Progress on three-dimensional visualizing skin architecture with multiple immunofluorescence staining and tissue-clearing approaches

The skin forms the external covering of the body and is its largest organ, comprising many different cell types. Although the diversity of these cells has been widely studied with various histological methods, our understanding of skin architecture is mainly established on thin tissue sections, which restricted the information available to two dimensions. The development of innovative techniques to induce optical transparency ("clearing") in biological tissues has enabled researchers to visualize the three-dimensional reconstruction of intact organs and thick tissue sections at a cellular resolution. With the aid of tissue-clearing treatment, the labeled cutaneous nerve fibers and blood vessels can be followed for a longer distance on the thicker skin section or the whole mount skin under a fluorescence microscopy or a confocal microscopy. It is beneficial for demonstrating the morphological characteristics of nerve fibers and blood vessels themselves, as well as their spatial interconnection. In this review, we provide a brief summary of the literature on the use of tissue optical clearing methods and describe our experience of multiple fluorescent staining and tissue clearing approaches on thicker skin sections and whole-mount skin in our laboratory. Given the existing conventional methods, we expected to provide a more effective approach to comprehensively study skin architecture.