Effect of tissue fixation on the optical properties of structural components assessed by non-linear microscopy imaging.

Fixation methods such as formalin are commonly used for the preservation of tissue with the aim of keeping their structure as close as possible to the native condition. However, fixatives chemically interact with tissue molecules, such as collagen in the extracellular matrix (ECM) or myosin, and may thus modify their structure. Taking advantage of the second- and third-harmonic generation (SHG and THG) emission capabilities of such components, we used nonlinear two-photon microscopy (NL2PM) to evaluate the effect that preservation methods, such as chemical fixatives, have on the nonlinear capabilities of protein components within mouse tissues. Our results show that depending on the preservation technique used, the nonlinear capabilities of collagen, lipid droplets and myosin microarchitecture are strongly affected. Parameters of collagen fibers, such as density and branch points, especially in collagen-sparse regions, e.g., in kidneys, were found to be altered upon formalin fixation. Moreover, cryo-freezing drastically reduced SHG signals from myosin. Our findings provide valuable information to select the best tissue fixation method for visualization and quantification of structural proteins, such as collagen and myosin by advanced NL2PM imaging techniques. This may advance the interpretation of the role these proteins play in disease.