In this blog post, we’ll explore the innovations CLARITY technology has brought to biological research, as well as its principles and potential applications.
What if the brain could be made transparent? If we could stain specific substances within a transparent brain, we could easily observe how those substances function and are distributed within the brain. In the past, such observations were extremely difficult in biological research. However, in 2013, a very simple yet groundbreaking technology was announced.
CLARITY. While it is an English word meaning “clarity,” it is also the name of a well-known tissue clearing technique that anyone involved in biological research has likely heard of at least once.
CLARITY is a technique that makes brain tissue transparent, allowing the interior of the tissue to be observed under a fluorescence microscope. What makes this technique special is that it overcomes the limitation of tissue size. In the past, biological tissues or organs had to be cut into sections thinner than a certain thickness to be observed. For example, to study the brain, researchers had to divide it into numerous sections, examine each one, and then piece the results back together like a puzzle. However, CLARITY makes the entire brain transparent, allowing the entire tissue to be observed without the need to create sections.
Furthermore, CLARITY is highly compatible with existing staining methods. Biologists have long used antibody staining to observe specific proteins. This method employs antibodies that selectively bind to specific proteins, much like a key fits a lock. By staining the target protein with antibodies on tissue made transparent by CLARITY, researchers can determine how that protein is distributed within the tissue.
What makes CLARITY even more groundbreaking is that the tissue structure remains intact even after repeated rounds of antibody staining. After staining the desired protein with an antibody, the unbound antibody can be removed, and another antibody can then be used to stain a different protein. Even if this process is repeated multiple times, the tissue’s structure remains intact, allowing for the sequential observation of the distribution of various proteins within a single tissue sample.
Despite the technology’s greatness, the principle is surprisingly simple. Formaldehyde is generally used to fix cells. In CLARITY, formaldehyde is used in combination with hydrogel monomers to embed intracellular proteins and nucleic acids within a hydrogel structure.
As a result, the cell’s structure remains intact even after the lipids that make up the cell membrane are removed. Subsequently, when lipids are removed from the tissue, the primary cause of light scattering disappears, and the tissue becomes transparent.
Even during this process, proteins, DNA, and RNA embedded in the hydrogel retain their original positions. Therefore, various biomolecules can be observed at their precise locations while preserving the tissue’s structure. This is the core principle of CLARITY technology.
CLARITY can be applied to nearly all tissues containing cells. Consequently, not only the central nervous system but also various other tissues, such as the thyroid gland, can be observed three-dimensionally at the cellular level. Its applications are not limited to neurobiology. For example, by selectively staining only cancer cells in the tissues of laboratory animals, researchers can determine exactly where the cancer cells are located and the pathways along which they spread.
Currently, CLARITY is being utilized in various forms in laboratories around the world, and with the subsequent development of numerous improvements, it has established itself as a key foundational technology in tissue clearing research. Biology has advanced significantly through technological innovations such as PCR, and CLARITY is also regarded as one of the leading technologies driving such innovation. This simple yet powerful technique provides scientists with a new perspective and helps them understand biological phenomena that were previously invisible with greater precision.
