20 KERATIN AND KERATINIZATION 



with an accessory molecule usually an aromatic polyphenol (Q) (Hackman, 

 1953 and 1959): 



h i r~ 



Q Q Q 



I I I 



The cross-linked protein may, in turn, embed a fibrous meshwork of a 

 different character. For example, in the arthropod cuticle, a chitinous 

 meshwork of fibrils, themselves stabilized by crystallite formation, is 

 embedded in a tanned ;8-type protein (-s) named arthropodin (Fraenkel 

 and Rudall, 1947). The result is a very rigid exoskeleton whose inflexibility 

 has had much to do with the evolution of these animals. 



Brown (1949a and b and 1950) has discussed the various chemical and 

 histochemical methods for recognizing the presence of a tanned protein. 

 There is no entirely satisfactory method. A hard, insoluble, darkened 

 material is usually presumed to be tanned. The actual bond between the 

 quinone and the polypeptide is still in dispute. Pryor's original proposal 

 (1940) that the link involved the amino end groups of the polypeptides: 



HO - ^ NHP 

 HO 



NH.P. . . 



[P . . . = polypeptide chain] 



and an o- or />-diphenol is not now accepted. If the material is not dissolved 

 by keratinolytic agents, i.e. those breaking disulphide bonds and hydrogen 

 bonds (see pp. 234 and 236) but is dissolved by sodium hypochlorite 

 solution (an unknown reaction) it may be tanned. The demonstration of 

 diphenols in the secreting cells is strong support. 



Hardness may also be influenced by the deposition in the protein matrix 

 of inorganic materials, usually calcium salts or silica. For example, the 

 crustaceans mix calcium salts with their chitinous cuticles. Probably for 

 reasons of weight, insect cuticles are usually free of calcium salts although 

 oxalates may be deposited in their egg cases (Rudall, 1955). The verte- 

 brates uniformly lay down calcium salts in association with their connective 

 tissue protein, collagen, to form bone as a rigid endoskeleton. Such 

 mineral deposits are rare in keratinized tissues. Pautard (in press) has 

 demonstrated by means of X-rays and by electron microscopy the presence 

 of apatite in " whalebone," which is not bone, as the name implies, but an 

 extensive horny proliferation of the oral epithelium in certain whales. 

 The enamel of teeth may also contain a keratinous component. See p. 78. 



Although calcium has many important biological roles its concentration 

 within cells is normally lower than that in the surrounding fluids. Never- 



