CLARITY (microscopy)

CLARITY (microscopy). CLARITY is a transformative Tissue clearing technique in Biomedical imaging that renders biological specimens optically transparent while preserving their molecular and structural integrity. Developed by a team led by Kwanghun Chung and Karl Deisseroth, it enables high-resolution, three-dimensional visualization of intact organs and organisms using Light-sheet fluorescence microscopy and Confocal microscopy. The method has revolutionized Connectomics and Systems neuroscience by allowing researchers to map complex neural circuits within the Mouse brain and Human brain.

Overview

The core innovation of CLARITY lies in its ability to remove light-scattering Lipids from tissue while supporting the Extracellular matrix and Proteins within a transparent Hydrogel mesh. This process converts opaque samples, such as an entire Rodent brain, into a transparent, macromolecule-permeable structure compatible with multiple rounds of Immunohistochemistry. The technique was first published in the journal Nature (journal) and has since been widely adopted by laboratories at institutions like the Allen Institute for Brain Science and the Janelia Research Campus. Its development addressed a major bottleneck in Neuroanatomy, where traditional Histology required destructive sectioning.

Method

The CLARITY protocol involves several key chemical and physical steps. First, tissue is infused with Acrylamide, Bis-acrylamide, and a Formaldehyde-based fixative, which are then thermally polymerized to form a hydrogel-tissue hybrid. This hybrid is then subjected to active Electrophoresis in a chamber filled with Sodium dodecyl sulfate to remove lipids, a process termed Eletrophoretic tissue clearing. Alternatively, a passive diffusion method can be used. The resulting cleared tissue is refractive-index matched using solutions like FocusClear or RIMS before imaging. Subsequent staining with Antibodies targeting proteins like Green fluorescent protein or Parvalbumin allows for specific labeling, and the hydrogel matrix permits antibody removal for multi-round staining.

Applications

CLARITY has found extensive application in Neuroscience, particularly for mapping the Connectome of model organisms like the Mouse and Zebrafish. It has been used to trace long-range Axonal projections, study the pathology of Alzheimer's disease in post-mortem Human brain tissue, and investigate Tumor microenvironments in Cancer research. Beyond the brain, it has been applied to whole organs such as the Heart, Lung, and Pancreas in studies of Diabetes and Developmental biology. Collaborations with advanced imaging platforms, such as those at the Howard Hughes Medical Institute, have enabled the creation of comprehensive cellular atlases.

Advantages and limitations

The primary advantage of CLARITY is its ability to provide integrated structural and molecular information from intact specimens, preserving delicate architectures like Dendritic spines and Blood vessel networks. It is compatible with a wide array of Fluorescence microscopy techniques and enables repeated interrogation of the same sample. However, limitations include the lengthy time required for lipid clearing and antibody penetration, potential for tissue expansion or distortion, and the technical complexity of the Electrophoresis setup. Compared to other clearing methods like CUBIC or IDISCO, CLARITY can be more labor-intensive but often offers superior protein retention for immunostaining.

Development and variants

Following its initial description by Kwanghun Chung's laboratory at Massachusetts Institute of Technology, numerous variants have been developed to improve speed, scalability, and compatibility. These include PACT and PARS, which modify the hydrogel composition and clearing mechanism for specific tissues. The Chung Lab also introduced SWITCH, a technique for controlling fixation and staining kinetics. Commercial kits, such as those from Logos Biosystems, have standardized the protocol. The ongoing development of CLARITY is closely linked with advances in Light-sheet fluorescence microscopy hardware, including systems from Carl Zeiss AG and LaVision BioTec, enabling faster imaging of large, cleared samples.

Category:Microscopy Category:Neuroimaging Category:Laboratory techniques