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Overcoming the limits of the microscope
The resolution of a light microscope is limited. Physicists have long since worked out what these limits are and which parameters determine the spatial resolution. Many groups have nevertheless made numerous attempts to overcome these resolution limits. Rather than improving the power and quality of the microscope, Chen et al. instead expanded the biological specimens under study (see the Perspective by Dodt). They introduced a polymer gel into fixed cells and tissues and chemically induced swelling of the polymer by almost two orders of magnitude. They could then produce much higher-resolution images of their samples, which included the mouse hippocampus.
In optical microscopy, fine structural details are resolved by using refraction to magnify images of a specimen. We discovered that by synthesizing a swellable polymer network within a specimen, it can be physically expanded, resulting in physical magnification. By covalently anchoring specific labels located within the specimen directly to the polymer network, labels spaced closer than the optical diffraction limit can be isotropically separated and optically resolved, a process we call expansion microscopy (ExM). Thus, this process can be used to perform scalable superresolution microscopy with diffraction-limited microscopes. We demonstrate ExM with apparent ~70-nanometer lateral resolution in both cultured cells and brain tissue, performing three-color superresolution imaging of ~107 cubic micrometers of the mouse hippocampus with a conventional confocal microscope.