Tissue clearing

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Tissue clearing refers to a group of chemical techniques used to turn tissues transparent.[1][2][3] This allows deep insight into these tissues, while preserving spatial resolution.[1] Many tissue clearing methods exist, each with different strengths and weaknesses.[2][4] Some are generally applicable, while others are designed for specific applications.[4] Tissue clearing is usually combined with one or more labeling techniques and subsequently imaged, most often by optical sectioning microscopy techniques.[1][5][6] Tissue clearing has been applied to many areas in biological research.[7]

History[edit]

In the early 1900s, Werner Spalteholz developed a technique that allowed the clarification of large tissues,[2][8] using Wintergrünöl (methyl salicylate) and benzyl benzoate.[9] Over the next hundred years, various scientists introduced their own variations on Spalteholz's technique.[8] Tuchin et al. introduced TOC in 1997, adding a new branch of tissue clearing that was hydrophilic instead of hydrophobic like Spalteholz's technique.[1][10] In 2007, Dodt et al. developed a two step process, wherein tissues were first dehydrated with ethanol and hexane and subsequently made transparent by immersion in benzyl alcohol and benzyl benzoate (BABB), a technique they coupled with light sheet fluorescence microscopy.[2][3] Hama et al. developed another hydrophilic approach, Scale, in 2011.[2][8] The following year, Ertürk et al. developed a hydrophobic approach called 3DISCO, in which they pretreated tissue with tetrahydrofuran and dichloromethane before clearing it in dibenzyl ether.[3][8] A year later, in 2013, Chung et al. developed CLARITY, the first approach to use hydrogel monomers to clear tissue.[2][3][8]

Principles[edit]

Tissue opacity is thought to be the result of light scattering due to heterogeneous refractive indices.[1][4][5] Tissue clearing methods chemically homogenize refractive indices, resulting in almost completely transparent tissue.[4][6]

Classifications[edit]

While multiple classification standards for tissue clearing exist, the most common classifications use the chemical principle and mechanism of clearing to group tissue clearing methods.[1] These include hydrophobic clearing methods,[1][2][6] which may also be known as organic,[3] solvent-based,[4][5] organic solvent-based,[11][12] or dehydration[13] clearing methods; hydrophilic clearing methods,[1][2][6] which may also be known as aqueous-based[5][11] or water-based[13] methods, and may be further sub-categorized into simple immersion[4] and hyperhydration[4] (also called delipidation/ hydration[5]); and hydrogel-based clearing methods, which may also be known as detergent[3] or hydrogel embedding[4][5][11] methods. Tissue-expansion clearing methods use hydrogel, and may be included under hydrogel-based clearing[2] or as their own category.[1]

Methods[edit]

Common methods include those of the DISCO family, including 3DISCO, and CLARITY and related protocols.[1][2][3] Others include BABB,[1][2][4] PEGASOS,[1][2][4] SHANEL,[1][5] SeeDB,[1][2][4] CUBIC,[1][2][4] ExM,[1][2][4] and SHIELD.[1][4][5]

Labeling[edit]

Tissue clearing methods have varying compatibility with different methods of fluorescent labeling.[1][5][6] Some are better suited to pre-clearing tagging approaches, such as genetic labeling.[1][5] while others require post-clearing tagging, such as immunolabeling and chemical dye labeling.[1][5]

Imaging[edit]

After clearing and labeling, tissues are typically imaged using confocal microscopy,[11][12][13] two-photon microscopy,[1][5][11] or one of the many variants of light-sheet fluorescence microscopy.[7][11][12] Other less commonly used methods include optical projection tomography[1][5] and stimulated Raman scattering.[5][7][11]

Data[edit]

Imaging cleared tissues generates massive volumes of complex data, which requires powerful computational hardware and software to store, process, analyze, and visualize.[1][6][13] A single mouse brain can generate terabytes of data.[2][6][13] Both commercial and open-source software exists to address this need, some of it adapted from solutions for two-dimensional images and some of it designed specifically for the three-dimensional images produced by imaging of cleared tissues.[1][11][12]

Applications[edit]

Tissue clearing has been applied to the nervous system,[1][2][3][4][5][6][7][11][14][15] bones (including teeth),[7][11][12][16][17][18] skeletal muscles,[7][18][19] hearts and vasculature,[7][11][20] gastrointestinal organs,[7][21] urogenital organs,[7][11][22] skin,[7][23] lymph nodes,[7] mammary glands,[7] lungs,[7] eyes,[7] tumors,[7][11] and adipose tissues.[7][11] Whole-body clearing is less common, but has been done in smaller animals, including rodents.[1][6][7] Tissue clearing has also been applied to human cancer tissues [24][25]

References[edit]

  1. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z Zhao J, Lai HM, Qi Y, He D, Sun H (January 2021). "Current Status of Tissue Clearing and the Path Forward in Neuroscience". ACS Chemical Neuroscience. 12 (1): 5–29. doi:10.1021/acschemneuro.0c00563. PMID 33326739. S2CID 229300600.
  2. ^ a b c d e f g h i j k l m n o p q Ueda HR, Dodt HU, Osten P, Economo MN, Chandrashekar J, Keller PJ (May 2020). "Whole-Brain Profiling of Cells and Circuits in Mammals by Tissue Clearing and Light-Sheet Microscopy". Neuron. 106 (3): 369–387. doi:10.1016/j.neuron.2020.03.004. PMC 7213014. PMID 32380050.
  3. ^ a b c d e f g h Vigouroux RJ, Belle M, Chédotal A (July 2017). "Neuroscience in the third dimension: shedding new light on the brain with tissue clearing". Molecular Brain. 10 (1): 33. doi:10.1186/s13041-017-0314-y. PMC 5520295. PMID 28728585.
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  9. ^ Spalteholz W (1914). Über das Durchsichtigmachen von menschlichen und tierischen Präparaten und seine theoretischen Bedingungen, nebst Anhang: Über Knochenfärbung. Leipzig: S. Hirzel.
  10. ^ Tuchin VV, Maksimova IL, Zimnyakov DA, Kon IL, Mavlyutov AH, Mishin AA (October 1997). "Light propagation in tissues with controlled optical properties". Journal of Biomedical Optics. 2 (4): 401–17. Bibcode:1997JBO.....2..401T. doi:10.1117/12.281502. PMID 23014964.
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