Optimization of Tissue Clearing Methods and Imaging Conditions for 3D Visualization of the Vasculature of the Adult Murine Knee.

The vasculature is an essential regulator of tissue oxygenation, metabolism, and immune surveillance under developmental and homeostatic conditions, as well as in regeneration and disease. Within the musculoskeletal system, blood vessels are involved in bone development and resorption, and they also mediate the inflammatory processes that contribute to diseases affecting the joints, including osteoarthritis. Historically, visualization of vascular networks in joints has been limited to conventional histological approaches due to technical limitations and the inherit structure of the skeletal muscle and bones. Recent advances in optical tissue clearing technologies and 3D fluorescence imaging approaches enabling three-dimensional analysis in whole, intact tissues offer an entrée to interrogate musculoskeletal development and disease at a cellular resolution. However, these techniques were not originally developed to image hard mineralized tissues, such as the femur and tibias. To circumvent these challenges, we have optimized tissue decalcification conditions and systematically compared aqueous- and solvent-based tissue clearing approaches to establish an optimal pipeline for clearing and imaging of mineralized tissues with superior resolution of the vasculature. Collectively, this work shows that optical clearing and lightsheet microscopy presents a viable method for generating high-resolution images of the murine hindlimb vasculature, with potential applications in aging and disease modeling.