In situ visualization of collagen architecture in biological tissues using polarization-resolved Second Harmonic microscopy

Multiphoton microscopy (MPM) has been shown to provide three-dimensional (3D) cell-scale contrasted images of biological tissues. In particular, Second Harmonic Generation (SHG) microscopy has emerged as a unique tool to probe the 3D distribution of collagen fibrils within unstained tissues because this second order coherent nonlinear signal is highly specific for dense non-centrosymmetrical macromolecular structures. Such a 3D imaging technique is of great interest for biological and biomedical studies since collagen is the main component of the extra-cellular matrix in mammals. This structural protein is characterized by triple helical domains, which self-assemble into fibrils that further form fibers, lamellae or other 3D networks responsible for the architecture of organs. This hierarchical organization of collagen is crucial to the biophysical and mechanical properties of tissues (for instance mechanical strength of the tendon, transparency of the cornea,.).