Optical second‐harmonic scattering in rat‐tail tendon

The angular dependence of optical second‐harmonic generation by native, wet, rat‐tail tendon is found to display a sharp, intense, forward peak superimposed on a broad background. The sharp peak is shown to imply long‐range polar order, while the broad background corresponds to that predicted for the random “up”/“down” array of collagen fibrils seen with the electron microscope. The degree of polar order is determined, and the dependence of the fibril diameter distribution on age and state of hydration is measured. The coherence length of tendon for harmonic generation and the absolute magnitude of the nonlinear susceptibility of the collagen fibril are also determined. The biological significance of these various findings is discussed.