10 



KERATIN AND KERATI NIZAT ION 



Birefringence 



The presence of oriented material, either fibrous or membranous, is 

 most conveniently recognized by its optical properties using the polarizing 

 microscope (Schmidt, 1924). The method is extremely sensitive, but 

 yields no information concerning molecular structure. 



All major biological fibres and, in particular, the keratinized epidermal 

 tissues are birefringent (double refracting) and the direction of maximum 

 refractive index coincides with the fibre axis as inferred from microscopic 

 inspection. The birefringence is quantitatively defined as: 

 At? = Vu - 7^ 



when r/ n and r] ± are the refractive indices in a direction parallel to the 

 fibre and at right-angles to it. In practice, usually a retardation R is 

 measured for a sample of material of thickness d and A17 = Rjd. Some 

 typical values for fibres are given in Table 4. 



Table 4. Optical Constants of Hair and Related 



Fibres: -q n Refractive Index Parallel to Fibre Axis; 



■q L Refractive Index Normal to Fibre Axis 



( Al 7 = Vw ~ vD- 



* Barnes (1933). 

 f Frey-Wyssling (1953). 

 I The value for wool taken as a typical, non-medullated, keratin with 

 a thin cuticle (i.e. consisting largely of oriented fibrous keratin) is 

 regarded as a maximum value. Other observers quote figures for A17 

 nearer to 0-01. Not infrequently lower values (<~ 0-008) can be measured, 

 but this may be presumed to be due to deterioration. Whether A77 

 approaches a quite definite figure, characteristic of pure, dry, undamaged 

 fibrous keratin has not been established. 



The actual values of A77 are not greatly used for identifying fibre-types 

 since, being influenced by many factors (such as swelling, imbibition of 

 liquids and tension), they vary greatly and further, although the retardation 

 is easily observed and measured with a compensator, for microscopic 

 objects in cells the thickness is not so easily determined. Nevertheless 

 the polarizing microscope is a most sensitive instrument for the recognition 

 of oriented material and for detecting changes in orientation produced by 

 deformation, chemical effects, heat, etc. It can be particularly useful when 

 studying the degree of stabilization of an oriented material. 



