Until widespread international travel is allowed again, you may find yourself looking longingly at photographs of your dream holiday destination. But as we all know, photographs are poor substitutes for the real thing. The same holds true for the study of skin anatomy.
Histology, the go-to technique for imaging skin, involves staining thin tissue slices to be viewed under a light microscope. Much like a photograph, this two-dimensional slice cannot fully capture the skin’s three-dimensional nature or how cells within interact, posing difficulties in studying structures like tumors that encompass multiple skin layers.
In contrast, three-dimensional (3D) tissue imaging can produce a more comprehensive view of the skin and its internal structures. However, these same structures are what make thicker skin sections trickier to image. Though light typically travels in a straight line, it can scatter upon contact with biomolecules for the same reason why a pencil looks bent in water—differences in what is called the refractive index. Accordingly, light scattering results in blurry, low-quality images.
To get around this, tissues can be prepared with optical clearing, a process that renders tissues transparent. In a review published in the Journal of Investigative Dermatology, researchers from A*STAR's Singapore Immunology Network (SIgN) in collaboration with the National Skin Centre (NSC) discuss a range of optical clearing methods and 3D imaging techniques.
“These techniques now make it possible for us to understand tissue structure and organization and the complexity of cell-cell and cell-tissue interactions in 3D,” said corresponding author Lai Guan Ng, a Principal Investigator at SIgN.
To turn tissues transparent, optical clearing methods replace the water in cells and tissue with substances that more closely match the refractive indices of biological components—allowing light to pass through without scattering. However, optical clearing is a laborious process that can take up to two weeks.
According to Ng, automating tissue preparation and finding faster staining alternatives, such as smaller dye particles that can diffuse faster into the tissue, could ease these limitations. As current approaches are also unable to clear light-absorbing pigments like melanin, Ng emphasized the need to bleach more pigmented skin samples without damaging them.
The images and atlases that these 3D imaging technologies can produce will be foundational in understanding the anatomy of the skin and other organs. “The structure of an organ is closely tied to its function,” said Ng. “Hence, viewing 3D tissue structure in homeostasis would help us comprehend how a disease affects organ function and how to address those changes in treatment.”
The A*STAR-affiliated researchers contributing to this research are from the Singapore Immunology Network (SIgN).
