TOKYO -- If you could make biological tissues transparent, it would open up a new world of scientific and medical advances. You could spot cancerous tumors at a glance, for example, and detect diseases at the earliest stage, at the cellular level.
Technologies to do just that are starting to appear, and the new methods look like they could be especially useful in basic research into the network of nerve cells within brains.
In July, a group of researchers led by the University of Tokyo professors Hiroki Ueda and Kohei Miyazono developed a technology to make the entire body of an adult mouse transparent. Ueda and others refined the transparency technique, called Cubic, announced in 2014, to make each organ more transparent. If it is programmed to produce a protein that fluoresces only in the presence of cancer cells, a researcher can tell immediately the location of cancerous tissue.
Until now, there has never been a whole-organ examination technology. With the current method, which examines samples of diseased tissues or organs taken from the patient's body for microscopic examination, cancer cells can easily be overlooked as the method only examines the most suspect areas with the unaided eye.
"Cubic will help us find the causes and develop treatments for diseases such as cancer and immune disorders," Ueda said.
Research into making biological specimens transparent dates back about 100 years. The method developed in Germany to create skeletal preparations by making muscles transparent used organic solvents that destroy protein.
After Japanese-born chemist Osamu Shimomura and others won the 2008 Nobel Prize in chemistry for research into fluorescent protein, a method that did not destroy protein was eagerly awaited.
A method known as Scale, which was developed in 2011 by a group of researchers led by Atsushi Miyawaki, deputy director of the Riken Brain Science Institute, marked a breakthrough. Composed of urea and glycerin, Scale uses water-soluble solvents and does not destroy protein.
An image showing a network of nerve cells inside a mouse brain down to several millimeters deeply shocked researchers around the world, and kicked off a race to develop a transparency technique.
Cubic, a sister version of Scale, turns glycerin into amino alcohol to increase the percentage of surfactants. It is now better able to make larger tissues transparent while protecting their protein.
In 2013, another Riken group developed SeeDB, a fructose-based solution.
New methods have been proposed by researchers worldwide -- Lucid, by a group of researchers led by University of Tokyo professor Hiroshi Onodera; Clarity, developed by Stanford University; and 3Disco, by a group of researchers at Vienna University of Technology, and others.
With Lucid tissue-clearing reagent, samples can be treated in 1-4 days. Scale and Cubic are not suitable for extended storage, as tissues dissolve gradually, but with Lucid tissues remain intact even after five years. Onodera, as a physician, said Lucid is a technology that can be used for pathological diagnosis.
Clarity received the most attention, as it was released by a well-known laboratory of Karl Deisseroth. It can only treat small samples, and is complicated and difficult to reproduce. Some research institutions prohibit its use, as it requires toxic solvents.
"The tissue-clearing technology market is getting crowded," Miyawaki said. Three-dimensional, whole-organ examination methods are likely to give a significant boost to medical and biological research, and the researcher who discovers them will receive worldwide acclaim if international standards can be established.
Cubic tissue-clearing reagents were released recently by Tokyo Chemical Industry. Japanese camera maker Olympus will cooperate in putting Scale to practical use, and Wako Pure Chemical Industries will handle the Scale reagents.
Many companies have shown interest, but Miyawaki has expressed skepticism about the market's rapid growth, citing many challenges.
For one, there are no objective indicators of whole-organ examination. The transparency of each technique is different, but cannot be compared with figures and other data. Furthermore, it is difficult to establish a standard method, since different samples are used in biological research.
Narrowing down the mass-production models, like those for semiconductor inspections, will make it easier for microscope makers to develop products. However, biological researchers usually refine light sources and lenses of microscopes according to their own subjects, making microscope manufacturers reluctant to develop new models.
Another technological breakthrough will be needed to spread the whole-organ clearing method.