Reactive oxygen FIB spin milling enables correlative workflow for 3D super-resolution light microscopy and serial FIB/SEM of cultured cells.

Correlative light and electron microscopy (CLEM) is a powerful tool for defining the ultrastructural context of molecularly-labeled biological specimens, particularly when superresolution fluorescence microscopy (SRM) is used for CLEM. Current CLEM, however, is limited by the stark differences in sample preparation requirements between the two modalities. For CLEM using SRM, the small region of interest (ROI) of either or both modalities also leads to low success rate and imaging throughput. To overcome these limitations, here we present a CLEM workflow based on a novel focused ion beam/scanning electron microscope (FIB/SEM) compatible with common SRM for imaging biological specimen with ultrahigh 3D resolution and improved imaging throughput. By using a reactive oxygen source in a plasma FIB (PFIB) and a rotating sample stage, the novel FIB/SEM was able to achieve several hundreds of micrometer large area 3D analysis of resin embedded cells through a process named oxygen serial spin mill (OSSM). Compared with current FIB mechanisms, OSSM offers gentle erosion, highly consistent slice thickness, reduced charging during SEM imaging, and improved SEM contrast without increasing the dose of post-staining and fixation. These characteristics of OSSM-SEM allowed us to pair it with interferometric photoactivated localization microscopy (iPALM), a recent SRM technique that affords 10-20 nm isotropic spatial resolution on hydrated samples, for 3D CLEM imaging. We demonstrate a CLEM workflow generalizable to using other SRM strategies using mitochondria in human osteosarcoma (U2OS) cells as a model system, where immunostained TOM20, a marker for the mitochondrial outer membrane, was used for iPALM. Owing to the large scan area of OSSM-SEM, it is now possible to select as many FOVs as needed for iPALM and conveniently re-locate them in EM, this improving the imaging throughput. The significantly reduced dose of post-fixation also helped to better preserve the sample ultrastructures as evidenced by the excellent 3D registration between OSSM-SEM and iPALM images and by the accurate localization of TOM20 (by iPALM) to the peripheries of mitochondria (by OSSM-SEM). These advantages make OSSM-SEM an ideal modality for CLEM applications. As OSSM-SEM is still in development, we also discuss some of the remaining issues and the implications to biological imaging with SEM alone or with CLEM.