Collagen fiber arrangement in normal and diseased cartilage studied by polarization sensitive nonlinear microscopy

Abstract

Second harmonic generation (SHG) and two-photon fluorescence (TPF) microscopy is used to image the intercellular and pericellular matrix in normal and degenerate equine articular cartilage. The polarization sensitivity of SHG can be used directly to determine fiber orientation in the superficial $10to20μm$ of tissue, and images of the ratio of intensities taken with two orthogonal polarization states reveal small scale variations in the collagen fiber organization that have not previously been reported. The signal from greater depths is influenced by the birefringence and biattenuance of the overlying tissue. An assessment of these effects is developed, based on the analysis of changes in TPF polarization with depth, and the approach is validated in tendon where composition is independent of depth. The analysis places an upper bound on the biattenuance of tendon of $2.65×10−4$. Normal cartilage reveals a consistent pattern of variation in fibril orientation with depth. In lesions, the pattern is severely disrupted and there are changes in the pericellular matrix, even at the periphery where the tissue appears microscopically normal. Quantification of polarization sensitivity changes with depth in cartilage will require detailed numerical models, but in the meantime, multiphoton microscopy provides sensitive indications of matrix changes in cartilage degeneration.