THE KERATINIZATION PROCESS 23$ 



before the closure of the disulphide bonds. Hydrogen bonds are usually 

 weakened by the introduction of urea into the solvent. The thermodynamic 

 factor involved here is the heat change accompanying the transfer of the 

 H-bond between two peptides to a pair of urea molecules which appears to 

 favour the peptide-urea association. In a sense the urea molecules prise 

 apart the chains and destroy the secondary structures maintained by them. 

 Useful discussions of this problem will be found in articles by Ward and 

 Lundgren (1954) and by O'Donnell and Woods (1956). 



Extracts from the Pre-keratinized Zone 



The idea of extracting the proteins from a keratinizing tissue before 

 they harden is attractive since the extract might be expected to contain 

 soluble precursors. Rudall (1946 and 1952) showed, however, that the 

 buffered aqueous solvents commonly used in biochemical extractions 

 removed very little protein from skin. He found, however, that the ad- 

 dition of urea to the solution suffices to dissolve copious amounts of 

 protein from a thick skin such as a cow's nose. He named this protein 

 " epidermin". 



When precipitated from solution by ammonium sulphate, epidermin 

 forms a voluminous, white, sticky, curd-like material, easily gathered 

 together and drawn into fibres. These fibres are somewhat elastic when 

 wet, are birefringent and yield an excellent a-type X-ray pattern. Stretch- 

 ing produces a fibre giving a /^-pattern. When heated in water (50-60°C) 

 oriented fibres contract, and the contracted material gives a disoriented 

 /3-pattern. Epidermin thus behaves as an unstabilized keratin, i.e. its basic 

 molecular framework is established but, in the absence of cross-linkages, 

 is readily disorganized. 



Rudall (1946) and others (Derksen et al., 1937) have demonstrated the 

 increasing thermal stability of strips of tissue cut from successively higher 

 layers of epidermis. The effect of keratinization can be imitated closely by 

 cross-linking epidermin fibres with formaldehyde and benzoquinone 

 (Rudall, 1946). 



Similar extracts may also be made from other keratinized tissues by 

 urea solutions. Here again the simple linear arrangement of the hair 

 follicle enables an exact location of the extracted protein to be determined 

 (Mercer, 1949b). When a plucked human head follicle bearing a papilla is 

 placed in concentrated urea solutions there is an immediate swelling 

 followed by dissolution of the lower half of the keratinization zone (Fig. 

 93). Above a rather definite level the precortex merely swells and to a 

 decreasing degree the higher the level (Fig. 94). The germinal tissues of 

 the bulb containing little protein are less affected; nuclei are not dissolved. 

 The relation of these events to other histochemical features is best seen in 

 the series of Figs. 91, 92 and 97. It will be noted that only the lower half 



