Achieving transient and reversible optical transparency in live mice with tartrazine

Optical imaging provides real-time visualization of tissues and cells at high spatial and temporal resolutions through techniques such as fluorescence microscopy, optical coherence tomography and photoacoustic imaging. However, overcoming light scattering, caused by mismatches in the refractive indices of tissue components such as water and lipids, still represents a major challenge, particularly when imaging through the thicker biological tissues of living animals. Despite advances in deep-tissue imaging, many optical methods struggle to achieve diffraction-limited resolution at depth or are unsuitable for use in live animals. Here we introduce a noninvasive approach to achieving transient and reversible optical transparency in live mice using absorbing dye molecules, using tartrazine as a representative example. Rooted in the fundamental physics of light-matter interactions, this approach enables reversible optical transparency in live animals and can be further applied ex vivo in freshly dissected tissues. In this Protocol, we detail the procedures for visualizing in vivo internal organs and muscle sarcomeres in the mouse abdomen and hindlimb through their respective transparency windows, showcasing a versatile approach for a variety of optical imaging applications in live animals. The entire protocol for an in vivo application can be implemented in just over 2 weeks by users with expertise in optical imaging and animal handling.