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, 115 (5), 861-866

Ultrafast Imaging of Cell Elasticity With Optical Microelastography

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Ultrafast Imaging of Cell Elasticity With Optical Microelastography

Pol Grasland-Mongrain et al. Proc Natl Acad Sci U S A.

Abstract

Elasticity is a fundamental cellular property that is related to the anatomy, functionality, and pathological state of cells and tissues. However, current techniques based on cell deformation, atomic force microscopy, or Brillouin scattering are rather slow and do not always accurately represent cell elasticity. Here, we have developed an alternative technique by applying shear wave elastography to the micrometer scale. Elastic waves were mechanically induced in live mammalian oocytes using a vibrating micropipette. These audible frequency waves were observed optically at 200,000 frames per second and tracked with an optical flow algorithm. Whole-cell elasticity was then mapped using an elastography method inspired by the seismology field. Using this approach we show that the elasticity of mouse oocytes is decreased when the oocyte cytoskeleton is disrupted with cytochalasin B. The technique is fast (less than 1 ms for data acquisition), precise (spatial resolution of a few micrometers), able to map internal cell structures, and robust and thus represents a tractable option for interrogating biomechanical properties of diverse cell types.

Keywords: cell biomechanics; cell biophysics; cell elasticity imaging; elastography imaging; shear wave imaging.

Conflict of interest statement

Conflict of interest statement: G.C., S.C., P.G.-M., and A.Z. filed a patent supported jointly by the University of Montreal and University of Montreal Hospital and INSERM on the technology reported in this manuscript.

Figures

Fig. 1.
Fig. 1.
Illustration of the experiment (Left: picture; Right: scheme). A cell placed in a Petri dish is held by a holding pipette and vibration is applied using a second pipette attached to a piezo-drive unit. Vibration is applied to the zona pellucida of the oocyte. Images of the cell are acquired by a high-speed camera through a microscope.
Fig. 2.
Fig. 2.
Experimental (A) and simulated (B) Y-displacement maps, at t = 15, 30, 45, and 60 μs, respectively, superimposed on the optical images of the cell. Displacements with amplitude approximately from −2.4 to 2.4 μm propagating from the left vibrating pipette toward the right side of the cell can be observed.
Fig. 3.
Fig. 3.
(A) Elasticity map estimated from experimental displacements superimposed on the microscopy image (A-1) and corresponding distribution of elasticity with median values (A-2) of a mouse oocyte. (B) Elasticity map estimated from simulated displacements superimposed on the microscopy image (B-1) and corresponding distribution of elasticity with median values (B-2) of a soft medium mimicking a mouse oocyte. Nucleus, cytoplasm, zona pellucida, and extracellular fluid can be easily distinguished on both images; artifacts are observed in the zona pellucida. (C) Median shear moduli for successive measures within different zones (cytoplasm, nucleus, zona pellucida, and extracellular fluid). No time evolution is observed. (D) Average median shear moduli of the whole cell among 23 successive measurements as a function of the number of optical images used. Error bars correspond to the SD among successive measurements. (E) Effect of the vibration amplitude on oocyte shear modulus median, obtained by averaging 10–23 measures. Error bars correspond to the SD among successive measurements. Pearson’s correlation coefficient R is too low to show any correlation between the measured shear modulus and the vibration amplitude.
Fig. 4.
Fig. 4.
Elasticity map superimposed on the microscopy image (A-1 and B-1) and corresponding distribution of elasticity with median values (A-2 and B-2) of a normal mouse oocyte (A) and a mouse oocyte softened by cytochalasin B (B). Elasticity decreased in all functional areas. (C) Box plot of whole oocyte shear modulus without and with cytochalasin B, showing a significant decrease in elasticity. Elasticity map superimposed on the microscopy image (1) and corresponding distribution of elasticity with median values (2) of a germinal vesicle-stage oocyte (D), a two-cell mouse embryo (E), and a four-cell mouse embryo (F).

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