Observing single cells in whole organs with optical imaging

Cells are the basic unit of human organs that are not fully understood. The revolutionary advancements of optical imaging allowed us to observe single cells in whole organs, revealing the complicated composition of cells with spatial information. Therefore, in this review, we revisit the principles of optical contrast related to those biomolecules and the optical techniques that transform optical contrast into detectable optical signals. Then, we describe optical imaging to achieve three-dimensional spatial discrimination for biological tissues. Due to the milky appearance of tissues, the spatial information blurred deep in the whole organ. Fortunately, strategies developed in the last decade could circumvent this issue and lead us into a new era of investigation of the cells with their original spatial information.