagnostic category, the impact of rehabilitative procedures on the integrity and health of articular cartilage should be a consideration. In this paper, I will review why articular cartilage breaks down, how cartilage lesions are classified in vitro and in vivo, as well as cartilage's capacity for repair and repair enhancement. The primary focus will be on processes and procedures that impact physical therapy. Review sources included common computer-based search instruments and literature in all languages. This research showed that most studies have been conducted on animals, which differ in important respects from humans. Such studies, however, provide guidelines for physical therapists. Unloading and overloading are detrimental to articular cartilage. Research indicates value in controlled, progressive regimes that alternate load and non-load conditions.

The influence of compressive loading on growth of cartilage of the mandibular condyle in vitro

The purpose of this study was to clarify the change in mandibular condyles under compressive loading. An organ-culture system of fetal rat mandibular condyles was used, and mechanical loading was generated by compressing the gas phase within a closed chamber. After the culture period, with compressive loading, type I collagen and fibronectin were observed in the lower half of the hypertrophic chondrocyte layer in the mandibular condyles; in contrast, without compressive loading, there was no such reaction. The size of the condyle was not increased by compressive loading. These results suggest that intermittent compressive loading could induce type I collagen and fibronectin production by chondrocytes.

Incompressibility of the solid matrix of articular cartilage under high hydrostatic pressures

The objective of this study was to test the hypothesis that the organic solid matrix of articular cartilage is incompressible under physiological levels of pressure. Due to its anisotropic swelling behavior, an anisotropic version of the biphasic theory was used to predict the deformation and internal stress fields. This theory predicts that, under hydrostatic loading of cartilage via a pressurized external fluid, a state of uniform hydrostatic fluid pressure exists within the tissue regardless of the anisotropic nature of the solid matrix. The theory also predicts that if the solid matrix is intrinsically incompressible, the tissue will not deform under hydrostatic loading conditions. This prediction, i.e., no deformation, was experimentally tested by subjecting specimens of normal bovine articular cartilage to hydrostatic pressures. A new high pressure hydrostatic loading chamber was designed and built for this purpose. It was found that normal bovine articular cartilage, when subject to hydrostatic pressures up to 12 M Pa, does not deform measurably. This experimental finding supports one of the fundamental assumptions of the biphasic theory for cartilage, i.e., the organic solid matrix of the tissue is intrinsically incompressible when loaded within the normal physiologic range of pressures. Hydrostatic loading has often heen used in cartilage explant cultures for tissue metabolism studies. The findings of this study provides an accurate method to calculate the states of stress acting on the fluid and solid phases of the tissue in these hydrostatically loaded explant culture experiments, and suggest that tissue deformation will be minimal under pure hydrostatic pressurization.

Hyperpolarisation of cultured human chondrocytes following cyclical pressure-induced strain: evidence of a role for alpha 5 beta 1 integrin as a chondrocyte mechanoreceptor

Mechanical stimuli influence chondrocyte metabolism, inducing changes in intracellular cyclic adenosine monophosphate and proteoglycan production. We have previously demonstrated that primary monolayer cultures of human chondrocytes have an electrophysiological response after intermittent pressure-induced strain characterised by a membrane hyperpolarisation of approximately 40%. The mechanisms responsible for these changes are not fully understood but potentially involve signalling molecules such as integrins that link extracellular matrix with cytoplasmic components. The results reported in this paper demonstrate that the transduction pathways involved in the hyperpolarisation response of human articular chondrocytes in vitro after cyclical pressure-induced strain involve alpha 5 beta 1 integrin. We have demonstrated, using pharmacological inhibitors of a variety of intracellular signalling pathways, that the actin cytoskeleton, the phospholipase C calmodulin pathway, and both tyrosine protein kinase and protein kinase C activities are important in the transduction of the electrophysiological response. These results suggest that alpha 5 beta 1 is an important chondrocyte mechanoreceptor and a potential regulator of chondrocyte function.

Structure and function of embryonic growth plate in the absence of functioning skeletal muscle

Normal growth and development of the skeleton require the presence of viable, actively contracting skeletal muscle throughout the fetal period. A chick embryo model of midgestation chemical paralysis and secondary muscle atrophy was used to test the hypothesis that functioning muscle stimulates the growth of long bones by influencing the proliferation, differentiation, and hypertrophy of chondrocytes in cartilage of the epiphysis and growth plate. Paralysis did not alter the overall developmental stage of the long bone or the organization of the growth plate. Compared with controls, however, uptake of bromodeoxyuridine in the paralyzed chick was reduced by 27-55% in the chondroepiphysis and uppermost zone of the tibial growth plate, indicating reduced proliferation of chondrocytes. A specific reduction in the size of the proliferative zone and a reduced number of proliferating cells were also observed. By contrast, in the second, post-proliferative zone of the growth plate, the height of the zone was unchanged and its area was only slightly reduced compared with controls. Finally, median hypertrophic cell profile area, a measure of cell size, was not significantly affected by paralysis, although frequency analysis revealed modest numerical reductions in the population of the largest hypertrophic chondrocytes in the paralyzed group. These data suggest that the role of functioning fetal muscle in maintaining proper skeletal growth may be mediated primarily through specific stimulation of the recruitment or proliferation of immature chondrocytes, or of both.

Meniscal tissue regeneration in porous 50/50 copoly(L-lactide/epsilon-caprolactone) implants

Porous materials of a high-molecular-weight 50/50 copolymer of L-lactide and epsilon-caprolactone with different compression moduli were used for meniscal repair. In contrast to the previously used 4,4'-diphenylmethane and 1,4-trans-cyclohexane diisocyanates containing polyurethanes, degradation products of the copolymer are non-toxic. Two series of porous materials with compression moduli of 40 and 100 kPa respectively were implanted in the knees of dogs using a new, less traumatizing suturing technique. A porous aliphatic polyurethane series with compression modulus of 150 kPa was implanted for comparison. Adhesion of the implant to meniscal tissue was found to be essential for healing of the longitudinal lesion. Copolymer implants showed better adhesion, probably due to the higher degradation rate of the copolymer. Fibrocartilage formation was found to be affected by the compression modulus of the implant. Implants with a modulus of 40 kPa did not show ingrowth of fibrocartilage, whereas implants with compression moduli of 100 and 150 kPa yielded 50-70 and 80-100% fibrocartilage respectively. During degradation the copolymer phase separated into a crystalline phase containing mainly L-lactide and an amorphous phase containing mainly epsilon-caprolactone. The copolymer degraded through bulk degradation.

Hydrostatic pressure influences mRNA expression of transforming growth factor-beta 1 and heat shock protein 70 in chondrocyte-like cell line

The present study investigated the influence of hydrostatic pressure on the expression of cytokines and heat shock protein 70 in a chondrocyte-like cell line. Chondrocyte-like cells (HCS-2/8) were exposed to hydrostatic pressure by a special pressure apparatus. Total RNA for cytokines (interleukin-1 beta, basic fibroblast growth factor, insulin-like growth factor-I, and transforming growth factor-beta 1) and for heat shock protein 70 was extracted and was analyzed by a polymerase chain reaction method and Northern blotting. An assay for incorporation of [35S]sulfate was performed to assess proteoglycan synthesis. The expression of transforming growth factor-beta 1 mRNA was enhanced after exposure to 5-MPa of hydrostatic pressure and was reduced after 50 MPa, whereas the expression of heat shock protein 70 was enhanced following exposure to 50 MPa of hydrostatic pressure. The incorporation of [35S]sulfate into the cultured cells increased following exposure to 1-5 MPa of hydrostatic pressure and decreased following 10-50 MPa of pressure. These results suggest that hydrostatic pressure at physiologic levels enhances the expression of transforming growth factor-beta 1 mRNA in addition to increasing proteoglycan synthesis in chondrocytes and that excessively high hydrostatic pressure reduces the expression of transforming growth factor-beta 1 mRNA and increases the expression of heat shock protein 70 mRNA while decreasing proteoglycan synthesis.

Gap junctions mediate intercellular calcium signalling in cultured articular chondrocytes

Gap junction-mediated intercellular communication has been implicated in a variety of cellular functions. Among these, signal transduction can be coordinated among several cells due to gap junctional permeability to intracellular second messengers. Chondrocytes from articular cartilage in primary culture respond to extracellular ATP by rhythmically increasing their cytosolic Ca2+ concentration. Digital imaging fluorescence microscopy of Fura-2 loaded cells was used to monitor Ca2+ in confluent and semi-confluent cell layers. Under these conditions, Ca2+ spikes propagate from cell to cell giving rise to intercellular Ca2+ waves. The functional expression of gap junctions was assessed, in confluent chondrocyte cultures, by the intercellular transfer of Lucifer yellow dye in scrape-loading experiments. Intercellular dye transfer was blocked by the gap junction inhibitor 18 alpha-glycyrrhetinic acid. In imaging experiments, the inhibitor caused the loss of synchrony of ATP-induced Ca2+ oscillations, and blocked the intercellular Ca2+ propagation induced by mechanical stimulation of a single cell in a monolayer. It is concluded that gap junctions mediate intercellular signal transduction in cartilage cells and may provide a mechanism for co-ordinating their metabolic activity.

[Physical exercise and the skeleton]

The skeleton provides more than only a framework for the body. Bone is a calcified conjunctive tissue sensitive to various mechanical stimuli, mainly to those resulting from gravity and muscular contractions. Numerous animal and human studies demonstrate the importance of weight-bearing physical activity as well as mechanical loading for maintaining skeletal integrity. Lack of weight-bearing activity is dangerous for the skeleton: a decrease in bone mineral density (BMD) has been demonstrated in animals and humans under conditions of weightlessness or immobilization. Other studies have also reported a lower vertebral BMD among young amenorrheic athletes than among athletes with regular cycles and/or non athletes. The main factor responsible for this lower BMD in the amenorrheic athletes is the persistent low level of endogenous estrogen observed among these women. However this does not represent a premature and irreversible loss of bone mass since the resumption of menses following a decrease in training is the primary factor for a significant increase in vertebral BMD in these formerly amenorrheic athletes. A weight-bearing exercise is likely to be more beneficial at weight-bearing than at non weight-bearing sites, and hypogonadism resulting from very intensive training and exercise is more detrimental to trabecular than cortical bone. Bone deficit at non weight-bearing sites may be attenuated by maintenance of body weight. Nevertheless the etiology of "stress fractures" among athletes remains poorly understood, and the exact relationship between soft tissue mass and BMD is not clear. Osteoporosis, the most common bone disorder in France, is a pathological condition associated with increased loss of bone mass, resulting in a greater risk of fracture. Although symptoms of osteoporosis do not generally occur until after menopause, recent evidence suggests that bone loss starts much earlier in life. Therefore osteoporosis might be prevented by increasing peak bone mass and/or by slowering bone loss after menopause. Exercise such as resistance training or weight-bearing activities like running or walking have an osteogenic effect on increasing BMD in young people, and the decrease in BMD is slower in exercised than in non-exercised post-menopausal women. Nevertheless the influence of the length and of the intensity of such physical activities remain to be determined.

Mechanotransduction and the functional response of bone to mechanical strain

Mechanotransduction plays a crucial role in the physiology of many tissues including bone. Mechanical loading can inhibit bone resorption and increase bone formation in vivo. In bone, the process of mechanotransduction can be divided into four distinct steps: (1) mechanocoupling, (2) biochemical coupling, (3) transmission of signal, and (4) effector cell response. In mechanocoupling, mechanical loads in vivo cause deformations in bone that stretch bone cells within and lining the bone matrix and create fluid movement within the canaliculae of bone. Dynamic loading, which is associated with extracellular fluid flow and the creation of streaming potentials within bone, is most effective for stimulating new bone formation in vivo. Bone cells in vitro are stimulated to produce second messengers when exposed to fluid flow or mechanical stretch. In biochemical coupling, the possible mechanisms for the coupling of cell-level mechanical signals into intracellular biochemical signals include force transduction through the integrin-cytoskeleton-nuclear matrix structure, stretch-activated cation channels within the cell membrane, G protein-dependent pathways, and linkage between the cytoskeleton and the phospholipase C or phospholipase A pathways. The tight interaction of each of these pathways would suggest that the entire cell is a mechanosensor and there are many different pathways available for the transduction of a mechanical signal. In the transmission of signal, osteoblasts, osteocytes, and bone lining cells may act as sensors of mechanical signals and may communicate the signal through cell processes connected by gap junctions. These cells also produce paracrine factors that may signal osteoprogenitors to differentiate into osteoblasts and attach to the bone surface. Insulin-like growth factors and prostaglandins are possible candidates for intermediaries in signal transduction. In the effector cell response, the effects of mechanical loading are dependent upon the magnitude, duration, and rate of the applied load. Longer duration, lower amplitude loading has the same effect on bone formation as loads with short duration and high amplitude. Loading must be cyclic to stimulate new bone formation. Aging greatly reduces the osteogenic effects of mechanical loading in vivo. Also, some hormones may interact with local mechanical signals to change the sensitivity of the sensor or effector cells to mechanical load.

Enlargement of the rabbit mandibular condyle after experimental induction of anterior disc displacement: a histomorphometric study

PURPOSE

Clinical and autopsy studies have shown that patients with temporomandibular joint dysfunction are more likely to have enlargement and deformity of the condyle and subsequently occlusal disharmony. However, it is not known what causes this enlargement. This study was designed to test the hypothesis that surgical induction of anterior disc displacement (ADD) in the rabbit craniomandibular joint (CMJ) could lead to enlargement and deformity of the condyle.

MATERIALS AND METHODS

The right CMJ was exposed surgically, and the discal attachments were severed except for the posterior discal attachment (bilaminar zone). Then, the disc was repositioned anteriorly and sutured to the zygomatic arch. The left joint served as a sham-operated control. CMJ tissues then were removed after fixation at 24 hours (5 rabbits), 1 week (10 rabbits), 2 weeks (10 rabbits), or 6 weeks (10 rabbits), processed, and stained with hematoxylineosin. Histomorphometric assessment was used to evaluate changes in condylar volume, and thickness of the fibrous, reserve cell, and condylar cartilage layers.

RESULTS

The results showed a progressive enlargement of the condylar volume in all experimental joints compared with controls (P < .01). The enlargement was attributable to a significant increase in the cartilage thickness and surface area of the nonarticulating portion of the condyle in the 1-week group (P < .01). In the 2- and 6-week groups, there were significant, progressive increases in cartilage thickness and surface area of the articulating portion of the condyle (P < .01). In all animals, increased cartilage thickness was associated with a decrease in the thickness of the fibrous and the reserve cell layers (P < .01).

CONCLUSION

It is concluded that surgical induction of ADD in the rabbit CMJ causes enlargement of the condyle, which is in part caused by hyperplasia of the condylar cartilage.

Glycosaminoglycan synthesis in the rat articular disk in response to mechanical stress

The mechanism by which compressive mechanical stress affects glycosaminoglycan synthesis in the articular disk was investigated with a modified organ culture technique. Forty-eight male Sprague-Dawley rats were divided into three experimental groups and one control group of 12 animals each, aged 7 and 9 weeks. The experimental groups followed different regimens of stress applied for 25%, 75%, or 100% of the time during the total test period of 24 hours. Articular disks were stressed with flexible bottomed dishes (Flex I dishes, Flexcell Corp., McKeesport, Pa.) using the Flexercell Strain Unit (Flexcell Corp., McKeesport, Pa.) and incubated with [3H]-glucosamine for 24 hours. Samples were then collected, digested with Pronase-E, and after precipitation with cetylpyridinium chloride (CPC) and ethanol, the different glycosaminoglycans (GAGs) were separated by using cellulose acetate electrophoresis. The significant GAG types with stress were chondroitin6sulfate (C6S), hyaluronic acid (HA), and dermatan sulfate (DS). There was no significant relationship in the experimental groups between age and regimen of stress applied in either age. Higher stress regimens showed significantly higher proportions of C6S when compared with the controls, whereas HA appeared to decrease slightly and DS was not affected. Since C6S is the major component of hyaline cartilage, the results of this study suggest that compressive forces in the articular disk may stimulate the development of more cartilagenous-like properties with respect to GAG content.

Mechanical loading triggers specific biochemical responses in mandibular condylar chondrocytes

The effect of mechanical loading on the phosphorylation state of condylar cartilage proteins was investigated by high resolution electrophoretic analysis of 32P-labelled proteins from mechanically stimulated rat mandibular condylar chondrocytes. Specific dephosphorylation (and/or loss) of an acidic, 35-36 kDa protein(s) and of proteins overlapping with members of the ras superfamily of small GTP-binding proteins was observed. These responses may constitute part of a mechanically induced transduction system which establishes the differentiated phenotype.

Chondrocyte cells respond mechanically to compressive loads

Many studies have illustrated the effect of mechanical loading on articular cartilage and the corresponding changes in chondrocyte metabolism, yet the mechanism through which the cells respond to loading still is unclear. The purpose of this study was to evaluate the change in shape of chondrocytes under a statically applied uniaxial compressive load. Isolated chondrocytes from rat chondrosarcoma were embedded in 2% agarose gel. Strains of 5, 10, and 15% were applied, and images of the cell were recorded from initial loading to equilibrium (15 minutes). A finite-element model was used to model the experimental setup and to estimate the mechanical properties of the chondrocyte at equilibrium. The transient behavior of the composite in the experiment was analyzed with use of a standard linear viscoelastic model. We found that all cells decreased in cross-sectional area under each of the applied compressive strains. In the finite-element model, the elasticity of the chondrocyte was similar to that of the surrounding agarose gel (4.0 kPa) and had a Poisson's ratio of 0.4. Viscoelastic analysis showed that the chondrocytes contributed a significant viscoelastic component to the behavior of the composite in comparison with the agarose gel alone. If a decrease in cell volume proportional to the decrease in cross-sectional area is assumed, the decrease observed was greater than would be predicted by a passive cellular response due to an equivalent osmotic pressure. This indicates that the chondrocyte may be altering its intracellular composition by cellular processes in response to mechanical loading.

The effects of induced dental malocclusion on the fibrocartilage disc of the adult rabbit temporomandibular joint

Unilateral cast occlusal splints were fitted to the mandibular posterior teeth of adult rabbits, for periods of 1-28 days. The reactivity of the mandibular disc was examined by the effect on cell proliferation across the anterior, intermediate and posterior discal bands, as measured by metaphase arrest using vincristine sulphate. The effect on the disc was to activate cell proliferation on the splinted side. Intensity of response varied according to the length of time after fitting the splint, and the site involved. The findings suggested that the adult mandibular disc may participate in compensatory change at a cellular level and thus respond to changing functional loads placed upon the adult temporomandibular joints.

Mechanical stress and osteogenesis in vitro

The use of hydrostatic pressure to apply mechanical stress to bone organ cultures is reviewed. Ossifying long bones and calvarial rudiments are sensitive to this type of stress. Intermittent hydrostatic compression of near physiologic magnitude (ICF) has anabolic effects on mineral metabolism in such rudiments, and continuous hydrostatic stress of high magnitude (CCP) has catabolic effects. The effects of ICF may be ascribed to shear stress generated at tissue interphases of different chemical and mechanical properties. Local factors, such as prostaglandins and growth factors, seem to be involved in the tissue response to mechanical stress.

Response of plasma membrane to applied hydrostatic pressure in chondrocytes and fibroblasts

Effects of applied hydrostatic pressure on transmembrane potentials were investigated in sheep articular chondrocytes and human skin fibroblasts in non-confluent monolayer cultures. Resting potentials in chondrocytes (about -12 mv) and in fibroblasts (about -15 mV) were increased and decreased respectively by over 40% after pressure was applied cyclically (0.33 Hz, 120 mm Hg, 20 minutes). Continuous pressure (120 mm Hg, 20 minutes) caused deplorization in both cell types. Low frequency pressure application (less than 0.08 Hz) caused depolarization in chondrocytes and hyperpolarization in fibroblasts. Quinidine (2 x 10(-5) M) blocked and verapamil (10(-5) M) reduced hyperpolarization responses, suggesting involvement of Ca(2+)-dependent K+ channels. A23187 (1.9 x 10(-6) M) caused hyperpolarization in chondrocytes, augmented further by subsequent pressure application (0.33 Hz). Tetrodotoxin (10(-6) M) blocked depolarization responses indicating that these were due to Na+ influx. Blockade of histamine H1 receptors by chlorpheniramine maleate (5.1 x 10(-6) M), H2 receptors by cimetidine (7.9 x 10(-6) M) and beta-adrenoreceptors by sotolol (1.3 x 10(-4) M) had no effect on hydrostatic pressure-induced hyperpolarization in chondrocytes. Cytochalasin B (2 x 10(-5) M and at 4 x 10(-6) M) abolished pressure-induced hyperpolarization in chondrocytes; in contrast, applied cyclical hydrostatic pressure to cytochalasin-treated fibroblasts caused hyper-polarization, suggesting that cytoskeletal changes were involved.

Comparison of the effects of hydrostatic compressive force on glycosaminoglycan synthesis and proliferation in rabbit chondrocytes from mandibular condylar cartilage, nasal septum, and spheno-occipital synchondrosis in vitro

We have developed a simple in vitro model whereby precise quantities of compressive force can be applied to cultured chondrocytes from craniofacial cartilage: mandibular condylar cartilage (MCC), nasal septal cartilage (NSC), and spheno-occipital synchondrosis (SOS). Using this model, we found that hydrostatic compressive force stimulated glycosaminoglycan (GAG) synthesis, a cartilage phenotype, in MCC and SOS chondrocytes and DNA synthesis in MCC, NSC, and SOS chondrocytes. These stimulations were dependent on force magnitude and duration, reaching maximal GAG synthesis at 27 hours and maximal DNA synthesis at 20 hours after application of force. The maximal increase of GAG synthesis induced by compressive force was about 60% at 100 gm/cm2 for 5 minutes in nonstimulated MCC chondrocytes and 40% at 50 gm/cm2 for 1 minute in nonstimulated SOS chondrocytes. The maximal increase in DNA synthesis, produced by a compressive force of 50 gm/cm2 for 1 minute, was 50% in NSC chondrocytes, 50% in SOS chondrocytes, and 30% in MCC chondrocytes. There was no stimulation of GAG synthesis in NSC chondrocytes. These observations suggest that extrinsic force regulates craniofacial growth by controlling the differentiation and proliferation of chondrocytes in the craniofacial skeleton and that the difference in their responses to compressive force may reflect differences in the characteristics of these cells and their physiologic function in vivo.

On the importance of cAMP and Ca++ in mandibular condylar growth and adaptation

The origin of the mandibular condylar cartilage is not periosteal, like that of the other secondary cartilages; this cartilage originates in a cellular blastema of its own. Despite the fact that the development of secondary cartilages, in general, is dependent on mechanical irritation, that of the condylar cartilage is not. The low level of function experienced postnatally seems to favor growth, but because the proliferation cells of the condylar cartilage are multipotential, they switch their differentiation pathway in the direction of osteoblasts in the absence of function, and growth of the cartilage ceases. This regulation of differentiation is mediated by maturation of the cartilage cells. If function is not present, maturation advances rapidly, and the mature cartilage induces bone formation instead of cartilage. Cyclic AMP and Ca are important mediators in this process, because they affect the advancement of maturation.

Modulation of osteogenesis in fetal bone rudiments by mechanical stress in vitro

Studies of organ cultures of developing bone subjected to intermittent mechanical stress are reviewed. Mineral metabolism in these bones is modulated by exposure to dynamic stress of physiological magnitude. Finite element stress analysis of long bone rudiments shows that hydrostatic pressure during organ culture produces significant shear stresses at mineralized/non-mineralized tissue interfaces, in addition to dilatational stress. Both matrix producing cells (chondrocytes, osteoblasts) and matrix resorbing cells (osteoclasts) are affected by mechanical stress in vitro. The organ culture model offers certain opportunities for studying effects of mechanical stress on skeletal tissue at the cell and tissue level.

The concept of osseointegration and bone matrix expression

Osseointegration has been defined as the direct structural and functional connection between ordered, living bone and the surface of a load-carrying implant. To date, this concept has been described by descriptive histological and ultrastructural criteria but not by biochemical means. This review evaluates the basic science work performed on this concept and then applies the concept to the principle of osseous healing. Specific studies are cited where alterations in the healing response are due to clinical management of implant placement and how studies of surface properties may lead to further insights on implant design and prognosis. In addition, a review of bone expression as a function of in vitro stress applications is given. This is followed by an indepth review of the collagens and noncollagenous proteins, described to date, within isolated bone matrix. It is this collagenous matrix (especially type I) that is described as being close to and oriented with a glycoprotein component next to the implant surface. In turn, the large family of noncollagenous proteins are important in mediating bone proliferation, matrix accumulation, orientation, mineralization, and turnover. This section is followed by a discussion of specific growth factors as they may relate to osseous healing around an implant.