218 



KERATIN AND KERATI NIZAT ION 



the oxidation may be reversed and the distribution of new SH groups will 

 be that of the former disulphide bonds. This distribution is found to be 

 complementary to the SH (Hardy, 1952; Rudall, 1952) as may be seen 

 from Fig. 97(c). 



In some pathological conditions oxidation may be incomplete, and a 

 positive thiol reaction persists into the normally-hardened layers. A 

 significant case of this was described by Marston (1946), in sheep reared 

 on a diet deficient in copper (p. 159). This element seems to play a role 



Fig. 96. The positive SH zones in the growing feather follicle (after 

 Giroud and Bulliard). Compare with figure of the hair follicle (Fig. 90). 



as a coenzyme in the oxidation of thiol groups (Flesch, 1949); when 

 deficient in copper the positive- SH reaction persists along the greater 

 part of the follicular shaft, the emerging wool fibre is less thoroughly 

 stabilized and the crimp has a longer wavelength (see also p. 159). 



Thus the outstanding fact relating to the progress of keratinization is 

 that it occurs substantially after fibril formation (zones D and E) and in 

 two steps. The two steps can be satisfactorily correlated with (a) a primary 

 stabilization effected by hydrogen bending (zone D) and rather readily 

 disorganized (Fig. 90D) followed by (b) a consolidation of the primary 

 structure by the progressive introduction of cystine bridges resulting from 

 the oxidative linking of cysteine (SH) side chains. These chemical 

 changes are accomplished without detectable change in the a-type structure 

 which is established and stabilized by hydrogen bonds in the first-formed 

 fibrils. The sudden increase in stability, particularly apparent in the tests 

 depicted in Figs. 92 and 93 (level (E)), seems to coincide with the appear- 

 ance of a dense material between the filaments seen in the electron 

 microscope and described on p. 224. 



