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Matrix viscoplasticity and its shielding by active mechanics in microtissue models: experiments and mathematical modeling

View Article: PubMed Central - PubMed

ABSTRACT

The biomechanical behavior of tissues under mechanical stimulation is critically important to physiological function. We report a combined experimental and modeling study of bioengineered 3D smooth muscle microtissues that reveals a previously unappreciated interaction between active cell mechanics and the viscoplastic properties of the extracellular matrix. The microtissues’ response to stretch/unstretch actuations, as probed by microcantilever force sensors, was dominated by cellular actomyosin dynamics. However, cell lysis revealed a viscoplastic response of the underlying model collagen/fibrin matrix. A model coupling Hill-type actomyosin dynamics with a plastic perfectly viscoplastic description of the matrix quantitatively accounts for the microtissue dynamics, including notably the cells’ shielding of the matrix plasticity. Stretch measurements of single cells confirmed the active cell dynamics, and were well described by a single-cell version of our model. These results reveal the need for new focus on matrix plasticity and its interactions with active cell mechanics in describing tissue dynamics.

No MeSH data available.


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Magnetic microtissue platform for the study of the dynamics of self-assembled smooth muscle tissue constructs.(a) Schematic three quarters view showing a microtissue suspended between two flexible PDMS micropillars whose deflections report the microtissue’s contractile force. The wells are 400 μm × 800 μm × 125 μm deep. The flexible lower sections of the pillars were 30 μm × 170 μm in cross section and 85 μm high, and the pillars had spring constants k = 0.59 μN/μm for small lateral deflections. (b) A magnetic force FMag applied via a magnetic tweezer to a magnetic Ni sphere bonded to one of the pillars is used to apply time-varying strains to the microtissue. (c,d) show top-views of a SMC microtissue with (c) as grown, and (d) subjected to a 2% strain under FMag. = 28 μN. The Ni sphere appears as a black circle, and the tip of the magnetic tweezer is visible at the right edge of the images. 2D projected fluorescence confocal images of a microtissue show (e) collagen type-I, (f) nuclei, and (g) F-actin.
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f1: Magnetic microtissue platform for the study of the dynamics of self-assembled smooth muscle tissue constructs.(a) Schematic three quarters view showing a microtissue suspended between two flexible PDMS micropillars whose deflections report the microtissue’s contractile force. The wells are 400 μm × 800 μm × 125 μm deep. The flexible lower sections of the pillars were 30 μm × 170 μm in cross section and 85 μm high, and the pillars had spring constants k = 0.59 μN/μm for small lateral deflections. (b) A magnetic force FMag applied via a magnetic tweezer to a magnetic Ni sphere bonded to one of the pillars is used to apply time-varying strains to the microtissue. (c,d) show top-views of a SMC microtissue with (c) as grown, and (d) subjected to a 2% strain under FMag. = 28 μN. The Ni sphere appears as a black circle, and the tip of the magnetic tweezer is visible at the right edge of the images. 2D projected fluorescence confocal images of a microtissue show (e) collagen type-I, (f) nuclei, and (g) F-actin.

Mentions: To measure the time-dependent biomechanical response of model tissues to external mechanical stimuli, we constructed arrays of engineered microtissues containing bovine pulmonary artery smooth muscle cells (SMCs), as shown in Fig. 1. Such microtissues have been used to measure a range of tissue properties using other cell types91011121314151617, but the potential for dynamic studies has not been fully explored. Arrays of microwells were fabricated in polydimethylsiloxane (PDMS). The microwells were 400 μm wide by 800 μm long by 125 μm deep, and each contained a pair of flexible T-shaped PDMS pillars that acted as microcantilevers, as shown in Fig. 1a. A magnetic Ni microsphere bonded to one cantilever in each well enabled application of force1115. A collagen/fibrin solution containing SMCs was introduced into the microwells at densities that yielded ~300 cells per well. The contractile action of the SMCs compacted the collagen/fibrin matrix, leading to “dogbone” shaped microtissues suspended between the pillars in each microwell, as shown in Fig. 1c. The microtissues demonstrated a generally uniform distribution of ECM and cells, as shown in Fig. 1e,f, and actin stress fiber bundles aligned predominantly along the long axis of the tissues, as shown in Fig. 1g. As the microtissues formed, an axial stress developed in each tissue due to the collective forces generated by the cells, and the resulting deflections of the PDMS pillars anchoring the microtissues reported these forces. These endogenous forces within the microtissues reached a plateau after approximately 48 hours with average stress in the tissues’ central region (N = 21) (Supplementary Figs 1 and 2a), after which the mechanical experiments were performed.


Matrix viscoplasticity and its shielding by active mechanics in microtissue models: experiments and mathematical modeling
Magnetic microtissue platform for the study of the dynamics of self-assembled smooth muscle tissue constructs.(a) Schematic three quarters view showing a microtissue suspended between two flexible PDMS micropillars whose deflections report the microtissue’s contractile force. The wells are 400 μm × 800 μm × 125 μm deep. The flexible lower sections of the pillars were 30 μm × 170 μm in cross section and 85 μm high, and the pillars had spring constants k = 0.59 μN/μm for small lateral deflections. (b) A magnetic force FMag applied via a magnetic tweezer to a magnetic Ni sphere bonded to one of the pillars is used to apply time-varying strains to the microtissue. (c,d) show top-views of a SMC microtissue with (c) as grown, and (d) subjected to a 2% strain under FMag. = 28 μN. The Ni sphere appears as a black circle, and the tip of the magnetic tweezer is visible at the right edge of the images. 2D projected fluorescence confocal images of a microtissue show (e) collagen type-I, (f) nuclei, and (g) F-actin.
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Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC5037370&req=5

f1: Magnetic microtissue platform for the study of the dynamics of self-assembled smooth muscle tissue constructs.(a) Schematic three quarters view showing a microtissue suspended between two flexible PDMS micropillars whose deflections report the microtissue’s contractile force. The wells are 400 μm × 800 μm × 125 μm deep. The flexible lower sections of the pillars were 30 μm × 170 μm in cross section and 85 μm high, and the pillars had spring constants k = 0.59 μN/μm for small lateral deflections. (b) A magnetic force FMag applied via a magnetic tweezer to a magnetic Ni sphere bonded to one of the pillars is used to apply time-varying strains to the microtissue. (c,d) show top-views of a SMC microtissue with (c) as grown, and (d) subjected to a 2% strain under FMag. = 28 μN. The Ni sphere appears as a black circle, and the tip of the magnetic tweezer is visible at the right edge of the images. 2D projected fluorescence confocal images of a microtissue show (e) collagen type-I, (f) nuclei, and (g) F-actin.
Mentions: To measure the time-dependent biomechanical response of model tissues to external mechanical stimuli, we constructed arrays of engineered microtissues containing bovine pulmonary artery smooth muscle cells (SMCs), as shown in Fig. 1. Such microtissues have been used to measure a range of tissue properties using other cell types91011121314151617, but the potential for dynamic studies has not been fully explored. Arrays of microwells were fabricated in polydimethylsiloxane (PDMS). The microwells were 400 μm wide by 800 μm long by 125 μm deep, and each contained a pair of flexible T-shaped PDMS pillars that acted as microcantilevers, as shown in Fig. 1a. A magnetic Ni microsphere bonded to one cantilever in each well enabled application of force1115. A collagen/fibrin solution containing SMCs was introduced into the microwells at densities that yielded ~300 cells per well. The contractile action of the SMCs compacted the collagen/fibrin matrix, leading to “dogbone” shaped microtissues suspended between the pillars in each microwell, as shown in Fig. 1c. The microtissues demonstrated a generally uniform distribution of ECM and cells, as shown in Fig. 1e,f, and actin stress fiber bundles aligned predominantly along the long axis of the tissues, as shown in Fig. 1g. As the microtissues formed, an axial stress developed in each tissue due to the collective forces generated by the cells, and the resulting deflections of the PDMS pillars anchoring the microtissues reported these forces. These endogenous forces within the microtissues reached a plateau after approximately 48 hours with average stress in the tissues’ central region (N = 21) (Supplementary Figs 1 and 2a), after which the mechanical experiments were performed.

View Article: PubMed Central - PubMed

ABSTRACT

The biomechanical behavior of tissues under mechanical stimulation is critically important to physiological function. We report a combined experimental and modeling study of bioengineered 3D smooth muscle microtissues that reveals a previously unappreciated interaction between active cell mechanics and the viscoplastic properties of the extracellular matrix. The microtissues’ response to stretch/unstretch actuations, as probed by microcantilever force sensors, was dominated by cellular actomyosin dynamics. However, cell lysis revealed a viscoplastic response of the underlying model collagen/fibrin matrix. A model coupling Hill-type actomyosin dynamics with a plastic perfectly viscoplastic description of the matrix quantitatively accounts for the microtissue dynamics, including notably the cells’ shielding of the matrix plasticity. Stretch measurements of single cells confirmed the active cell dynamics, and were well described by a single-cell version of our model. These results reveal the need for new focus on matrix plasticity and its interactions with active cell mechanics in describing tissue dynamics.

No MeSH data available.


Related in: MedlinePlus