Researchers at Stanford University in the United States were able to look inside the body of a living mouse by dissolving a common, FDA-approved food dye, tartrazine, in water and applying it to the mouse’s abdomen and head to temporarily make the skin, muscles and tissues transparent. The mouse’s body then returns to normal and its exposed organs disappear once the dye is washed away with water.
Experiments
The researchers experimented with tissue-mimicking gels as well as with biological tissues outside the body. Using absorbent dye molecules, the researchers were able to transform the normally opaque abdomen of a living mouse into a transparent wall. This “transparent abdomen” allows direct visualization of enteric neurons labeled with fluorescent proteins, and enables monitoring of their movement, which mirrors the bowel movements of living mice.
The researchers also applied the dye solution topically to the mouse’s scalp to image blood vessels in the brain, and to the mouse’s hind limb to obtain high-resolution microscopic images of muscle fibers.
According to the Eurek Alert website, this approach provides opportunities to image the structure, functions and activities of deep tissues and organs without the need for surgical procedures to remove superficial tissue.
Optical imaging
Optical imaging is defined as a technique for examining internal body parts without surgery. Optical imaging relies on visible light and the properties of photons (light particles) to obtain detailed images of organs and tissues as well as small structures such as cells and molecules.
The challenge in trying to image biological material is that its complex structure causes opacity as a result of unwanted light scattering.
This scattering arises from the mismatch of refractive index between the components of biological tissue—remember, the refractive index is what determines the speed at which light travels through a medium other than a vacuum—which limits the depth of penetration of optical imaging. The desire to see inside biological tissues and uncover the fundamental processes of life has spurred extensive research into deep-tissue optical imaging methods, such as two-photon microscopy and optical tissue resolution.
However, these methods either lack sufficient penetration depth and resolution, or are not suitable for live animals. Therefore, the ability to achieve optical transparency in live animals holds the promise of transforming many optical imaging techniques.
The results of the study, published in the journal Science on September 6, provided an unexpected observation that molecules that strongly absorb light can achieve optical transparency in living animals.
Researchers explored the physical phenomenon behind this phenomenon, and found that light-absorbing molecules, when strongly dissolved in water, can modify the refractive index of the aqueous medium to match that of high-refractive-index tissue components such as fat.
Optical imaging greatly reduces patients’ exposure to harmful ionizing radiation such as gamma rays or X-rays. Because the use of non-ionizing radiation, which includes visible light, ultraviolet, and infrared light, is safer than techniques that require ionizing radiation, optical imaging can be used in repeat procedures to monitor disease progression or treatment outcomes.