Third harmonic generation microscopy of cells and tissue organization.

The interaction of cells within their microenvironmental niche is fundamental to cell migration, positioning, growth and differentiation in order to form and maintain complex tissue organization and function. Third harmonic generation (THG) microscopy is a label-free scatter process that is elicited by water-lipid and water-protein interfaces, including intra- and extracellular membranes, and extracellular matrix structures. In applied life sciences, THG delivers a versatile contrast modality to complement multi-parameter fluorescence, second harmonic generation and fluorescence lifetime microscopy, which allows detection of cellular and molecular cell functions in three-dimensional tissue culture and small animals. In this Commentary, we review the physical and technical basis of THG, and provide considerations for optimal excitation, detection and interpretation of THG signals. We further provide an overview on how THG has versatile applications in cell and tissue research, with a particular focus on analyzing tissue morphogenesis and homeostasis, immune cell function and cancer research, as well as the emerging applicability of THG in clinical practice.