Expansion microscopy : improving imaging through uniform tissue expansion

Abstract

Until the past decade, optical microscopy of biological specimens was strongly limited by diffraction and scattering, affecting imaging resolution and depth, respectively. Now, numerous methods are available to overcome each of these limitations, but sub-diffraction limited resolution imaging over large volumes of scattering tissue is still a challenge. This work concerns the development of a new method, Expansion Microscopy (ExM) for achieving effect sub-diffraction-limited optical images in biological specimens. In ExM, the specimen is embedded in a swellable gel material to which fluorescent probes are chemically anchored. The embedded tissue is strongly digested so that it will not hinder uniform expansion driven by the gel. The gel with embedded, fragmented tissue is washed in water, triggering expansion of around 4-fold in each dimension. A variant of the method, ExM with Protein Retention (proExM) is presented that allows proteins themselves, rather than fluorescent probes, to be anchored by a small molecule cross-linker to the gel, so that the method may be carried out entirely with commercial components and standard antibodies.