Application of mechanoregulatory models to simulate peri-implant tissue formation in an in vivo bone chamber

Several mechanoregulatory tissue differentiation models have been proposed over the last decade. Corroboration of these models by comparison with experimental data is necessary to determine their predictive power. So far, models have been applied with various success rates to different experimental set-ups investigating mainly secondary fracture healing. In this study, the mechanoregulatory models are applied to simulate the implant osseointegration process in a repeated sampling in vivo bone chamber, placed in a rabbit tibia. This bone chamber provides a mechanically isolated environment to study tissue differentiation around titanium implants loaded in a controlled manner. For the purpose of this study, bone formation around loaded cylindrical and screw-shaped implants was investigated. Histologically, no differences were found between the two implant geometries for the global amount of bone formation in the entire chamber. However, a significantly larger amount of bone-to-implant contact was observed for the screw-shaped implant compared to the cylindrical implant. In the simulations, a larger amount of bone was also predicted to be in contact with the screw-shaped implant. However, other experimental observations could not be predicted. The simulation results showed a distribution of cartilage, fibrous tissue and (im)mature bone, depending on the mechanoregulatory model that was applied. In reality, no cartilage was observed. Adaptations to the differentiation models did not lead to a better correlation between experimentally observed and numerically predicted tissue distribution patterns. The hypothesis that the existing mechanoregulatory models were able to predict the patterns of tissue formation in the in vivo bone chamber could not be fully sustained.     

Numerical simulation of tissue differentiation around loaded titanium implants in a bone chamber

The application of a bone chamber provides a controlled environment for the study of tissue differentiation and bone adaptation. The influence of different mechanical and biological factors on the processes can be measured experimentally. The goal of the present work is to numerically model the process of peri-implant tissue differentiation inside a bone chamber, placed in a rabbit tibia. 2D and 3D models were created of the tissue inside the chamber. A number of loading conditions, corresponding to those applied in the rabbit experiments, were simulated. Fluid velocity and maximal distortional strain were considered as the stimuli that guide the differentiation process of mesenchymal cells into fibroblasts, chondrocytes and osteoblasts. Mesenchymal cells migrate through the chamber from the perforations in the chamber wall. This process is modelled by the diffusion equation. The predicted tissue phenotypes as well as the process of tissue ingrowth into the chamber show a qualitative agreement with the results of the rabbit experiments. Due to the limited number of animal experiments (four) and the observed inter-animal differences, no quantitative comparison could be made. These results however are a strong indication of the feasibility of the implemented theory to predict the mechano-regulation of the differentiation process inside the bone chamber.     

Bone tissue formation from human embryonic stem cells in vivo

Although the use of embryonic stem cells in the assisted repair of musculoskeletal tissues holds promise, a direct comparison of this cell source with adult marrow-derived stem cells has not been undertaken. Here we have compared the osteogenic differentiation potential of human embryonic stem cells (hESC) with human adult-derived stem cells in vivo. hESC lines H7, H9, the HEF-1 mesenchymal-like, telomerized H1 derivative, the human embryonic kidney epithelial cell line HEK293 (negative control), and adult human mesenchymal stem cells (hMSC) were either used untreated or treated with osteogenic factors for 4 days prior to injection into diffusion chambers and implantation into nude mice. After 11 weeks in vivo chambers were removed, frozen, and analyzed for evidence of bone, cartilage, and adipose tissue formation. All hESCs, when pretreated with osteogenic (OS) factors gave rise exclusively to bone in the chambers. In contrast, untreated hESCs (H9) formed both bone and cartilage in vivo. Untreated hMSCs did not give rise to bone, cartilage, or adipose tissue in vivo, while pretreatment with OS factors engendered both bone and adipose tissue. These data demonstrate that hESCs exposed to OS factors in vitro undergo directed differentiation toward the osteogenic lineage in vivo in a similar fashion to that produced by hMSCs. These findings support the potential future use of hESC-derived cells in regenerative medicine applications.     

In silico study of bone tissue regeneration in an idealised porous hydrogel scaffold using a mechano-regulation algorithm

Mechanical stimulation, in the form of fluid perfusion or mechanical strain, enhances osteogenic differentiation and overall bone tissue formation by mesenchymal stems cells cultured in biomaterial scaffolds for tissue engineering applications. In silico techniques can be used to predict the mechanical environment within biomaterial scaffolds, and also the relationship between bone tissue regeneration and mechanical stimulation, and thereby inform conditions for bone tissue engineering experiments. In this study, we investigated bone tissue regeneration in an idealised hydrogel scaffold using a mechano-regulation model capable of predicting tissue differentiation, and specifically compared five loading cases, based on known experimental bioreactor regimes. These models predicted that low levels of mechanical loading, i.e. compression (0.5% strain), pore pressure of 10 kPa and a combination of compression (0.5%) and pore pressure (10 kPa), could induce more osteogenic differentiation and lead to the formation of a higher bone tissue fraction. In contrast greater volumes of cartilage and fibrous tissue fractions were predicted under higher levels of mechanical loading (i.e. compression strain of 5.0% and pore pressure of 100 kPa). The findings in this study may provide important information regarding the appropriate mechanical stimulation for in vitro bone tissue engineering experiments.     

A repeated sampling bone chamber methodology for the evaluation of tissue differentiation and bone adaptation around titanium implants under controlled mechanical conditions

A repeated sampling bone chamber methodology was developed for the study of the influence of the mechanical environment on skeletal tissue differentiation and bone adaptation around titanium implants. Via perforations, bone grows into the implanted outer bone chamber, containing an inner bone chamber with a central test implant. An actuator—easily mounted on the outer bone chamber—allows a controlled mechanical stimulation of the test implant. After each experiment, the inner bone chamber—with its content—can be harvested and analysed. A new inner bone chamber with a central implant can be inserted consecutively in the outer bone chamber and a new experiment can start. Pilot studies led to a reliable surgical protocol and showed the applicability of the methodology, offering the possibility to study skeletal tissue differentiation and adaptation around implants under well-controlled mechanical conditions, and this protected from external loading. Repeated sampling of the bone chamber allows conducting several experiments within the same animal at the same site, thereby excluding subject- and site-dependent variability and reducing the amount of experimental animals.     

A mechano-regulatory bone-healing model incorporating cell-phenotype specific activity

Phenomenological computational models of tissue regeneration and bone healing have been only partially successful in predicting experimental observations. This may be a result of simplistic modeling of cellular activity. Furthermore, phenomenological models are limited when considering the effects of combined physical and biological interventions. In this study, a new model of cell and tissue differentiation, using a more mechanistic approach, is presented and applied to fracture repair. The model directly couples cellular mechanisms to mechanical stimulation during bone healing and is based on the belief that the cells act as transducers during tissue regeneration. In the model, the cells within the matrix proliferate, differentiate, migrate, and produce extracellular matrix, all at cell-phenotype specific rates, based on the mechanical stimulation they experience. The model is assembled from coupled partial differentiation equations, which are solved using a newly developed finite element formulation. The evolution of four cell types, i.e. mesenchymal stem cells, fibroblasts, chondrocytes and osteoblasts, and the production of extracellular matrices of fibrous tissue, cartilage and bone are calculated. The material properties of the tissues are iteratively updated based on actual amounts of extracellular matrix in material elements at progressive time points. A two-dimensional finite element model of a long bone osteotomy was used to evaluate the model's potential. The additional value of the presented model and the importance of including cell-phenotype specific activities when modeling tissue differentiation and bone healing, were demonstrated by comparing the predictions with phenomenological models. The model's capacity was established by showing that it can correctly predict several aspects of bone healing, including cell and tissue distributions during normal fracture healing. Furthermore, it was able to predict experimentally established alterations due to excessive mechanical stimulation, periosteal stripping and impaired effects of cartilage remodeling.     

Soft tissue movement and stress shielding do not affect bone ingrowth in the bone conduction chamber

A variety of bone chambers are used in orthopedic research to study bone and tissue ingrowth in small and large animals. If different bone chambers are placed in one species, differences in bone ingrowth are observed. For instance, bone ingrowth in the bone conduction chamber (BCC) is high, but is low or absent in the repeated sampling bone chamber (RSBC). This difference may be explained by the design and fixation of these chambers. It is known that stress shielding and micromovement can influence bone formation. The objective of the study reported here was to determine whether stress shielding or soft tissue movement affected bone ingrowth in the BCC in the goat. Two types of caps were made, with fixation similar to that of the fixation plate of the RSBC. By placing the caps over the BCCs and fixating the caps directly to the tibial bone, the effect of stress shielding was studied. One cap was in direct contact with the bone chamber underneath, the other cap did not touch the chamber. This difference was used to observe whether movement of the soft tissue on top of the chamber and cap would affect bone ingrowth. Each limb received one control chamber without a cap and a chamber with a cap, either with or without contacting the BCC, yielding four implants per goat. After 12 weeks, bone and total tissue ingrowths were measured. Bone ingrowth was seen in 38 of 40 chambers. Total tissue and bone ingrowths were comparable between control chambers and BCCs with a cap, irrespective of type. Neither stress shielding, nor lack of movement of soft tissue affected bone ingrowth. Other factors in the design of the chambers were responsible for the difference in bone ingrowth between the BCC and the RSBC.     

The fate of mechanically induced cartilage in an unloaded environment.

According to mechanobiologic theories, persistent intermittent mechanical stimulation is required to maintain differentiated cartilage. In a rat model for bone repair, we studied the fate of mechanically induced cartilage after unloading. In three groups of rats, regenerating mesenchymal tissue was submitted to different loading conditions in bone chambers. Two groups were immediately killed after loading periods of 3 or 6 weeks (the 3-group and the 6-group). The third group was loaded for 3 weeks and then kept unloaded for another 3 weeks (the (3 + 3)-group). Cartilage was found in all loaded groups. Without loading, cartilage does not appear in this model. In the 3-group there was no clear ongoing endochondral ossification, the 6-group showed ossification in 2 out of 5 cartilage containing specimens, and in the (3 + 3)-group all cartilage was undergoing ossification. These results suggest a tendency of the cartilage to be maintained also under unloaded conditions until it is reached by bone that can replace it through endochondral ossification.Additional measurements showed less amount of new bone in the loaded specimens. In most of the loaded specimens in the 3-group, necrotic bone fragments were seen embedded in the fibrous tissue layer close to the loading piston, indicating that bone tissue had been resorbed due to the hydrostatic compressive load. In some specimens, a continuous cartilage layer covered the end of the specimen and seemed to protect the underlying bone from pressure-induced resorption. We suggest that one of the functions of the cartilage forming in the compressive loaded parts of a bone callus is to protect the surrounding bone callus from pressure-induced fluid flow leading to resorption.     

The circumfusion system for multipurpose culture chambers. I. Introduction to the mechanics, techniques, and basic results of a 12-chamber (in vitro) closed circulatory system

A self-contained mechanical system for circulating nutrient fluid through 12 tissue culture chambers is described in detail. This system utilizes nonperforated cellophane membranes in the chambers which separate the circulating nutrient from the tissue culture environments. The nutrient, therefore, is dialyzed through the cellophane of each chamber; some cell products are retained in the microenvironment between the closely apposed cellophane and cover slip, whereas the other cell products move from chamber to chamber in the circulating nutrient. The resultant environmental conditions directed by the circumfusion systems are highly favorable for maintaining the differentiation of chick embryo tissues over protracted periods; a number of micrographs are shown.     

Correlations between local strains and tissue phenotypes in an experimental model of skeletal healing

Defining how mechanical cues regulate tissue differentiation during skeletal healing can benefit treatment of orthopaedic injuries and may also provide insight into the influence of the mechanical environment on skeletal development. Different global (i.e., organ-level) mechanical loads applied to bone fractures or osteotomies are known to result in different healing outcomes. However, the local stimuli that promote formation of different skeletal tissues have yet to be established. Finite element analyses can estimate local stresses and strains but require many assumptions regarding tissue material properties and boundary conditions. This study used an experimental approach to investigate relationships between the strains experienced by tissues in a mechanically stimulated osteotomy gap and the patterns of tissue differentiation that occur during healing. Strains induced by the applied, global mechanical loads were quantified on the mid-sagittal plane of the callus using digital image correlation. Strain fields were then compared to the distribution of tissue phenotypes, as quantified by histomorphometry, using logistic regression. Significant and consistent associations were found between the strains experienced by a region of the callus and the tissue type present in that region. Specifically, the probability of encountering cartilage increased, and that of encountering woven bone decreased, with increasing octahedral shear strain and, to a lesser extent, maximum principal strain. Volumetric strain was the least consistent predictor of tissue type, although towards the end of the four-week stimulation timecourse, cartilage was associated with increasingly negative volumetric strains. These results indicate that shear strain may be an important regulator of tissue fate during skeletal healing.     

Assessment of mechanobiological models for the numerical simulation of tissue differentiation around immediately loaded implants

Nowadays, there is a growing consensus on the impact of mechanical loading on bone biology. A bone chamber provides a mechanically isolated in vivo environment in which the influence of different parameters on the tissue response around loaded implants can be investigated. This also provides data to assess the feasibility of different mechanobiological models that mathematically describe the mechanoregulation of tissue differentiation. Before comparing numerical results to animal experimental results, it is necessary to investigate the influence of the different model parameters on the outcome of the simulations. A 2D finite element model of the tissue inside the bone chamber was created. The differentiation models developed by Prendergast, et al. ["Biophysical stimuli on cells during tissue differentiation at implant interfaces", Journal of Biomechanics, 30(6), (1997), 539-548], Huiskes et al. ["A biomechanical regulatory model for periprosthetic fibrous-tissue differentiation", Journal of Material Science: Materials in Medicine, 8 (1997) 785-788] and by Claes and Heigele ["Magnitudes of local stress and strain along bony surfaces predict the course and type of fracture healing", Journal of Biomechanics, 32(3), (1999) 255-266] were implemented and integrated in the finite element code. The fluid component in the first model has an important effect on the predicted differentiation patterns. It has a direct effect on the predicted degree of maturation of bone and a substantial indirect effect on the simulated deformations and hence the predicted phenotypes of the tissue in the chamber. Finally, the presence of fluid also causes time-dependent behavior. Both models lead to qualitative and quantitative differences in predicted differentiation patterns. Because of the different nature of the tissue phenotypes used to describe the differentiation processes, it is however hard to compare both models in terms of their validity.     

Chip-Based Comparison of the Osteogenesis of Human Bone Marrow- and Adipose Tissue-Derived Mesenchymal Stem Cells under Mechanical Stimulation

Adipose tissue-derived stem cells (ASCs) are considered as an attractive stem cell source for tissue engineering and regenerative medicine. We compared human bone marrow-derived mesenchymal stem cells (hMSCs) and hASCs under dynamic hydraulic compression to evaluate and compare osteogenic abilities. A novel micro cell chip integrated with microvalves and microscale cell culture chambers separated from an air-pressure chamber was developed using microfabrication technology. The microscale chip enables the culture of two types of stem cells concurrently, where each is loaded into cell culture chambers and dynamic compressive stimulation is applied to the cells uniformly. Dynamic hydraulic compression (1 Hz, 1 psi) increased the production of osteogenic matrix components (bone sialoprotein, oateopontin, type I collagen) and integrin (CD11b and CD31) expression from both stem cell sources. Alkaline phosphatase and Alrizarin red staining were evident in the stimulated hMSCs, while the stimulated hASCs did not show significant increases in staining under the same stimulation conditions. Upon application of mechanical stimulus to the two types of stem cells, integrin (β1) and osteogenic gene markers were upregulated from both cell types. In conclusion, stimulated hMSCs and hASCs showed increased osteogenic gene expression compared to non-stimulated groups. The hMSCs were more sensitive to mechanical stimulation and more effective towards osteogenic differentiation than the hASCs under these modes of mechanical stimulation.     

Cell condensation in chondrogenic differentiation.

Reduction of intercellular spaces in the areas of prospective cartilage and bone formation (precartilage condensation) precedes chondrogenesis and may represent an important step in the process of cartilage differentiation during limb skeletogenesis. We have attempted to clarify the role of the microenvironment established during cell condensation, taking advantage of a tissue culture model system that allows condensation (i.e., increased cell density due to cell aggregation) and chondrogenic differentiation (i.e., synthesis of cartilage-specific extracellular matrix proteins, such as type II collagen and acquisition of a chondrocyte morphology) of chick embryo cartilage-derived undifferentiated cells. To prevent condensation cells were grown in carboxymethylcellulose and changes in the differentiation pathway were evaluated. In another series of experiments, we have separated single cells from the aggregated cells and analyzed their differentiation properties. Morphological analyses and the evaluation of type II collagen expression, at both the protein and the mRNA level, show that a reduced rate of cell clustering and cell to cell contact parallels a reduction of cell recruitment into the differentiation program. On the basis of our results, we suggest that the following cascade of events regulates the early stages of chondrocyte differentiation: (a) the acquisition of the ability to establish cell to cell contacts, (b) the formation of a permissive environment capable of activating the differentiation program, and (c) the expression of differentiation markers.     

Development of osteogenic tissue in diffusion chambers from early precursor cells in bone marrow of adult rats

Diffusion chambers containing bone marrow cells from adult rats were implanted intraperitoneally into rat hosts and cultured in vivo for up to 64 days. Biochemical and histological analyses of the contents of the chambers demonstrate that a connective tissue consisting of bone, cartilage and fibrous tissues is formed by precursor cells present in marrow stroma. The amounts of osteogenic tissue and DNA are directly correlated with time of implantation and with number of cells inoculated. In the chambers there is initial formation of fibrous tissue which is strongly reactive to collagen type III, laminin and fibronectin. In areas of osteogenesis which appear later within this fibrous anlage, expression of collagen type III, laminin and fibronectin decrease and collagen types I and II increase in association with bone and cartilage respectively. Where osteogenesis does not develop, fibrous tissue continues to express collagen type III. The sequential expression of the different extracellular matrix components is similar to that previously observed during osteogenic differentiation in embryonic and adult developmental systems. It is concluded that the formation of fibrous and osteogenic tissues in diffusion chambers by precursor cells present in adult marrow, resembles the normal developmental process.     

A mechano-regulation model for tissue differentiation during fracture healing: analysis of gap size and loading

Bone has a capability to repair itself when it is fractured. Repair involves the generation of intermediate tissues, such as fibrous connective tissue, cartilage and woven bone, before final bone healing can occur. The intermediate tissues serve to stabilise the mechanical environment and provide a scaffold for differentiation of new tissues. The repair process is fundamentally affected by mechanical loading and by the geometric configuration of the fracture fragments. Biomechanical analyses of fracture healing have previously computed the stress distribution within the callus and identified the components of the stress tensor favouring or inhibiting differentiation of particular tissue phenotypes. In this paper, a biphasic poroelastic finite element model of a fracture callus is used to simulate the time-course of tissue differentiation during fracture healing. The simulation begins with granulation tissue (post-inflammation phase) and finishes with bone resorption. The biomechanical regulatory model assumes that tissue differentiation is controlled by a combination of shear strain and fluid flow acting within the tissue. High shear strain and fluid flows are assumed to deform the precursor cells stimulating formation of fibrous connective tissue, lower levels stimulate formation of cartilage, and lower again allows ossification. This mechano-regulatory scheme was tested by simulating healing in fractures with different gap sizes and loading magnitudes. The appearance and disappearance of the various tissues found in a callus was similar to histological observation. The effect of gap size and loading magnitude on the rate of reduction of the interfragmentary strain was sufficiently close to confirm the hypothesis that tissue differentiation phenomena could be governed by the proposed mechano-regulation model.     

A computational model to explore the role of angiogenic impairment on endochondral ossification during fracture healing

While it is well established that an adequate blood supply is critical to successful bone regeneration, it remains poorly understood how progenitor cell fate is affected by the altered conditions present in fractures with disrupted vasculature. In this study, computational models were used to explore how angiogenic impairment impacts oxygen availability within a fracture callus and hence regulates mesenchymal stem cell (MSC) differentiation and bone regeneration. Tissue differentiation was predicted using a previously developed algorithm which assumed that MSC fate is governed by oxygen tension and substrate stiffness. This model was updated based on the hypothesis that cell death, chondrocyte hypertrophy and endochondral ossification are regulated by oxygen availability. To test this, the updated model was used to simulate the time course of normal fracture healing, where it successfully predicted the observed quantity and spatial distribution of bone and cartilage at 10 and 20 days post-fracture (dpf). It also predicted the ratio of cartilage which had become hypertrophic at 10 dpf. Following this, three models of fracture healing with increasing levels of angiogenic impairment were developed. Under mild impairment, the model predicted experimentally observed reductions in hypertrophic cartilage at 10 dpf as well as the persistence of cartilage at 20 dpf. Models of more severe impairment predicted apoptosis and the development of fibrous tissue. These results provide insight into how factors specific to an ischemic callus regulate tissue regeneration and provide support for the hypothesis that chondrocyte hypertrophy and endochondral ossification during tissue regeneration are inhibited by low oxygen.     

The spatio-temporal arrangement of different tissues during bone healing as a result of simple mechanobiological rules

During secondary bone healing, different tissue types are formed within the fracture callus depending on the local mechanical and biological environment. Our aim was to understand the temporal succession of these tissue patterns for a normal bone healing progression by means of a basic mechanobiological model. The experimental data stemmed from an extensive, previously published animal experiment on sheep with a 3?mm tibial osteotomy. Using recent experimental data, the development of the hard callus was modelled as a porous material with increasing stiffness and decreasing porosity. A basic phenomenological model was employed with a small number of simulation parameters, which allowed comprehensive parameter studies. The model distinguished between the formation of new bone via endochondral and intramembranous ossification. To evaluate the outcome of the computer simulations, the tissue images of the simulations were compared with experimentally derived tissue images for a normal healing progression in sheep. Parameter studies of the threshold values for the regulation of tissue formation were performed, and the source of the biological stimulation (comprising e.g. stem cells) was varied. It was found that the formation of the hard callus could be reproduced in silico for a wide range of threshold values. However, the bridging of the fracture gap by cartilage on the periosteal side was observed only (i) for a rather specific choice of the threshold values for tissue differentiation and (ii) when assuming a strong source of biological stimulation at the periosteum.     

The deformation behavior and viscoelastic properties of chondrocytes in articular cartilage

Chondrocytes in articular cartilage utilize mechanical signals in conjunction with other environmental factors to regulate their metabolic activity. However, the sequence of biomechanical and biochemical events involved in the process of mechanical signal transduction has not been fully deciphered. A fundamental step in determining the role of various factors in regulating chondrocyte activity is to characterize accurately the biophysical environment within the tissue under physiological conditions of mechanical loading. Microscopic imaging studies have revealed that chondrocytes as well as their nuclei undergo shape and volume changes in a coordinated manner with deformation of the tissue matrix. Through micromechanical experiments, it has been shown that the chondrocyte behaves as a viscoelastic solid material with a mechanical stiffness that is several orders of magnitude lower than that of the cartilage extracellular matrix. These properties seem to be due to the structure of the chondrocyte cytoskeleton, and in part, the viscoelastic properties of the cell nucleus. The mechanical properties of the pericellular matrix that immediately surrounds the chondrocyte significantly differ from those of the chondrocyte and the extracellular matrix, suggesting that the pericellular matrix plays an important role in defining the mechanical environment of the chondrocyte. These experimentally measured values for chondrocyte and cartilage mechanical properties have been used in combination with theoretical constitutive modeling of the chondrocyte within articular cartilage to predict the non-uniform and time-varying stress-strain and fluid flow environment of the cell. The ultimate goal of these studies has been to elucidate the sequence of biomechanical and biochemical events through which mechanical stress influences chondrocyte activity in both health and in disease.     

Assessment of Mechanobiological Models for the Numerical Simulation of Tissue Differentiation around Immediately Loaded Implants

Nowadays, there is a growing consensus on the impact of mechanical loading on bone biology. A bone chamber provides a mechanically isolated in vivo environment in which the influence of different parameters on the tissue response around loaded implants can be investigated. This also provides data to assess the feasibility of different mechanobiological models that mathematically describe the mechanoregulation of tissue differentiation. Before comparing numerical results to animal experimental results, it is necessary to investigate the influence of the different model parameters on the outcome of the simulations. A 2D finite element model of the tissue inside the bone chamber was created. The differentiation models developed by Prendergast, et al. [“Biophysical stimuli on cells during tissue differentiation at implant interfaces”, Journal of Biomechanics, 30(6), (1997), 539–548], Huiskes et al. [“A biomechanical regulatory model for periprosthetic fibrous-tissue differentiation”, Journal of Material Science: Materials in Medicine, 8 (1997) 785–788] and by Claes and Heigele [“Magnitudes of local stress and strain along bony surfaces predict the course and type of fracture healing”, Journal of Biomechanics, 32(3), (1999) 255–266] were implemented and integrated in the finite element code. The fluid component in the first model has an important effect on the predicted differentiation patterns. It has a direct effect on the predicted degree of maturation of bone and a substantial indirect effect on the simulated deformations and hence the predicted phenotypes of the tissue in the chamber. Finally, the presence of fluid also causes time-dependent behavior.

Both models lead to qualitative and quantitative differences in predicted differentiation patterns. Because of the different nature of the tissue phenotypes used to describe the differentiation processes, it is however hard to compare both models in terms of their validity.     

Directing bone marrow-derived stromal cell function with mechanics

Because bone marrow-derived stromal cells (BMSCs) are able to generate many cell types, they are envisioned as source of regenerative cells to repair numerous tissues, including bone, cartilage, and ligaments. Success of BMSC-based therapies, however, relies on a number of methodological improvements, among which better understanding and control of the BMSC differentiation pathways. Since many years, the biochemical environment is known to govern BMSC differentiation, but more recent evidences show that the biomechanical environment is also directing cell functions. Using in vitro systems that aim to reproduce selected components of the in vivo mechanical environment, it was demonstrated that mechanical loadings can affect BMSC proliferation and improve the osteogenic, chondrogenic, or myogenic phenotype of BMSCs. These effects, however, seem to be modulated by parameters other than mechanics, such as substrate nature or soluble biochemical environment. This paper reviews and discusses recent experimental data showing that despite some knowledge limitation, mechanical stimulation already constitutes an additional and efficient tool to drive BMSC differentiation.