In vivo measurement of age-related stiffening in the crystalline lens by Brillouin optical microscopy

Abstract

The biophysical and biomechanical properties of the crystalline lens (e.g., viscoelasticity) have long been implicated in accommodation and vision problems, such as presbyopia and cataracts. However, it has been difficult to measure such parameters noninvasively. Here, we used in vivo Brillouin optical microscopy to characterize material acoustic properties at GHz frequency and measure the longitudinal elastic moduli of lenses. We obtained three-dimensional elasticity maps of the lenses in live mice, which showed biomechanical heterogeneity in the cortex and nucleus of the lens with high spatial resolution. An in vivo longitudinal study of mice over a period of 2 months revealed a marked age-related stiffening of the lens nucleus. We found remarkably good correlation (log-log linear) between the Brillouin elastic modulus and the Young's modulus measured by conventional mechanical techniques at low frequencies (~1 Hz). Our results suggest that Brillouin microscopy is potentially useful for basic and animal research and clinical ophthalmology.

Figure 1

Brillouin light scattering microscopy: principle, setup, and characterization. (a) Probing volume. (b) Conceptual energy diagram of photon-phonon interaction in spontaneous Brillouin scattering. (c) Schematic of the confocal microscope setup.

Figure 2

In vivo Brillouin imaging of the mouse eye. (a) Setup. (b) Representative Brillouin spectra taken along the optics axis of the eyeball at depths of 1550 μm (i), 1200 μm (ii), 950 μm (iii), and 550 μm (iv). (c) Brillouin elasticity map of a murine eye in vivo. Scale bar: 1 mm.

Figure 3

Age-related stiffening of the crystalline lens. (a) Axial Brillouin profile of the eye in 1-month-old and 18-month-old mice. (b) Peak frequency shifts of the lens nucleus in vivo measured from individual mice at various ages (solid circles) and one mouse over time (open circles).

Figure 4

Brillouin depth profiles along the optic axis of the porcine lens harvested from young (<1 month) and old (>6 months) animals.

Figure 5

Elastic moduli of young and old bovine lenses (central column) measured by Brillouin (a), longitudinal stress-strain (b), and dynamic shear (1 Hz) rheological instruments (c); p-value = <0.0001, 0.0012, and 0.0016, respectively.

Figure 6

Comparison of Brillouin longitudinal and quasi-static Young's moduli of tissue specimens cut from porcine lenses (a) and bovine lenses (b). Circles, experimental data; solid line, log-log linear fit.