248 KERATIN AND KERATINIZATION 



histochemistry (p. 214), by chemical (p. 236) and mechanical separation 

 (p. 231), followed in some cases by analyses proving a low-sulphur content. 

 Alexander has also pointed out that the existence of several distinct 

 methods of producing supercontraction, which are discussed on p. 259 et 

 seq., also shows that keratin is divided into cystine-stabilized and hydrogen- 

 bond-stabilized fractions (1951). 



A summary of the outstanding properties of the two components of what 

 may be termed the filament-plus-matrix model (Fig. 102) is as follows: 



(a) Filaments (cc-component) 



Dimensions. About 60-80 A diameter ; length indefinite, at least > 2000 

 A in wool and hair. 



Internal structure. Composed of a-type fibrous protein (see Chapter 5) 

 (Figs. 77, 78 and 79). 



Composition. Cystine content is less than whole keratin (Table 15). 

 Serine, threonine, proline lower, acidic residues higher than whole fibre. 



Macromolecular composition. May contain a primary peptide of molecu- 

 lar weight between 50,000-80,000. Its relation to the filament is obscure. 



(b) Matrix (y-coMPONENT) 



Amorphous, shorter chain polypeptides having a higher content of 

 cystine, serine, threonine and proline. 



Molecular structure. Probably not a-type, may be irregular. The ratios 

 of <x:y is of the order of 1:1 in wool and hair to judge from electron 

 micrographs. From the analysis of extracts of oxidized wool it would 

 appear that the a-component may amount to 60% of the total. 



It seems reasonable to suppose that all the hard keratins will possess a 

 similar fine structure (Rudall, 1952). Fibrils essentially similar to those 

 noted in wool and hair have been demonstrated in feather both in the germ- 

 inal layers of the follicle and in the fully keratinized material (Mercer, 1958 ; 

 Mercer, unpub.). Porcupine quill possesses a most regular structure 

 (Rogers, 1959a and b). In the present state of our knowledge it is possible 

 to suppose that the y-matrix protein is an entirely new protein which the 

 cells of the tissue commence to make in the keratinization zone in response 

 to the altered conditions prevailing there and that it secondarily deposits 

 on the bundles of a-filaments to form a cementing matrix. Or it may be 

 thought to be synthesized directly on the filaments establishing an actual 

 peptide linkage with existing polypeptides. The first alternative would seem 

 the more probable since the two proteins are separable when the cystine 

 links are severed. It is suggestive that, in the hair cuticle cells at this same 

 level, a very similar amorphous high-sulphur keratin is also synthesized. 



With the soft keratin of the epidermis the situation is by no means so 

 clear. This material, composed largely of transformed keratohyalin, has an 



