Chemical Processing Of Brain Tissues For Large-Volume, High-Resolution Optical Imaging

Imaging the brain circuitry in intact, three-dimensional context has been challenging due to the physical constraints including light scattering and slow-molecular diffusion inside biological tissues. Over the past few years, advanced clearing and labeling methods-broadly defined as 'chemical processing' of brain tissues-have emerged as promising strategies for multiplexed, large-scale imaging of brains. Tissue clearing techniques improve the accessibility of light and molecular probes, allowing for volumetric imaging without sectioning. Multiplexed labeling and signal amplification strategies enable precise detection, localization, and sometimes quantification of various biomolecules (e.g., RNA, protein) within a single tissue. The newly emerging, size-modulating clearing techniques increase imaging speed (via size reduction) or effective imaging resolution (via expansion) while rendering the samples transparent. Here we review the recent progress in the chemical techniques for brain tissue processing and overview the underlying principles for each technique. We further discuss important challenges and suggest directions for the future chemical tissue-processing techniques for high-resolution, large-scale brain imaging.