Harmonic microscopy of isotropic and anisotropic microstructure of the human cornea

In this study we present combined third-harmonic generation (THG) and second-harmonic generation (SHG) microscopy images of intact human corneas, and we analyze experimentally and theoretically the origin of the THG signal. Multiharmonic microscopy provides detailed images of the cornea microstructure over its entire thickness. A component of the THG signal originates from cellular structures and another one originates from anisotropy changes between successive collagen lamellae in the stroma. This anisotropy-related signal can be specifically detected using circular incident polarization, and provide contrasted images of the stacking and tissue-scale heterogeneity of stromal lamellae. Forward-radiated THG and SHG signals are generally anticorrelated, indicating that maximum THG is obtained from lamellar interfaces whereas maximum SHG is obtained from within lamellae. Polarization-resolved THG imaging reflects the alternate anisotropy directions of the lamellae. We present a model for THG imaging of layered anisotropic samples and numerical calculations that account for our observations.