Many biological tissues are optically anisotropic. Tissue birefringence results primarily from the linear anisotropy of fibrous structures, which form extracellular media. The refractive index of a medium is higher along the length of a fiber than along the cross section [Fig. 45(a)]. A specific tissue structure is a system composed of parallel cylinders that create a uniaxial birefringent medium, with the optic axis parallel to the cylinder axes. This is called birefringence of form. A large variety of tissues, such as eye cornea, tendon, cartilage, eye sclera, dura mater, testis, muscle, nerve, retina, bone, tooth, myelin, etc., exhibit form birefringence. All of these tissues contain uniaxial andâor biaxial birefringent structures. For instance, in bone and tooth, these are mineralized structures originating from hydroxyapatite crystals, which play an important role in hard tissue birefringence. In particular, dental enamel is an ordered array of such crystals surrounded by a protein/lipid/water matrix.
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