Protein-retention expansion microscopy: Improved sub-cellular imaging resolution through physical specimen expansion.

Super-resolution microscopy methods circumvent the classical diffraction limit of optical microscopy using combinations of specially engineered excitation light, fluorescent dyes, highly sensitive detectors, and reconstruction algorithms. Protein-retention expansion microscopy (ExM) is a method to physically expand biological specimens, enabling effectively sub-diffraction limited imaging on standard microscopes with standard staining reagents. Specimen expansion is driven by a swellable gel material that can be synthesized in situ using off-the-shelf chemicals and materials. The expansion material and process are robust and amenable to further development, which has enabled the emergence of numerous ExM variants with extended capabilities from multiple independent labs. The method presented here is useful for routine expansion of tissue slices and adherent or floating cultured cells, and also forms the basis for these variant methods.