Spatial mapping of the mechanical properties of the living retina using scanning force microscopy

The retina is an active soft material, in which mechanosensitive cells are thought to respond to the local mechanical heterogeneity they encounter during development and adult physiological functioning. The retina is also constantly exposed to mechanical stress with shear and traction forces acting at its inner surface. Consequences of these forces depend on the tissue's resistance to deformation, which is characterized by its stiffness. However, currently there is a lack of high-resolution data on retinal mechanical properties. Here, we used scanning force microscopy to determine the apparent elastic modulus K of the retinal inner surface along the length of the eye with sub-millimetre resolution, and compared characteristic K values of the retinal quadrants. We found that the inner retina is a mechanically inhomogeneous tissue. Most elastic moduli were in the range of 940 to 1800 Pa; significant differences were found between areas less than 50 mu m apart. To identify the origin of this mechanical inhomogeneity, we investigated the size and distribution of structures comprising the retinal surface: large cell bodies in the ganglion cell layer, nerve fibers, inner limiting membrane, and Muller cell endfeet. Our data suggest that the distribution of compliant nerve fiber bundles and stiff neuronal cell bodies contributes most to the mechanical properties of the inner retina. These data offer a basis for understanding cellular mechanoresistivity and -sensitivity in the retina as a mechanically active tissue, and they may help to understand mechanisms and consequences of a variety of retino-pathological processes and their surgical treatment.