Second harmonic generation and orientational order in connective tissue: a mosaic model for fibril orientational ordering in rat-tail tendon

Orientational ordering of collagen fibrils in rat-tail tendon is studied by means of second harmonic generation (SHG). The polarization dependence of this nonlinear optical process is shown to provide information about the orientation distribution of the fibrils which is comparable to that obtainable from X-ray diffraction. It is also shown that SHG provides new information about the spatial dependence of this distribution which is not easily obtainable by other means. In rat-tail tendon the collagen fibrils are ordered in units whose dimensions are large compared to the wavelength of light, and within each such unit there exists a high degree of fibril parallelism. The major source of orientational disorder is due to the relative misalignment of these units. It is thus demonstrated for the first time that the correct description of orientational order in this widely studied model system requires the use of a mosaic model.