The discrimination of type I and type II collagen and the label-free imaging of engineered cartilage tissue

Using excitation polarization-resolved second harmonic generation (SHG) microscopy, we measured SHG intensity as a function of the excitation polarization angle for type I and type II collagens. We determined the second order susceptibility (χ(2)) tensor ratios of type I and II collagens at each pixel, and displayed the results as images. We found that the χ(2) tensor ratios can be used to distinguish the two types of collagen. In particular, we obtained χzzz/χzxx = 1.40 ± 0.04 and χxzx/χzxx = 0.53 ± 0.10 for type I collagen from rat tail tendon, and χzzz/χzxx = 1.14 ± 0.09 and χxzx/χzxx = 0.29 ± 0.11 for type II collagen from rat trachea cartilage. We also applied this methodology on the label-free imaging of engineered cartilage tissue which produces type I and II collagen simultaneously. By displaying the χ(2) tensor ratios in the image format, the variation in the χ(2) tensor ratios can be used as a contrast mechanism for distinguishing type I and II collagens.