Microscale Interrogation of 3D Tissue Mechanics

Abstract

Cells in vivo live in a complex microenvironment composed of the extracellular matrix (ECM) and other cells. Growing evidence suggests that the mechanical interaction between the cells and their microenvironment is of critical importance to their behaviors under both normal and diseased conditions, such as migration, differentiation, and proliferation. The study of tissue mechanics in the past two decades, including the assessment of both mechanical properties and mechanical stresses of the extracellular microenvironment, has greatly enriched our knowledge about how cells interact with their mechanical environment. Tissue mechanical properties are often heterogeneous and sometimes anisotropic, which makes them difficult to obtain from macroscale bulk measurements. Mechanical stresses were first measured for cells cultured on two-dimensional (2D) surfaces with well-defined mechanical properties. While 2D measurements are relatively straightforward and efficient, and they have provided us with valuable knowledge on cell-ECM interactions, that knowledge may not be directly applicable to in vivo systems. Hence, the measurement of tissue stresses in a more physiologically relevant three-dimensional (3D) environment is required. In this mini review, we will summarize and discuss recent developments in using optical, magnetic, genetic, and mechanical approaches to interrogate 3D tissue stresses and mechanical properties at the microscale.

Keywords: active microrheology; elasticity; extracellular matrix; stress sensor; tension sensor; traction stress.

Figure 1

Overview of technologies for probing microscale cellular and tissue mechanics. (A) Schematic of a tissue composed of different types of cells (brown, yellow, blue, green) and ECMs (pink). (B) Schematic of AFM probing of surface mechanics. (C) Schematic of optical tweezer probing of ECM mechanics. (D) Schematic of probing tissue mechanics from propagating waves within the tissue either directly by measuring wave speed through imaging or indirectly by measuring the BFS from light scattering. (E) Schematics demonstrating the measurement of released solid stresses after physical tissue incision (top-left). The deformations of the cut plane (top-right) are visualized with high-resolution ultrasonic or optical imaging, and the normal stresses perpendicular to the cut plane (σ_zz) are computed from finite element modeling (bottom). (F) Schematic of ECM deformations/displacements (green arrows) measured using embedded fluorescent beads (red dots) around a cell in 3D (blue). (G) Schematics of a cell-sized elastic bead or incompressible droplet (red) embedded within a cell cluster (top-left) and the normal stresses experienced by the bead/droplet sensor (bottom-right). (H) A representative image showing the distribution of optical retardance in an invading ex vivo breast tumor organoid as revealed by quantitative polarized microscopy, adapted from Wang et al. (2018a) with permission. Scale bar, 25 μm. (I) Schematic showing fluctuating tracers (green) in the cytoplasm (pink) or nucleus (blue) with superimposed trajectories (black), from which an MSD-lag time (τ) plot characteristic of intracellular forces can be obtained (bottom-left). (J) Schematic of FRET-based stress sensors. (K) Schematic of the FliptR membrane tension probe (blue, left) which planarizes under pressure (red, right) from the neighboring lipid chains (green/ yellow).