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

Abstract

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.

Figure 1. AFM indentation of single cells

Indentation with a spherical-tip AFM probe (A) occurred at the center of the cell for the spherical morphology (inset pictures) and over the nucleus for the spread morphology. Polarized light images show a chondrocyte (B), osteoblast (C), adipocyte (D), ADAS cell (E), and MSC (F) highlighted by a dashed line surrounding their periphery.

Figure 2. E_elastic distribution for spherical cells

Elastic property distributions showed that osteoblasts, ADAS cells, and MSCs had similar variations within their populations. Chondrocytes and adipocytes had less variation and exhibited distribution peaks (lognormal fits) at lower elastic moduli than the other cell types.

Figure 3. E_elastic distribution for spread cells

Unlike the spherical morphology data, elastic properties for spread cells showed a distinct difference between osteoblasts and the other cell types. ADAS cells and MSCs exhibited similar population profiles, while chondrocytes did not change appreciably from their spherical morphology distributions. Adipocytes, which only exhibited one morphology, are shown for comparison purposes. Osteoblasts had the broadest distribution of elastic properties when spread, followed by MSCs then chondrocytes/ADAS cells.

Figure 4. E_elastic and height for spherical cells

Chondrocytes and adipocytes possessed E_elastic values that were significantly lower than the other cell types (A). Osteoblasts, ADAS cells, and MSCs all exhibited similar elastic properties when in a rounded cell shape. Cell heights were significantly different among all cell types, except between chondrocytes and ADAS cells (B). In particular, adipocytes were much larger than any of the other cell types. Data shown as mean ± standard deviation of log-normalized values.

Figure 5. E_R, E₀, and μ for spherical cells

Few differences existed for E_R (A), E₀ (B), or μ (C) among cell types tested in the spherical morphology. This result could indicate that cellular viscoelastic characteristics are not apparent until a firm attachment to a surrounding matrix has been established. Data shown as mean ± standard deviation of log-normalized values.

Figure 6. E_elastic and height for spread cells

Elastic modulus varied significantly among cell types, with osteoblasts possessing the highest and adipocytes the lowest (A). Adult stem cells exhibited similar elastic properties that were intermediate between primary cell types. Cell heights indicated that osteoblasts, ADAS cells, and MSCs all spread to approximately the same height while chondrocytes remained much taller (B). Adipocytes, which were tested in just one morphology, are included for comparison purposes. Data shown as mean ± standard deviation of log-normalized values.

Figure 7. E_R, E₀, and μ for spread cells

Viscoelastic properties for the spread morphology showed significant differences for all comparisons of E_R except between chondrocytes and adipocytes and between ADAS cells and MSCs (A). All primary cell types were significantly different when comparing E₀ (B). The apparent viscosity of osteoblasts was significantly higher than all other cell types but exhibited extremely large variations, indicating that the preciseness of this property might not be sufficient for cell-to-cell comparisons (C). Data shown as mean ± standard deviation of log-normalized values.

Figure 8. E_elastic / E_R ratio for spherical cells

Adult stem cells exhibited a distinct trait when compared to primary cell types. Both ADAS cells and MSCs possessed high elastic moduli in comparison to their relaxed moduli. This result is shown most clearly by calculating a ratio of E_elastic to E_R. Physically, this value indicates that, in comparison to primary cells, stem cells are initially very stiff but cannot resist deformation due to load over time. Data shown as mean ± standard deviation of log-normalized ratios.