THE KERATINIZATION PROCESS 265 



reaction disappears in the lower part of the keratinizing zone (Fig. 97 (b)). 

 We have seen that protein synthesis is substantially complete at this same 

 level. 



Mitochondria and various vacuoles (Golgi apparatus) are also lost sight 

 of as distinct structures between the condensing fibrils. Their remnants 

 persist as modified membranes and seem responsible for various gaps 

 which are to be seen in the otherwise almost uniformly-fused mass of 

 keratin fibrils (Plates 15, 16 and 17). It is not known what enzymes 

 are involved in the oxidative closure of the sulphur bridges, although 

 copper is concerned as it is with the oxidases in melanin formation (p. 279). 



The hair cuticle 



The peculiar type of amorphous keratin of the hair cuticle cells seems to 

 possess a limited extensibility, since these cells usually part company or 

 split when the hair is stretched more than 50% (Lehmann, 1943), and when 

 a many-layered cuticle is present as in fur hairs (Stoves, 1947 and 1943) the 

 elastic behaviour of the whole hair is considerably modified. Owing to its 

 fibrous texture the hair cortex may fray and split; against this tendency 

 the cuticle forms a retaining sheath whose laminated structure is adapted 

 to this end. The keratin of the cuticle is, in fact, so different from that of 

 the cortex as to require separate consideration; it represents yet another 

 distinct product of epidermal cells. 



Proceeding inwards from the surface of the fibre (Fig. 110) we encounter 

 (a) a thickened and altered external cell membrane, which is strongly 

 attached, through the intermediary of an intercellular sheet, to the mem- 

 branes of contiguous cells. The whole of this formation of external 

 membranes forms a strong, chemically resistant skin of such distinct 

 character that it is referred to as an epicuticle (Lindberg, 1949). (b) Beneath 

 the epicuticle lies the layer of keratin, its compact amorphous structure 

 being responsible for much of the mechanical protection. This may be 

 called an exocnticle. It is not uniform in texture. A layer immediately 

 beneath the epicuticle stains more deeply after reduction and fixation and 

 thus seems more cross-linked (Sikorski, 1960). (c) A layer of modified 

 cellular residue (p. 270) (3 in Fig. 110) (endocuticle) lies next. Its high 

 resistance to keratinolytic solvents (see pp. 261 and 268) supplements that 

 of the exocuticle and thus increases the total protection against chemical 

 action. 



Cuticular keratin is distinct from the other products of the cells of the 

 hair follicle in the following respects. 



(a) It is not fibrous. It has rather, during its formative stages (Plate 

 20A), the appearance of a viscous liquid condensing to give a coherent 

 amorphous lamella closely adhering to the external cell membrane (Fig. 



