Circular dichroism second-harmonic generation microscopy probes the polarity distribution of collagen fibrils

Second-harmonic generation (SHG) microscopy is currently the preferred technique for visualizing collagen in intact tissues, but the usual implementations struggle to reveal collagen fibrils oriented out of the imaging plane. Recently, an advanced SHG modality, circular dichroism SHG (CD-SHG), has been proposed to specifically highlight out-of-plane fibrils. In this study, we present a theoretical analysis of CD-SHG signals that goes beyond the electric dipolar approximation to account for collagen chirality. We demonstrate that magnetic dipolar contributions are necessary to analyze CD-SHG images of human cornea sections and other collagen-rich samples. We show that the sign of CD-SHG signals does not reveal whether collagen fibrils point upwards or downwards as tentatively proposed previously. CD-SHG instead probes the polarity distribution of out-of-plane fibril assemblies at submicrometer scale, namely homogeneous polarity versus a mix of antiparallel fibrils. This makes CD-SHG a powerful tool for characterizing collagen organization in tissues, specifically the degree of disorder, which is affected during pathological remodeling. CD-SHG may thus serve to discriminate healthy and diseased collagen-rich tissues.