Combining microscopy methods achieves superresolution imaging without fluorescent labels. Lu Wei and coworkers at the California Institute of Technology have combined stimulated Raman scattering (SRS) with expansion microscopy (Nat. Commun. 2021, DOI: 10.1038/s41467-021-23951-x). They call the approach vibrational imaging of swelled tissues and analysis (VISTA). It combines the molecular specificity of SRS with the spatial resolution of expansion microscopy.
In VISTA, the researchers embed biological tissue samples in a polymer hydrogel, expand the hydrogel in water, and image the sample using CH3 vibrations in proteins. When they followed conventional expansion microscopy protocols, which involves chemical cross-linking and protein digestion, a large fraction of the protein in the sample was lost. Instead of digesting the proteins, they denatured them, which preserved the Raman signal. The method achieved spatial resolution of 78 nm.
The researchers used VISTA to image various biological samples, including cells undergoing division, zebrafish embryonic retinas, and a mouse hippocampus. By using machine learning for data analysis, they were able to image multiple components in the mouse hippocampus.
The work is “a creative and effective combination of SRS with expansion microscopy,” says Renee F. Frontiera, a Raman microscopy expert at the University of Minnesota Twin Cities. “While SRS microscopy is likely not readily available for labs currently working in expansion microscopy, ideally this work will spur the development of easy-to-use commercial systems.”