Sometimes you spend years working on a project and then, right as you are about to share your progress with the world, someone else beats you to it. I’d imagine Meng-Tsen Ke, Satoshi Fujimoto, and Takeshi Imai were feeling pretty disgruntled in June when the Deisseroth lab published their technique for making brain tissue optically clear. The press coverage of CLARITY was immense, I even wrote a post about it. But it turns out while we were all drooling over clear brains, another group was coming up with a cheaper and easier way to make brain tissue see-through.
The Imai group at RIKEN in Japan developed a new technique called SeeDB (See Deep Brain) that makes brain tissue optically clear. This technique relied on finding an aqueous (water-based) solution with a refractive index close to that of fixed tissue. In the past, solutions with high concentrations of sucrose have been used to reduce light scattering in brain tissue. To optimize this process they changed the sugar to fructose and found even more success. The high fructose solution provided optimal tissue clearance and caused very little tissue shrinkage.
However, in order to get the fructose solution to soak into very large tissues, some heat must be applied. An unfortunate side effect of this heat is the activation of the Maillard reaction (browning) which we happen to love on our steaks, but is not so great in tissue we are trying to make as clear as possible. To stop this from happening, a reducing agent is added to the high fructose solution.
Soaking the tissue in increasing amounts of fructose, and finally in a fructose/reducing agent solution until the tissue is clear takes approximately three days. Other passive tissue clearing methods can take weeks. This technique is far more simple and much cheaper than the CLARITY method. While the results are not quite as perfect, they are certainly sufficient to image to great depths in larger pieces of brain tissue. This technique will enable labs with less money and equipment to continue to explore neural structures deep within the brain. So even if all the fanfare over clear brains has passed, I think this second fiddle technique will be important to the field of neuroscience and widely adopted very soon.