We have proposed an optical probe that can be used to characterize the orientation of collagen fibers in human dermis. A specific probing ability for collagen results from the use of second-harmonic-generation (SHG) light induced by collagen molecules in the tissue. Based on the concept of collagen SHG light, a reflection-type polarization measurement system (named SHG polarimetry) with a probe light spot of 15 microm in diameter has been constructed, and the human reticular dermis has been measured using this system. Resultant data exhibit that the reticular dermis possesses approximately uniaxial orientation of the collagen fibers. Furthermore, we demonstrated a nondestructive measurement of the collagen orientation in the papillary dermis across an epidermis layer. For distribution measurement of the collagen fiber orientation in the reticular dermis, we have extended the SHG polarimetry to one- (1-D) and two-dimensional (2-D) measurement. By the macroscopic 2-D SHG polarimetry, we have observed that the orientation angle and organization degree of collagen fibers vary widely depending on the discrete probing positions in the reticular dermis. Furthermore, microscopic 1-D SHG polarimetry indicated a swell of the orientation angle and a large variance of the organization degree in the collagen fibers in the microscopic region. These results imply that the reticular dermis posses a tangled structure of collagen fibers, which is highly consistent with the result of the anatomical examination of the skin. The proposed method will be a powerful tool for monitoring the microscopic distribution of the collagen fiber orientation in the human dermis.

Based on the reflection-type polarization measurement of second-harmonic-generation (SHG) light induced by collagen molecules, we are able to determine the collagen fiber orientation in human tissues taken from a cadaver. The resulting SHG radar graph shows the direction of the absolute orientation and the degree of organization of collagen fibers. To evaluate the probing sensitivity to the collagen orientation, we compared the proposed method with other polarimetric methods. Use of the proposed method revealed characteristic orientation differences among collagen fibers and demonstrated significant inhomogeneity with respect to the distribution of collagen orientation in human dentin. The proposed method provides a powerful research and diagnostic tool for examining the collagen orientation in human tissues.

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A human calcaneus bone, consisting of hydroxyapatite and collagen fibers, was successively sliced into samples in a direction perpendicular to the long axis of the bone and parallel to the long axis of the human lower limb. The transmitted microwave intensities of 12 GHz, reflecting the dielectric property, were measured for the slice samples using Osaki's microwave method (Tappi J., 1987; 70:105-108). The complex dielectric constant of 12 GHz of the collagen fiber film was much greater than that of hydroxyapatite disc, which demonstrated that the dielectric anisotropy observed for the sliced bone was mainly affected by the collagen fibers. The angular dependence of the transmitted microwave intensity gives the orientation angle reflecting the direction of the collagen-fiber orientation, and the degree of orientation reflecting the anisotropic property of collagen fibers. The orientation angle and the degree of orientation for the slice samples changed with changing position along the long axis of the calcaneus bone. The direction of orientation deviated to the lateral side at the heel part of the left calcaneus, and to the medial side at the middle part. The degree of orientation is relatively high at the heel part and low at the middle. Such results give a two-dimensional distribution of collagen-fiber orientation in the left calcaneus, and suggest that the direction and degree of orientation are closely related to the direction and magnitude of the stress applied to the bone, respectively. Anat

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