Osteoblast Elastic Modulus Measured by Atomic Force Microscopy Is Substrate Dependent

Evaluation of stem cell-to-tenocyte differentiation by atomic force microscopy to measure cellular elastic moduli

In the present study, we evaluated whether stem cell-to-tenocyte differentiation could be evaluated via measurement of the mechanical properties of the cell. We used mechanical uniaxial cyclic stretching to induce the differentiation of human bone marrow mesenchymal stem cells into tenocytes. The cells were subjected to cyclic elongation of 10 or 15 % at a cyclic frequency of 1 Hz for 24 or 48 h, and differentiation was assessed by real-time PCR (rtPCR) determination of messenger RNA expression levels for four commonly used markers of stem cell-to-tenocyte differentiation: type I collagen, type III collagen, tenascin-C, and scleraxis. The rtPCR results showed that cells subjected to 10 % cyclic elongation for 24 or 48 h differentiated into tenocytes. Atomic force microscopy (AFM) was then used to measure the force curves around the cell nuclei, and the AFM data were used to calculate the elastic moduli of the cell surfaces. The elastic modulus values of the control (non-stretched) cells differed significantly from those of cells stretched at 10 % for 24 or 48 h (P < 0.01). Confocal fluorescence microscopic observations of actin stress fibers suggested that the change in elastic modulus was ascribable to the development of the cellular cytoskeleton during the differentiation process. Therefore, we conclude that the atomic force microscopic measurement of the elastic modulus of the cell surface can be used to evaluate stem cell-to-tenocyte differentiation.

The influence of physical and physiological cues on atomic force microscopy-based cell stiffness assessment

Atomic force microscopy provides a novel technique for differentiating the mechanical properties of various cell types. Cell elasticity is abundantly used to represent the structural strength of cells in different conditions. In this study, we are interested in whether physical or physiological cues affect cell elasticity in Atomic force microscopy (AFM)-based assessments. The physical cues include the geometry of the AFM tips, the indenting force and the operating temperature of the AFM. All of these cues show a significant influence on the cell elasticity assessment. Sharp AFM tips create a two-fold increase in the value of the effective Young's modulus (E(eff)) relative to that of the blunt tips. Higher indenting force at the same loading rate generates higher estimated cell elasticity. Increasing the operation temperature of the AFM leads to decreases in the cell stiffness because the structure of actin filaments becomes disorganized. The physiological cues include the presence of fetal bovine serum or extracellular matrix-coated surfaces, the culture passage number, and the culture density. Both fetal bovine serum and the extracellular matrix are critical for cells to maintain the integrity of actin filaments and consequently exhibit higher elasticity. Unlike primary cells, mouse kidney progenitor cells can be passaged and maintain their morphology and elasticity for a very long period without a senescence phenotype. Finally, cell elasticity increases with increasing culture density only in MDCK epithelial cells. In summary, for researchers who use AFM to assess cell elasticity, our results provide basic and significant information about the suitable selection of physical and physiological cues.

Probing cytoskeletal structures by coupling optical superresolution and AFM techniques for a correlative approach

In this article, we describe and show the application of some of the most advanced fluorescence superresolution techniques, STED AFM and STORM AFM microscopy towards imaging of cytoskeletal structures, such as microtubule filaments. Mechanical and structural properties can play a relevant role in the investigation of cytoskeletal structures of interest, such as microtubules, that provide support to the cell structure. In fact, the mechanical properties, such as the local stiffness and the elasticity, can be investigated by AFM force spectroscopy with tens of nanometers resolution. Force curves can be analyzed in order to obtain the local elasticity (and the Young's modulus calculation by fitting the force curves from every pixel of interest), and the combination with STED/STORM microscopy integrates the measurement with high specificity and yields superresolution structural information. This hybrid modality of superresolution-AFM working is a clear example of correlative multimodal microscopy.

Mechanical properties of human amniotic fluid stem cells using nanoindentation

The aim of this study was to obtain nanomechanical properties of living cells focusing on human amniotic fluid stem (hAFS) cell using nanoindentation techniques. We modified the conventional method of atomic force microscopy (AFM) in aqueous environment for cell imaging and indentation to avoid inherent difficulties. Moreover, we determined the elastic modulus of murine osteoblast (OB6) cells and hAFS cells at the nucleus and cytoskeleton using force-displacement curves and Hertz theory. Since OB6 cell line has been widely used, it was selected to validate and compare the obtained results with the previous research studies. As a result, we were able to capture high resolution images through utilization of the tapping mode without adding protein or using fixation methods. The maximum depth of indentation was kept below 15% of the cell thickness to minimize the effect of substrate hardness. Nanostructural details on the surface of cells were visualized by AFM and fluorescence microscopy. The cytoskeletal fibers presented remarkable increase in elastic modulus as compared with the nucleus. Furthermore, our results showed that the elastic modulus of hAFS cell edge (31.6 kPa) was lower than that of OB6 cell edge (42.2 kPa). In addition, the elastic modulus of nucleus was 13.9 kPa for hAFS cell and 26.9 kPa for OB6 cells. Differences in cell elastic modulus possibly resulted from the type and number of actin cytoskeleton organization in these two cell types.

A multi-structural single cell model of force-induced interactions of cytoskeletal components

Several computational models based on experimental techniques and theories have been proposed to describe cytoskeleton (CSK) mechanics. Tensegrity is a prominent model for force generation, but it cannot predict mechanics of individual CSK components, nor explain the discrepancies from the different single cell stimulating techniques studies combined with cytoskeleton-disruptors. A new numerical concept that defines a multi-structural 3D finite element (FE) model of a single-adherent cell is proposed to investigate the biophysical and biochemical differences of the mechanical role of each cytoskeleton component under loading. The model includes prestressed actin bundles and microtubule within cytoplasm and nucleus surrounded by the actin cortex. We performed numerical simulations of atomic force microscopy (AFM) experiments by subjecting the cell model to compressive loads. The numerical role of the CSK components was corroborated with AFM force measurements on U2OS-osteosarcoma cells and NIH-3T3 fibroblasts exposed to different cytoskeleton-disrupting drugs. Computational simulation showed that actin cortex and microtubules are the major components targeted in resisting compression. This is a new numerical tool that explains the specific role of the cortex and overcomes the difficulty of isolating this component from other networks in vitro. This illustrates that a combination of cytoskeletal structures with their own properties is necessary for a complete description of cellular mechanics.

The effect of scaffold-cell entrapment capacity and physico-chemical properties on cartilage regeneration

An important tenet in designing scaffolds for regenerative medicine consists in mimicking the dynamic mechanical properties of the tissues to be replaced to facilitate patient rehabilitation and restore daily activities. In addition, it is important to determine the contribution of the forming tissue to the mechanical properties of the scaffold during culture to optimize the pore network architecture. Depending on the biomaterial and scaffold fabrication technology, matching the scaffolds mechanical properties to articular cartilage can compromise the porosity, which hampers tissue formation. Here, we show that scaffolds with controlled and interconnected pore volume and matching articular cartilage dynamic mechanical properties, are indeed effective to support tissue regeneration by co-cultured primary and expanded chondrocyte (1:4). Cells were cultured on scaffolds in vitro for 4 weeks. A higher amount of cartilage specific matrix (ECM) was formed on mechanically matching (M) scaffolds after 28 days. A less protein adhesive composition supported chondrocytes rounded morphology, which contributed to cartilaginous differentiation. Interestingly, the dynamic stiffness of matching constructs remained approximately at the same value after culture, suggesting a comparable kinetics of tissue formation and scaffold degradation. Cartilage regeneration in matching scaffolds was confirmed subcutaneously in vivo. These results imply that mechanically matching scaffolds with appropriate physico-chemical properties support chondrocyte differentiation.

The effects of confluency on cell mechanical properties

Mechanical properties of cells depend on various external and internal factors, like substrate stiffness and surface modifications, cell ageing and disease state. Some other currently unknown factors may exist. In this study we used force spectroscopy by AFM, confocal microscopy and flow cytometry to investigate the difference between single non-confluent and confluent (in monolayer) Vero cells. In all cases the stiffness values were fitted by log-normal rather than normal distribution. Log-normal distribution was also found for an amount of cortical actin in cells by flow cytometry. Cells in the monolayer were characterized by a significantly lower (1.4-1.7 times) Young's modulus and amount of cortical actin than in either of the single non-confluent cells or cells migrating in the experimental wound. Young's modulus as a function of indentation speed followed a weak power law for all the studied cell states, while the value of the exponent was higher for cells growing in monolayer. These results show that intercellular contacts and cell motile state significantly influence the cell mechanical properties.

Biomechanical evaluation by AFM of cultured human cell-multilayered periosteal sheets

We previously demonstrated that thicker periosteal sheets with enhanced cell layering maintain their component cells at relatively immature stages of differentiation but express a high in vivo osteogenic potential. As it has been recently proposed that stiff scaffolds provide a mechanical cue to various cell types that promotes differentiation, we postulated that the maintenance of immature cells in our periosteal sheets is due to the mechanical stiffness of the multilayered-cell architecture. To demonstrate the biomechanical characteristics of our periosteal sheets, we have determined their stiffnesses with atomic force microscopy (AFM) and evaluated the expression of extracellular matrix (ECM) components specifically by both immunocytochemistry and a complementary DNA microarray technology. Compared to osteoblastic Saos2 cells, the cytoskeletal fibers were developed more in the periosteal cells, but the periosteal cells in monolayer culture developed before either the cells in the peripheral or central regions of the periosteal sheets developed. However, the nanoindentation by AFM distinguished the central region from the peripheral region. The peak stiffness values of cells were ordered as follows: tissue culture polystyrene (1.66GPa)≫dispersed (9.99kPa)>central region (5.20kPa)>peripheral regions (3.67kPa). Similarly, the degree of development of α-smooth muscle actin (αSMA) filaments within cells was dispersed>central region>peripheral region. In conjunction with the abundantly deposited ECM in the periosteal sheets, these findings suggest that the order of cell stiffness may depend on the integration of the stiffness of individual ECM components and the extent of cytoskeletal fiber formation. Because recently published data have demonstrated that the optimal stiffness for osteogenic differentiation is 25-40kPa, it is plausible that the periosteal cells residing in the less-stiff multilayer regions could be maintained at relatively immature stages under the control of the stem-cell medium in vitro but start differentiating when exposed to the proper stiffness upon release from the culture conditions at the implantation site.

Stem-cell niche based comparative analysis of chemical and nano-mechanical material properties impacting ex vivo expansion and differentiation of hematopoietic and mesenchymal stem cells

The ability of stem cells to self-renew with minimal or no differentiation and, when appropriately cued, to give rise to many types of progenitor and mature cells, is the basis for applications in regenerative and transfusion medicine, but also in drug discovery and in vitro toxicology. Inspired by the complex interactions between stem cells and their microenvironment, the so-called stem-cell niche, the properties of supporting biomaterials, including surface biochemistry, topography (type, size, organization, and geometry of nanostructures), and mechanical properties, have been identified as important determinants of stem-cell fate in vitro. 3D culture environments that could recapitulate the complexity of the in vivo stem-cell microenvironment could further expand the complexity and repertoire of engineered environments with exciting translational applications. Herein, the material aspects that affect the expansion and differentiation fate of adult hematopoietic stem/progenitor cells (HSPCs) and mesenchymal stem cells (MSCs), two powerful cell types that co-reside in the bone-marrow niche, but with distinct, sometime complementary, differentiation fates, properties, and translational applications, are examined. Although MSCs are adherent cells and, in contrast, HSPCs are non- or weakly adherent cells, both can sense and respond to material properties, including surface (bio)chemistry, ECM composition, topography, and matrix elasticity, possibly through similar molecular mechanisms.

Cell mechanosensitivity: mechanical properties and interaction with gravitational field

This paper addressed the possible mechanisms of primary reception of a mechanical stimulus by different cells. Data concerning the stiffness of muscle and nonmuscle cells as measured by atomic force microscopy are provided. The changes in the mechanical properties of cells that occur under changed external mechanical tension are presented, and the initial stages of mechanical signal transduction are considered. The possible mechanism of perception of different external mechanical signals by cells is suggested.

Control of adipogenesis by ezrin, radixin and moesin-dependent biomechanics remodeling

We have recently shown that altered stem cell biomechanics can regulate the lineage commitment through a family of the membrane-cytoskeleton linker proteins (ERM; ezrin, radixin, moesin). The ERM proteins not only modulate the cell stiffness and actin cytoskeleton organization, but also rearrange focal adhesions and therefore influence the biochemically-directed stem cell differentiation. Combining silencing RNA, atomic force microscopy, and fluorescence microscopy, the role of the ERM proteins involved in the regulation of stem cell biomechanics and adipogenic differentiation was quantitatively determined. Transient ERM knockdown by RNAi caused disassembly of actin stress fibers and focal adhesions and a decrease in the cell stiffness. The silencing RNA treatment not only induced mechanical changes in stem cells but impaired adipogenesis in a time-dependent manner. While siRNA ERM treatment at day 0 substantially interfered with adipogenesis, the same treatment at day 3 of adipogenic differentiation significantly facilitated adipogenesis, as assessed by the expression of adipocyte-specific markers. The intact biomechanics homeostasis appears to be critical for the adipogenic induction. These findings may lead to potential biomechanical intervention techniques and methodologies to control the fate and extent of adipogenesis that would likely be involved in stem cell-based therapeutics for soft tissue repair and regeneration.

Mapping the mechanome of live stem cells using a novel method to measure local strain fields in situ at the fluid-cell interface

During mesenchymal condensation, the initial step of skeletogenesis, transduction of minute mechanical forces to the nucleus is associated with up or down-regulation of genes, ultimately resulting in formation of the skeletal template and appropriate cell lineage commitment. The summation of these biophysical cues affects the cell's shape and fate. Here, we predict and measure surface strain, in live stem cells, in response to controlled delivery of stresses, providing a platform to direct short-term structure - function relationships and long-term fate decisions. We measure local strains on stem cell surfaces using fluorescent microbeads coated with Concanavalin A. During delivery of controlled mechanical stresses, 4-Dimensional (x,y,z,t) displacements of the bound beads are measured as surface strains using confocal microscopy and image reconstruction. Similarly, micro-particle image velocimetry (μ-piv) is used to track flow fields with fluorescent microspheres. The measured flow velocity gradient is used to calculate stress imparted by fluid drag at the surface of the cell. We compare strain measured on cell surfaces with those predicted computationally using parametric estimates of the cell's elastic and shear modulus. Finally, cross-correlating stress - strain data to measures of gene transcription marking lineage commitment enables us to create stress - strain - fate maps, for live stem cells in situ. The studies show significant correlations between live stem cell stress - strain relationships and lineage commitment. The method presented here provides a novel means to probe the live stem cell's mechanome, enabling mechanistic studies of the role of mechanics in lineage commitment as it unfolds.

Estradiol influences the mechanical properties of human fetal osteoblasts through cytoskeletal changes

Estrogen is known to have a direct effect on bone forming osteoblasts and bone resorbing osteoclasts. The cellular and molecular effects of estrogen on osteoblasts and osteoblasts-like cells have been extensively studied. However, the effect of estrogen on the mechanical property of osteoblasts has not been studied yet. It is important since mechanical property of the mechanosensory osteoblasts could be pivotal to its functionality in bone remodeling. This is the first study aimed to assess the direct effect of estradiol on the apparent elastic modulus (E∗) and corresponding cytoskeletal changes of human fetal osteoblasts (hFOB 1.19). The cells were cultured in either medium alone or medium supplemented with β-estradiol and then subjected to Atomic Force Microscopy indentation (AFM) to determine E∗. The underlying changes in cytoskeleton were studied by staining the cells with TRITC-Phalloidin. Following estradiol treatment, the cells were also tested for proliferation, alkaline phosphatase activity and mineralization. With estradiol treatment, E∗ of osteoblasts significantly decreased by 43-46%. The confocal images showed that the changes in f-actin network observed in estradiol treated cells can give rise to the changes in the stiffness of the cells. Estradiol also increases the inherent alkaline phosphatase activity of the cells. Estradiol induced stiffness changes of osteoblasts were not associated with changes in the synthesized mineralized matrix of the cells. Thus, a decrease in osteoblast stiffness with estrogen treatment was demonstrated in this study, with positive links to cytoskeletal changes. The estradiol associated changes in osteoblast mechanical properties could bear implications for bone remodeling and its mechanical integrity.

Cellular mechanical properties reflect the differentiation potential of adipose-derived mesenchymal stem cells

The mechanical properties of adipose-derived stem cell (ASC) clones correlate with their ability to produce tissue-specific metabolites, a finding that has dramatic implications for cell-based regenerative therapies. Autologous ASCs are an attractive cell source due to their immunogenicity and multipotent characteristics. However, for practical applications ASCs must first be purified from other cell types, a critical step which has proven difficult using surface-marker approaches. Alternative enrichment strategies identifying broad categories of tissue-specific cells are necessary for translational applications. One possibility developed in our lab uses single-cell mechanical properties as predictive biomarkers of ASC clonal differentiation capability. Elastic and viscoelastic properties of undifferentiated ASCs were tested via atomic force microscopy and correlated with lineage-specific metabolite production. Cell sorting simulations based on these "mechanical biomarkers" indicated they were predictive of differentiation capability and could be used to enrich for tissue-specific cells, which if implemented could dramatically improve the quality of regenerated tissues.

Experiments in Nanomechanical Properties of Live Osteoblast Cells and Cell–Biomaterial Interface

Characterizing the mechanical characteristics of living cells and cell–biomaterial composite is an important area of research in bone tissue engineering. In this work, an in situ displacement-controlled nanoindentation technique (using Hysitron Triboscope) is developed to perform nanomechanical characterization of living cells (human osteoblasts) and cell–substrate constructs under physiological conditions (cell culture medium; 37 °C). In situ elastic moduli (E) of adsorbed proteins on tissue culture polystyrene (TCPS) under cell culture media were found to be ∼4 GPa as revealed by modulus mapping experiments. The TCPS substrates soaked in cell culture medium showed significant difference in surface nanomechanical properties (up to depths of ∼12 nm) as compared to properties obtained from deeper indentations. Atomic force microscopy (AFM) revealed the cytoskeleton structures such as actin stress fiber networks on flat cells which are believed to impart the structural integrity to cell structure. Load-deformation response of cell was found to be purely elastic in nature, i.e., cell recovers its shape on unloading as indicated by linear loading and unloading curves obtained at 1000 nm indentation depth. The elastic response of cells is obtained during initial cell adhesion (ECell, 1 h, 1000 nm = 4.4–12.4 MPa), cell division (ECell, 2 days, 1000 nm = 1.3–3.0 MPa), and cell spreading (ECell, 2 days, 1000 nm = 6.9–11.6 MPa). Composite nanomechanical responses of cell–TCPS constructs were obtained by indentation at depths of 2000 nm and 3000 nm on cell-seeded TCPS. Elastic properties of cell–substrate composites were mostly dominated by stiff TCPS (EBulk = 5 GPa) lying underneath the cell.

Impact of substrate elasticity on human hematopoietic stem and progenitor cell adhesion and motility

In the bone marrow, hematopoietic stem cells (HSCs) reside in endosteal and vascular niches. The interactions with the niches are essential for the maintenance of HSC number and properties. Although the molecular nature of these interactions is quite well understood, little is known about the role of physical parameters such as matrix elasticity. Osteoblasts, the major cellular component of the endosteal HSC niche, flatten during HSC mobilization. We show that this process is accompanied by osteoblast stiffening, demonstrating that not only biochemical signals but also mechanical properties of the niche are modulated. HSCs react to stiffer substrates with increased cell adhesion and migration, which could facilitate the exit of HSCs from the niche. These results indicate that matrix elasticity is an important factor in regulating the retention of HSCs in the endosteal niche and should be considered in attempts to propagate HSCs in vitro for clinical applications.

Altered osteogenic commitment of human mesenchymal stem cells by ERM protein-dependent modulation of cellular biomechanics

Cellular mechanics is known to play an important role in many cellular functions including adhesion, migration, proliferation, and differentiation. Human mesenchymal stem cells (hMSCs) demonstrate unique mechanical properties distinct from fully differentiated cells. This observation suggests that the stem cell mechanics may be modulated to regulate the hMSCs' lineage commitment. Specifically, ERM (ezrin, radixin, moesin) proteins are known to mediate the membrane-cytoskeleton adhesion, cell elasticity, actin cytoskeleton organization, and therefore could serve as potential targets for modulation of the cellular mechanics. Combining silencing RNA, atomic force microscopy, and laser optical tweezers, the role of the ERM proteins involved in the regulation of stem cell biomechanics and osteogenic differentiation was quantitatively determined. Transient ERM knockdown by RNAi causes disassembly of actin stress fibers and focal adhesions, a decrease in the cell stiffness, and membrane separation from the cytoskeleton. The silencing RNA treatment not only induced mechanical changes in stem cells but impaired biochemically-directed osteogenic differentiation. The intact actin cytoskeleton and focal adhesions of hMSCs appear critical for the osteogenic induction. Thus, ERM knockdown modulates the dynamics of cell mechanical changes during hMSC differentiation and regulates the expression of tissue specific molecular markers. These findings are of particular interest for modulation of the cellular biomechanics to control hMSCs' activities and fate in tissue engineering, regenerative medicine, and other stem cell-based therapeutic applications.

Stiffness and heterogeneity of the pulmonary endothelial glycocalyx measured by atomic force microscopy

The mechanical properties of endothelial glycocalyx were studied using atomic force microscopy with a silica bead (diameter ∼18 μm) serving as an indenter. Even at indentations of several hundred nanometers, the bead exerted very low compressive pressures on the bovine lung microvascular endothelial cell (BLMVEC) glycocalyx and allowed for an averaging of stiffness in the bead-cell contact area. The elastic modulus of BLMVEC glycocalyx was determined as a pointwise function of the indentation depth before and after enzymatic degradation of specific glycocalyx components. The modulus-indentation depth profiles showed the cells becoming progressively stiffer with increased indentation. Three different enzymes were used: heparinases III and I and hyaluronidase. The main effects of heparinase III and hyaluronidase enzymes were that the elastic modulus in the cell junction regions increased more rapidly with the indentation than in BLMVEC controls, and that the effective thickness of glycocalyx was reduced. Cytochalasin D abolished the modulus increase with the indentation. The confocal profiling of heparan sulfate and hyaluronan with atomic force microscopy indentation data demonstrated marked heterogeneity of the glycocalyx composition between cell junctions and nuclear regions.

Bone cell elasticity and morphology changes during the cell cycle

The mechanical properties of cells are reported to be regulated by a range of factors including interactions with the extracellular environment and other cells, differentiation status, the onset of pathological states, as well as the intracellular factors, for example, the cytoskeleton. The cell cycle is considered to be a well-ordered sequence of biochemical events. A number of processes reported to occur during its progression are inherently mechanical and, as such, require mechanical regulation. In spite of this, few attempts have been made to investigate the putative regulatory role of the cell cycle in mechanobiology. In the present study, Atomic Force Microscopy (AFM) was employed to investigate the elastic modulus of synchronised osteoblasts. The data obtained confirm that osteoblast elasticity is regulated by cell cycle phase; specifically, cells in S phase were found to have a modulus approximately 1.7 times that of G1 phase cells. Confocal microscopy studies revealed that aspects of osteoblast morphology, namely F-actin expression, were also modulated by the cell cycle, and tended to increase with phase progression from G0 onwards. The data obtained in this study are likely to have implications for the fields of tissue- and bio-engineering, where prior knowledge of cell mechanobiology is essential for the effective replacement and repair of tissue. Furthermore, studies focused on biomechanics and the biophysical properties of cells are important in the understanding of the onset and progression of disease states, for example cancer at the cellular level. Our study demonstrates the importance of the combined use of traditional and relatively novel microscopy techniques in understanding mechanical regulation by crucial cellular processes, such as the cell cycle.

Elastic modulus determination of normal and glaucomatous human trabecular meshwork

PURPOSE

Elevated intraocular pressure (IOP) is a risk factor for glaucoma. The principal outflow pathway for aqueous humor in the human eye is through the trabecular meshwork (HTM) and Schlemm's canal (SC). The junction between the HTM and SC is thought to have a significant role in the regulation of IOP. A possible mechanism for the increased resistance to flow in glaucomatous eyes is an increase in stiffness (increased elastic modulus) of the HTM. In this study, the stiffness of the HTM in normal and glaucomatous tissue was compared, and a mathematical model was developed to predict the impact of changes in stiffness of the juxtacanalicular layer of HTM on flow dynamics through this region.

METHODS

Atomic force microscopy (AFM) was used to measure the elastic modulus of normal and glaucomatous HTM. According to these results, a model was developed that simulated the juxtacanalicular layer of the HTM as a flexible membrane with embedded pores.

RESULTS

The mean elastic modulus increased substantially in the glaucomatous HTM (mean = 80.8 kPa) compared with that in the normal HTM (mean = 4.0 kPa). Regional variation was identified across the glaucomatous HTM, possibly corresponding to the disease state. Mathematical modeling suggested an increased flow resistance with increasing HTM modulus.

CONCLUSIONS

The data indicate that the stiffness of glaucomatous HTM is significantly increased compared with that of normal HTM. Modeling exercises support substantial impairment in outflow facility with increased HTM stiffness. Alterations in the biophysical attributes of the HTM may participate directly in the onset and progression of glaucoma.

Quasi-3D cytoskeletal dynamics of osteocytes under fluid flow

Osteocytes respond to dynamic fluid shear loading by activating various biochemical pathways, mediating a dynamic process of bone formation and resorption. Whole-cell deformation and regional deformation of the cytoskeleton may be able to directly regulate this process. Attempts to image cellular deformation by conventional microscopy techniques have been hindered by low temporal or spatial resolution. In this study, we developed a quasi-three-dimensional microscopy technique that enabled us to simultaneously visualize an osteocyte's traditional bottom-view profile and a side-view profile at high temporal resolution. Quantitative analysis of the plasma membrane and either the intracellular actin or microtubule (MT) cytoskeletal networks provided characterization of their deformations over time. Although no volumetric dilatation of the whole cell was observed under flow, both the actin and MT networks experienced primarily tensile strains in all measured strain components. Regional heterogeneity in the strain field of normal strains was observed in the actin networks, especially in the leading edge to flow, but not in the MT networks. In contrast, side-view shear strains exhibited similar subcellular distribution patterns in both networks. Disruption of MT networks caused actin normal strains to decrease, whereas actin disruption had little effect on the MT network strains, highlighting the networks' mechanical interactions in osteocytes.

Atomic force microscopy of biological samples

The ability to evaluate structural-functional relationships in real time has allowed scanning probe microscopy (SPM) to assume a prominent role in post genomic biological research. In this mini-review, we highlight the development of imaging and ancillary techniques that have allowed SPM to permeate many key areas of contemporary research. We begin by examining the invention of the scanning tunneling microscope (STM) by Binnig and Rohrer in 1982 and discuss how it served to team biologists with physicists to integrate high-resolution microscopy into biological science. We point to the problems of imaging nonconductive biological samples with the STM and relate how this led to the evolution of the atomic force microscope (AFM) developed by Binnig, Quate, and Gerber, in 1986. Commercialization in the late 1980s established SPM as a powerful research tool in the biological research community. Contact mode AFM imaging was soon complemented by the development of non-contact imaging modes. These non-contact modes eventually became the primary focus for further new applications including the development of fast scanning methods. The extreme sensitivity of the AFM cantilever was recognized and has been developed into applications for measuring forces required for indenting biological surfaces and breaking bonds between biomolecules. Further functional augmentation to the cantilever tip allowed development of new and emerging techniques including scanning ion-conductance microscopy (SICM), scanning electrochemical microscope (SECM), Kelvin force microscopy (KFM) and scanning near field ultrasonic holography (SNFUH).

The applications of atomic force microscopy to vision science

The atomic force microscope (AFM) is widely used in materials science and has found many applications in biological sciences but has been limited in use in vision science. The AFM can be used to image the topography of soft biological materials in their native environments. It can also be used to probe the mechanical properties of cells and extracellular matrices, including their intrinsic elastic modulus and receptor-ligand interactions. In this review, the operation of the AFM is described along with a review of how it has been thus far used in vision science. It is hoped that this review will serve to stimulate vision scientists to consider incorporating AFM as part of their research toolkit.

Atomic force microscopy in mechanobiology: measuring microelastic heterogeneity of living cells

Recent findings clearly demonstrate that cells feel mechanical forces, and respond by altering their -phenotype and modulating their mechanical environment. Atomic force microscope (AFM) indentation can be used to mechanically stimulate cells and quantitatively characterize their elastic properties, providing critical information for understanding their mechanobiological behavior. This review focuses on the experimental and computational aspects of AFM indentation in relation to cell biomechanics and pathophysiology. Key aspects of the indentation protocol (including preparation of substrates, selection of indentation parameters, methods for contact point detection, and further post-processing of data) are covered. Historical perspectives on AFM as a mechanical testing tool as well as studies of cell mechanics and physiology are also highlighted.

Electrical power free, low dead volume, pressure-driven pumping for microfluidic applications

This paper presents a simple-to-construct, low dead volume pump capable of generating a wide range of positive and negative pressures for microfluidic applications. The pump generates pressure or vacuum by changing the volume of air confined inside a syringe and is able to generate pressures between -95 and +300 kPa with a resolution as high as 1 Pa. Different from syringe pumps and electrokinetic pumping, which are capable of controlling flow rates only, our pump can be used to generate constant flow rates or constant pressures, which are required for certain applications such as the aspiration of biological cells for biophysical characterization. Compared to syringe pumps, the new pump has almost zero dead volume and does not exhibit pulsatile flows. Additionally, the system does not require electrical power and is cost effective (∼$100). To demonstrate the capabilities of the pump, we used it to aspirate osteoblasts (MC3T3-E1 cells) and to determine Young's modulus of the cells, to generate a concentration gradient, and to produce variable-sized droplets in microchannels using hydrodynamic focusing.

Macroscopic stiffening of embryonic tissues via microtubules, RhoGEF and the assembly of contractile bundles of actomyosin

During morphogenesis, forces generated by cells are coordinated and channeled by the viscoelastic properties of the embryo. Microtubules and F-actin are considered to be two of the most important structural elements within living cells accounting for both force production and mechanical stiffness. In this paper, we investigate the contribution of microtubules to the stiffness of converging and extending dorsal tissues in Xenopus laevis embryos using cell biological, biophysical and embryological techniques. Surprisingly, we discovered that depolymerizing microtubules stiffens embryonic tissues by three- to fourfold. We attribute tissue stiffening to Xlfc, a previously identified RhoGEF, which binds microtubules and regulates the actomyosin cytoskeleton. Combining drug treatments and Xlfc activation and knockdown lead us to the conclusion that mechanical properties of tissues such as viscoelasticity can be regulated through RhoGTPase pathways and rule out a direct contribution of microtubules to tissue stiffness in the frog embryo. We can rescue nocodazole-induced stiffening with drugs that reduce actomyosin contractility and can partially rescue morphogenetic defects that affect stiffened embryos. We support these conclusions with a multi-scale analysis of cytoskeletal dynamics, tissue-scale traction and measurements of tissue stiffness to separate the role of microtubules from RhoGEF activation. These findings suggest a re-evaluation of the effects of nocodazole and increased focus on the role of Rho family GTPases as regulators of the mechanical properties of cells and their mechanical interactions with surrounding tissues.

Can common adhesion molecules and microtopography affect cellular elasticity? A combined atomic force microscopy and optical study

The phenomenon that cells respond to chemical and topographic cues in their surroundings has been widely examined and exploited in many fields ranging from basic life science research to biomedical therapeutics. Adhesion promoting molecules such as poly-L-lysine (PLL) and fibronectin (Fn) are commonly used for in vitro cell assays to promote cell spreading/proliferation on tissue culture plastic and to enhance the biocompatibility of biomedical devices. Likewise, engineered topography is often used to guide cell growth and differentiation. Little is known about how these cues affect the biomechanical properties of cells and subsequent cell function. In this study we have applied atomic force microscopy (AFM) to investigate these biomechanical properties. In the first stage of the study we formulated a rigorous approach to quantify cellular elasticity using AFM. Operational factors, including indentation depth and speed, and mathematical models for data fitting have been systematically evaluated. We then quantified how PLL, Fn and microtopography affected cellular elasticity and the organization of the cytoskeleton. Cellular elasticity after 1 day in culture was greater on a Fn-coated surface as compared to PLL or glass. These statistically significant differences disappeared after two more days in culture. In contrast, the significantly higher elasticity associated with cells grown on micrometric grooves remained for at least 3 days. This work sheds light on the apparently simple but debatable questions: "Are engineered chemical cues eventually masked by a cell's own matrix proteins and so only exert short-term influence? Does engineered topography as well as engineered chemistry affect cell elasticity?"

Combining mechanical and optical approaches to dissect cellular mechanobiology

Mechanical force modulates a wide array of cell physiological processes. Cells sense and respond to mechanical stimuli using a hierarchy of structural complexes spanning multiple length scales, including force-sensitive molecules and cytoskeletal networks. Understanding mechanotransduction, i.e., the process by which cells convert mechanical inputs into biochemical signals, has required the development of novel biophysical tools that allow for probing of cellular and subcellular components at requisite time, length, and force scales and technologies that track the spatio-temporal dynamics of relevant biomolecules. In this review, we begin by discussing the underlying principles and recent applications of atomic force microscopy, magnetic twisting cytometry, and traction force microscopy, three tools that have been widely used for measuring the mechanical properties of cells and for probing the molecular basis of cellular mechanotransduction. We then discuss how such tools can be combined with advanced fluorescence methods for imaging biochemical processes in living cells in the context of three specific problem spaces. We first focus on fluorescence resonance energy transfer, which has enabled imaging of intra- and inter-molecular interactions and enzymatic activity in real time based on conformational changes in sensor molecules. Next, we examine the use of fluorescence methods to probe force-dependent dynamics of focal adhesion proteins. Finally, we discuss the use of calcium ratiometric signaling to track fast mechanotransductive signaling dynamics. Together, these studies demonstrate how single-cell biomechanical tools can be effectively combined with molecular imaging technologies for elucidating mechanotransduction processes and identifying mechanosensitive proteins.

Biomineralization of a self-assembled extracellular matrix for bone tissue engineering

Understanding how biomineralization occurs in the extracellular matrix (ECM) of bone cells is crucial to the understanding of bone formation and the development of a successfully engineered bone tissue scaffold. It is still unclear how ECM mechanical properties affect protein-mineral interactions in early stages of bone mineralization. We investigated the longitudinal mineralization properties of MC3T3-E1 cells and the elastic modulus of their ECM using shear modulation force microscopy, synchrotron grazing incidence X-ray diffraction (GIXD), scanning electron microscopy, energy dispersive X-ray spectroscopy, and confocal laser scanning microscopy (CLSM). The elastic modulus of the ECM fibers underwent significant changes for the mineralizing cells, which were not observed in the nonmineralizing cells. On substrates conducive to ECM network production, the elastic modulus of mineralizing cells increased at time points corresponding to mineral production, whereas that of the nonmineralizing cells did not vary over time. The presence of hydroxyapatite in mineralizing cells and the absence thereof in the nonmineralizing ones were confirmed by GIXD, and CLSM showed that a restructuring of actin occurred only for mineral-producing cells. These results show that the correct and complete development of the ECM network is required for osteoblasts to mineralize. This in turn requires a suitably prepared synthetic substrate for bone development to succeed in vitro.

Measuring Viscoelasticity of Soft Samples Using Atomic Force Microscopy

Relaxation indentation experiments using atomic force microscopy (AFM) are used to obtain viscoelastic material properties of soft samples. The quasilinear viscoelastic (QLV) model formulated by Fung (1972, “Stress Strain History Relations of Soft Tissues in Simple Elongation,” in Biomechanics, Its Foundation and Objectives, Prentice-Hall, Englewood Cliffs, NJ, pp. 181–207) for uniaxial compression data was modified for the indentation test data in this study. Hertz contact mechanics was used for the instantaneous deformation, and a reduced relaxation function based on continuous spectrum is used for the time-dependent part in the model. The modified QLV indentation model presents a novel method to obtain viscoelastic properties from indentation data independent of relaxation times of the test. The major objective of the present study is to develop the QLV indentation model and implement the model on AFM indentation data for 1% agarose gel and a viscoelastic polymer using spherical indenter.

Role of cytoskeletal components in stress-relaxation behavior of adherent vascular smooth muscle cells

A number of recent studies have demonstrated the effectiveness of atomic force microscopy (AFM) for characterization of cellular stress-relaxation behavior. However, this technique's recent development creates considerable need for exploration of appropriate mechanical models for analysis of the resultant data and of the roles of various cytoskeletal components responsible for governing stress-relaxation behavior. The viscoelastic properties of vascular smooth muscle cells (VSMCs) are of particular interest due to their role in the development of vascular diseases, including atherosclerosis and restenosis. Various cytoskeletal agents, including cytochalasin D, jasplakinolide, paclitaxel, and nocodazole, were used to alter the cytoskeletal architecture of the VSMCs. Stress-relaxation experiments were performed on the VSMCs using AFM. The quasilinear viscoelastic (QLV) reduced-relaxation function, as well as a simple power-law model, and the standard linear solid (SLS) model, were fitted to the resultant stress-relaxation data. Actin depolymerization via cytochalasin D resulted in significant increases in both rate of relaxation and percentage of relaxation; actin stabilization via jasplakinolide did not affect stress-relaxation behavior. Microtubule depolymerization via nocodazole resulted in nonsignificant increases in rate and percentage of relaxation, while microtubule stabilization via paclitaxel caused significant decreases in both rate and percentage of relaxation. Both the QLV reduced-relaxation function and the power-law model provided excellent fits to the data (R(2)=0.98), while the SLS model was less adequate (R(2)=0.91). Data from the current study indicate the important role of not only actin, but also microtubules, in governing VSMC viscoelastic behavior. Excellent fits to the data show potential for future use of both the QLV reduced-relaxation function and power-law models in conjunction with AFM stress-relaxation experiments.

Effect of silicon content on the sintering and biological behaviour of Ca10(PO4)(6-x)(SiO4)x(OH)(2-x) ceramics

Silicated hydroxyapatite powders (Ca10(PO4)(6-x)(SiO4)x(OH)(2-x); Si(x)HA) were synthesized using a wet precipitation method. The sintering of Si(x)HA ceramics with 0 < or = x < or = 1 was investigated. For 0 < or = x < or = 0.5, the sintering rate and grain growth decreased slightly with the amount of silicate. For larger amounts, the sintering behaviour differed with the formation of secondary phases before total densification. Sintering parameters (temperature and time) were adjusted to each composition to produce dense materials having similar microstructure without formation of these secondary phases. Dense ceramics made of pure hydroxyapatite and Si(x)HA containing various amounts of silicate (up to x = 0.6) were biologically tested in vitro with human osteoblast-like cells. The proliferation of cells on the surface of the ceramics increased up to 5 days of culture, indicating that the materials were biocompatible. However, the silicon content did not influence the cell proliferation.

Determining the mechanical properties of human corneal basement membranes with atomic force microscopy

Biophysical cues such as substrate modulus have been shown to influence a variety of cell behaviors. We have determined the elastic modulus of the anterior basement membrane and Descemet's membrane of the human cornea with atomic force microscopy (AFM). A spherical probe was used with a radius approximating that of a typical cell focal adhesion. Values obtained for the elastic modulus of the anterior basement membrane range from 2 to 15 kPa, with a mean of 7.5+/-4.2 kPa. The elastic modulus of Descemet's membrane was found to be slightly higher than those observed for the anterior basement membrane, with a mean of 50+/-17.8 kPa and a range of 20-80 kPa. The topography of Descemet's membrane has been shown to be similar to that of the anterior basement, but with smaller pore sizes resulting in a more tightly packed structure. This structural difference may account for the observed modulus differences. The determination of these values will allow for the design of a better model of the cellular environment as well as aid in the design and fabrication of artificial corneas.

Cyclic Hydraulic Pressure and Fluid Flow Differentially Modulate Cytoskeleton Re-Organization in MC3T3 Osteoblasts

Mechanical loads are essential towards maintaining bone mass and skeletal integrity. Such loads generate various stimuli at the cellular level, including cyclic hydraulic pressure (CHP) and fluid shear stress (FSS). To gain insight into the anabolic responses of osteoblasts to CHP and FSS, we subjected MC3T3-E1 preosteoblasts to either FSS (12 dynes/cm(2)) or CHP varying from 0 to 68 kPa at 0.5 Hz. As with FSS, CHP produced a significant increase in ATP release over static controls within 5 min of onset. Cell stiffness examined by atomic force microscopy increased after 15 min of either CHP or FSS stimulation, which was attenuated when extracellular ATP was hydrolyzed with apyrase. As previously shown FSS induced polymerization of actins into stress fibers. However, the microtubule network was completely disrupted under FSS. In contrast, CHP appeared to maintain strong microtubule and f-actin networks. The purinergic signaling was found to be involved in the remodeling of f-actin, but not microtubule. Both CHP and FSS applied for 1 hour increased expression of COX-2. These data indicate that, while CHP and FSS produce similar anabolic responses, these stimuli have very different effects on the cytoskeleton remodeling and could contribute to loss of mechanosensitivity with extended loading.

Contribution of actin filaments and microtubules to quasi-in situ tensile properties and internal force balance of cultured smooth muscle cells on a substrate

The effects of actin filaments (AFs) and microtubules (MTs) on quasi-in situ tensile properties and intracellular force balance were studied in cultured rat aortic smooth muscle cells (SMCs). A SMC cultured on substrates was held using a pair of micropipettes, gradually detached from the substrate while maintaining in situ cell shape and cytoskeletal integrity, and then stretched up to approximately 15% and unloaded three times at the rate of 1 mum every 5 s. Cell stiffness was approximately 20 nN per percent strain in the untreated case and decreased by approximately 65% and approximately 30% following AF and MT disruption, respectively. MT augmentation did not affect cell stiffness significantly. The roles of AFs and MTs in resisting cell stretching and shortening were assessed using the area retraction of the cell upon noninvasive detachment from thermoresponsive gelatin-coated dishes. The retraction was approximately 40% in untreated cells, while in AF-disrupted cells it was <20%. The retraction increased by approximately 50% and decreased by approximately 30% following MT disruption and augmentation, respectively, suggesting that MTs resist intercellular tension generated by AFs. Three-dimensional measurements of cell morphology using confocal microscopy revealed that the cell volume remained unchanged following drug treatment. A concomitant increase in cell height and decrease in cell area was observed following AF disruption and MT augmentation. In contrast, MT disruption significantly reduced the cell height. These results indicate that both AFs and MTs play crucial roles in maintaining whole cell mechanical properties of SMCs, and that while AFs act as an internal tension generator, MTs act as a tension reducer, and these contribute to intracellular force balance three dimensionally.

Cell viability viscoelastic measurement in a rheometer used to stress and engineer tissues at low sonic frequencies

Effects of vibration on human vocal fold extracellular matrix composition and the resultant tissue viscoelastic properties are difficult to study in vivo. Therefore, an in vitro bioreactor, simulating the in vivo physiological environment, was explored. A stress-controlled commercial rheometer was used to administer shear vibrations to living tissues at stresses and frequencies corresponding to male phonation, while simultaneously measuring tissue viscoelastic properties. Tissue environment was evaluated and adjustments made in order to sustain cell life for short term experimentation up to 6 h. Cell nutrient medium evaporation, osmolality, pH, and cell viability of cells cultured in three-dimensional synthetic scaffolds were quantified under comparably challenging environments to the rheometer bioreactor for 4 or 6 h. The functionality of the rheometer bioreactor was demonstrated by applying three vibration regimes to cell-seeded three-dimensional substrates for 2 h. Resulting strain was quantified throughout the test period. Rheologic data and cell viability are reported for each condition, and future improvements are discussed.

Researching into the cellular shape, volume and elasticity of mesenchymal stem cells, osteoblasts and osteosarcoma cells by atomic force microscopy

Within the bone lie several different cell types, including osteoblasts (OBs) and mesenchymal stem cells (MSCs). The MSCs are ideal targets for regenerative medicine of bone due to their differentiation potential towards OBs. Human MSCs exhibit two distinct morphologies: rapidly self-renewing cells (RS) and flat cells (FC) with very low proliferation rates. Another cell type found in pathological bone conditions is osteosarcoma. In this study, we compared the topographic and morphometric features of RS and FC cells, human OBs and MG63 osteosarcoma cells by atomic force microscopy (AFM). The results demonstrated clear differences: FC and hOB cells showed similar ruffled topography, whereas RS and MG63 cells exhibited smoother surfaces. Furthermore, we investigated how selected substrates influence cell morphometry. We found that RS and MG63 cells were flatter on fibrous substrates such as polystyrene and collagen I, but much more rounded on glass, the smoothest surface. In contrast, cells with large area, namely FC and hOB cells, did not exhibit pronounced changes in flatness with regards to the different substrates. They were, however, remarkably flatter in comparison to RS and MG63 cells. We could explain the differences in flatness by the extent of adhesion. Indeed, FC and hOB cells showed much higher content of focal adhesions. Finally, we used the AFM to determine the cellular Young's modulus. RS, FC and hOB cells showed comparable stiffness on the three different substrates, while MG63 cells demonstrated the unique feature of increased elasticity on collagen I. In summary, our results show, for the first time, a direct comparison between the morphometric and biophysical features of different human cell types derived from normal and pathological bone. Our study manifests the opinion that along with RNA, proteomic and functional research, morphological and biomechanical characterization of cells also reveals novel cell features and interrelationships.

Modeling the effect of cell-associated polymeric fluid layers on force spectroscopy measurements. Part I: model development

The mechanical response, the force-indentation relationship, in normal force spectroscopy measurements carried out on individual polysaccharide encapsulated bacteria is modeled using three increasingly refined approaches that consider the elastic response of the bacterium and cantilever in combination with a fluid (hydrodynamic) model for the polysaccharide layer. For the hydrodynamic description of the polysaccharide layer, several increasingly realistic models are described in detail, together with numerical solution techniques. These models range from one-dimensional, Newtonian, to two-dimensional, axisymmetric, fully viscoelastic (Phan-Thien/Tanner). In all cases, the models rigorously consider the time-dependent rheological-mechanical coupling between the elastic and fluid viscoelastic physical components of the experimental setup. Effects of inherent variability in geometrical and material properties of the bacterium and polysaccharide layer on the measurable response are quantified. A parametric investigation of the force-indentation relationship highlights the importance of accurate knowledge of the rheology of the extracellular polysaccharides. We also draw conclusions about the design and evaluation of force spectroscopy experiments on single encapsulated bacteria. Supported by model calculations, we also point the way to methods of in vivo rheological characterization of the extracellular polysaccharide as a preferable alternative to characterization after its removal from the native environment.

Atomic force microscopy probing of cell elasticity

Atomic force microscopy (AFM) has recently provided the great progress in the study of micro- and nanostructures including living cells and cell organelles. Modern AFM techniques allow solving a number of problems of cell biomechanics due to simultaneous evaluation of the local mechanical properties and the topography of the living cells at a high spatial resolution and force sensitivity. Particularly, force spectroscopy is used for mapping mechanical properties of a single cell that provides information on cellular structures including cytoskeleton structure. This entry is aimed to review the recent AFM applications for the study of dynamics and mechanical properties of intact cells associated with different cell events such as locomotion, differentiation and aging, physiological activation and electromotility, as well as cell pathology. Local mechanical characteristics of different cell types including muscle cells, endothelial and epithelial cells, neurons and glial cells, fibroblasts and osteoblasts, blood cells and sensory cells are analyzed in this paper.

Cell mechanics: integrating cell responses to mechanical stimuli

Forces are increasingly recognized as major regulators of cell structure and function, and the mechanical properties of cells are essential to the mechanisms by which cells sense forces, transmit them to the cell interior or to other cells, and transduce them into chemical signals that impact a spectrum of cellular responses. Comparison of the mechanical properties of intact cells with those of the purified cytoskeletal biopolymers that are thought to dominate their elasticity reveal the extent to which the studies of purified systems can account for the mechanical properties of the much more heterogeneous and complex cell. This review summarizes selected aspects of current work on cell mechanics with an emphasis on the structures that are activated in cell-cell contacts, that regulate ion flow across the plasma membrane, and that may sense fluid flow that produces low levels of shear stress.

Direct mechanical measurement of geodesic structures in rat mesenchymal stem cells

During numerous biological processes, cell adhesion, cell migration and cell spreading are vital. These basic biological functions are regulated by the interaction of cells with their extracellular environment. To examine the morphology and mechanical changes occurring in mesenchymal stem cells cultured on a mechanically rigid substrate, atomic force microscopy and fluorescence microscopy were employed. Investigations of the cells revealed both linear and geodesic F-actin configurations. No particular cell characteristics or intra-cellular location were implicated in the appearance of the geodesic structures. However, the length of time the cells were cultured on the substrate correlated with the percentage appearance of the geodesic structures. Calculating energy dissipation from cell images acquired by dynamic mode atomic force microscopy, it was observed that the vertices of the geodesic structures had significantly higher energy dissipation compared to the linear F-actin and the glass. This supports work by Lazarides [J. Cell Biol. 68, 202-219 (1976)], who postulated that the vertices of these geodesic structures should have a greater flexibility. Our results also support predictions based on the microfilament tensegrity model. By understanding the basic principles of cell ultrastructure and cell mechanics in relation to different extracellular environments, a better understanding of physiological and pathological process will be elicited.

Viscoelastic properties of human mesenchymally-derived stem cells and primary osteoblasts, chondrocytes, and adipocytes

The mechanical properties of single cells play important roles in regulating cell-matrix interactions, potentially influencing the process of mechanotransduction. Recent studies also suggest that cellular mechanical properties may provide novel biological markers, or "biomarkers," of cell phenotype, reflecting specific changes that occur with disease, differentiation, or cellular transformation. Of particular interest in recent years has been the identification of such biomarkers that can be used to determine specific phenotypic characteristics of stem cells that separate them from primary, differentiated cells. The goal of this study was to determine the elastic and viscoelastic properties of three primary cell types of mesenchymal lineage (chondrocytes, osteoblasts, and adipocytes) and to test the hypothesis that primary differentiated cells exhibit distinct mechanical properties compared to adult stem cells (adipose-derived or bone marrow-derived mesenchymal stem cells). In an adherent, spread configuration, chondrocytes, osteoblasts, and adipocytes all exhibited significantly different mechanical properties, with osteoblasts being stiffer than chondrocytes and both being stiffer than adipocytes. Adipose-derived and mesenchymal stem cells exhibited similar properties to each other, but were mechanically distinct from primary cells, particularly when comparing a ratio of elastic to relaxed moduli. These findings will help more accurately model the cellular mechanical environment in mesenchymal tissues, which could assist in describing injury thresholds and disease progression or even determining the influence of mechanical loading for tissue engineering efforts. Furthermore, the identification of mechanical properties distinct to stem cells could result in more successful sorting procedures to enrich multipotent progenitor cell populations.