In this blog post, we will look at how high-resolution imaging will change brain science and disease research through tissue transparency technology.
The human brain is one of the most complex organs in the body. The number of neurons (the basic unit of cells in the nervous system) in the brain is about 100 billion, which is similar to the number of stars in our galaxy, the Milky Way. As such, the human brain is a complex network of a very large number of cells, and its structure and function are not fully understood. The brain also plays a central role in regulating higher-order cognitive functions such as sensation, memory, learning, and thinking, and plays an important role in the formation of human identity. Therefore, understanding the principles of how the brain works is the same as understanding the principles of human thought, emotion, and behavior. To understand such a complex biological system, it is necessary to identify the characteristics and functions of the cells that make up the brain, as well as their arrangement and connection structure. In other words, a high-resolution three-dimensional map, which is a human body schematic with cell-level resolution, is needed. We would like to take a look at tissue transparency technology, which is being hailed as a breakthrough technology in the creation of such high-resolution three-dimensional maps.
Before looking at what tissue transparency technology is, let’s take a look at the background of how such technology was developed. Various scientists have been working on creating high-resolution human body maps for tissue research. Previously, tissues such as the brain have been imaged using methods such as computed tomography (CT), magnetic resonance imaging (MRI), or optical coherence tomography (OCT). These methods were effective for studying the structure and function of living human tissues because they could be imaged in three dimensions. However, they have limitations in that they cannot show the characteristics of each cell or the structure of the connections because the resolution is not high enough, and it is difficult to obtain information at the molecular level. This is because a kind of barrier is formed due to the high density of cells, preventing light and chemicals from entering the tissue. In particular, when cells in human tissues are too densely packed, it is difficult to see fine cell-to-cell interactions or neural network connectivity at a glance due to the lack of permeability. Therefore, at present, the method of using a high-resolution microscope that can observe even the smallest cell units in living tissues is considered the most advanced method for creating three-dimensional maps.
However, when using a high-resolution microscope to observe large, opaque tissues such as the brain, there is a limit to the ability to cut the tissue into very thin slices and image each slice. This is because the entire process of reassembling thousands of two-dimensional images into a three-dimensional map is time-consuming. Many studies have been conducted to solve this problem, and tissue transparency technology has recently been developed. In this context, the potential of tissue transparency technology is enormous. It has opened up the possibility of more sophisticated basic research and diagnosis of complex diseases such as cancer, cardiovascular disease, and brain disease through precise imaging at the cellular and molecular levels, as well as life science research.
Tissue transparency technology is a technology that makes opaque tissues transparent with a special chemical substance, which has paved the way to overcome the limitations of methods using high-resolution microscopes. Among them, the recently developed CLARITY technology allows for observations with a resolution that is about 2,000 times better than that of magnetic resonance imaging. CLARITY technology is a technology that can synthesize a transparent and porous polymeric mesh called “hydrogel” within the tissue to preserve the tissue structure and molecules in three dimensions, while completely removing the lipids that make the tissue opaque. It not only makes the tissue very transparent, but also completely removes the barriers that prevent the penetration of light and molecular probes, allowing light and molecular probes to easily penetrate the tissue. The key to CLARITY technology is that the detailed shape of cells and the structure of connections between cells are well preserved because the hydrogel preserves the three-dimensional information of tissues at the molecular level, even though barriers such as cell walls have been completely removed.
Therefore, CLARITY-treated tissues become optically transparent, allowing light to penetrate deeply, and even thick tissues such as the brain can be imaged with a high-resolution microscope without sectioning. Another advantage of CLARITY technology is that it can image the three-dimensional distribution of specific molecules by staining molecules in tissues that are linked to the hydrogel by covalent bonds with organic dyes. In addition, since the organic dyes can be removed without destroying the structure or molecules of the tissue, various molecular phenotypes can be analyzed through repeated analysis. This allows researchers to observe more vividly how cells and tissues interact in a living environment, and in particular, to capture changes in tissues when a specific disease develops.
Furthermore, CLARITY technology can make most organs transparent, enabling three-dimensional imaging, which can be used not only in brain science but also in organ disease research through tissue testing. In particular, it will be possible to use CLARITY to record the activity of specific disease-causing agents in the organ and the resulting tissue response in real time. Therefore, the development of these technologies will not only greatly expand the scope of medical and scientific research, but will also play an important role in the development of precision medical techniques.
The CLARITY technology does not have only rosy prospects. There are still some challenges to be solved. The first challenge is to speed up the processing. Unlike analyzing thin sections, it takes months to process large samples such as mouse brains. This is because the process of deeply delivering the chemicals necessary for tissue preservation, transparency, and dyeing is very slow. Follow-up studies are being conducted to solve this problem, and recent studies in Korea have shown results for the Active Clarity Technique (ACT), which is up to 30 times faster than CLARITY. The second challenge is to reduce costs. As the size of the organization to be analyzed increases, not only does more compounding become necessary to achieve transparency, but the amount of organic dye required to obtain molecular information also increases. In addition, the technical burden of processing large amounts of data remains a challenge to be addressed. Overcoming these limitations of low speed and high cost will enable the commercialization of 3D map technology.
So far, we have looked at the background of the development of tissue transparency technology, CLARITY technology, and the challenges it faces. What impact will this CLARITY technology have on humanity? Since the double helix structure of DNA was revealed by Watson and Crick, the Human Genome Project has now made it possible to complete the human genetic map by revealing the gene sequence. Furthermore, CLARITY technology will greatly contribute to the completion of high-resolution human body maps, which will provide answers to how genetically expressed cells are connected to each other. Scientists expect that the development of such technology will further reveal the secrets of the brain. In the near future, the structure and function of brain neurons will be elucidated, and a future in which incurable brain diseases such as Alzheimer’s and Parkinson’s are treated will unfold.
