Collagen microarchitecture from polarized light imaging: a biomechanics perspective.

Significance: Collagen, the main load-bearing component in tissue, is present in all animals and forms a variety of networks from the fibrils, fibers, bundles, and lamellae into which it self-assembles. The collagen microstructure is different among tissue types, and the different microstructures give rise to tissue-specific mechanical properties. Therefore, methods for visualizing collagen fibers and their orientation are essential for understanding the biomechanical properties of tissue. Aim: Our aim in this review is to provide the basis for understanding the methodology of polarized light imaging methods and how they can be used to characterize collagen microstructure. Approach: We begin with a description of collagen microstructure and its relationship to tissue biomechanics, a basic formalism of polarized light, and how collagen interacts with polarized light. We then describe polarized light microscopy and its various forms, particularly instant polarized light microscopy, then polarization-sensitive optical coherence tomography, and last, polarization-resolved second-harmonic generation microscopy. Results: We describe methods for imaging collagen microstructure with polarized light from in vivo methods to high-resolution volumetric imaging of tissue sections. Conclusions: We intend to help those interested in using polarized light to image and understand the relationship between collagen microstructure and biomechanics.