Yu, C.-C. et al. Elife 9, e46249 (2020).
Expansion microscopy (ExM) is a method whereby samples are physically expanded using embedded, swellable hydrogels. Upon imaging using standard microscopes, the image resolution achieved is improved by the degree of change in sample size, meaning, for example, that a fourfold linear expansion improves resolution fourfold, yielding super-resolution images. ExM has been used extensively on tissues and cells. Now Yu et al. have developed a workflow to apply ExM to Caenorhabditis elegans. The method, called ExCel, modifies previous protocols and incorporates steps that deal with the worm’s cuticle, which is too stiff to allow expansion and is impenetrable to many small molecules. The researchers show they can expand intact worms and achieve ~70-nm resolution using a protocol with ~4.5-fold linear expansion. They further show they can image proteins, RNA and DNA, and present protocols that are compatible with fluorescent protein imaging and endogenous epitope imaging. They also describe iterative ExCel, which allows 20-fold linear expansion.