Biomedical applications of tissue clearing and three-dimensional imaging in health and disease

Three-dimensional (3D) optical imaging techniques can expand our knowledge about physiological and pathological processes that cannot be fully understood with 2D approaches. Standard diagnostic tests frequently are not sufficient to unequivocally determine the presence of a pathological condition. Whole-organ optical imaging requires tissue transparency, which can be achieved by using tissue clearing procedures enabling deeper image acquisition and therefore making possible the analysis of large-scale biological tissue samples. Here, we review currently available clearing agents, methods, and their application in imaging of physiological or pathological conditions in different animal and human organs. We also compare different optical tissue clearing methods discussing their advantages and disadvantages and review the use of different 3D imaging techniques for the visualization and image acquisition of cleared tissues. The use of optical tissue clearing resources for large-scale biological tissues 3D imaging paves the way for future applications in translational and clinical research.This work was supported by Ministerio de Ciencia, Innovación y Universidades, ISCIII-FIS grants PI18/00462 co-financed by ERDF, European Union (FEDER) Funds from the European Commission, European Union,"A way of making Europe"; the CNIC is supported by the Ministerio de Ciencia, Innovación y Universidades y the Pro CNIC Foundation, Severo Ochoa Center ofExcellence (SEV-2015-0505), CIBER de Salud Mental(CIBERSAM), and COST-action CA16124. J.R. acknowledges funding from EU H2020 FET Open projectSENSITIVE, ID 801347, and Ministerio de Ciencia, Innovación y Universidades Grant FIS2016-77892-R. Allapplicable international, national, and/or institutional guidelines for the care and use of animals were fol-lowed. All procedures performed in studies involving animals were in accordance with the ethical standardsof the institution or practice at which the studies were conducted