Polarization MultiFocus Microscopy for volumetric super-resolution and orientation imaging of biofilaments
Accessing molecular orientation in single molecule localization microscopy (SMLM) offers valuable insights into molecular ordering and organization in biological structures. Conventional single-molecule orientation-localization microscopy (SMOLM) methods typically rely on either engineering the microscopes point-spread function (PSF) to encode the orientation information or on polarization resolved detection. While PSF engineering enables detailed orientation analysis, it often requires complex computational analysis and suffers from reduced performance in dense cellular environments due to PSF spreading and overlap. In contrast, polarization-based approaches are easier to implement and are more fit when imaging dense samples but are unable to retrieve the axial information of single molecules. To overcome this limitation, we introduce the Polarization MultiFocus Microscope (PolMFM), a novel method for simultaneously retrieving the orientation and 3D position of single molecules. PolMFM combines the orientation measurement capabilities of a 4-polarization splitting scheme with a 3-planes multifocus microscope (MFM) enabling the reconstruction of molecular 2D orientation, wobble, and axial localization in a single acquisition. Through simulations, we demonstrate that PolMFM accurately recovers both orientation and 3D position, despite PSF defocusing. Experimental validation with reference samples shows that PolMFM matches the orientation precision of 4-Polar STORM, while uniquely adding axial information. We demonstrate the power of PolMFM by resolving the orientation and 3D positions of molecules in actin filaments in fixed cells, and by revealing that chromatin in crickets undergoes major reorganization and increased ordering during spermiogenesis. These findings highlight the potential of PolMFM for high-precision, multidimensional super-resolution imaging in complex and crowded biological environments.