THE KERATINIZATION PROCESS 249 



irregular fibrous texture (Fig. 99), in which a definite fibrillar and non- 

 fibrillar component can be discerned. Epidermin, the precursor of skin 

 keratin, shows several components in the analytical ultracentrifuge, none of 

 which can be related with certainty to the a- and y-components of hair. 

 Moreover, the lower sulphur content, the sudden nature of the change 

 from amorphous keratohyalin (or trichohyalin) granules into the fibrous 

 form and its consolidation as a resistant birefringent fibrous layer, all are 

 in some contrast with the consolidation of the prefabricated fibrils of 

 a-keratin in the hair. If a y-component exists it must be smaller in amount 

 and perhaps not so easily distinguished on account of the lesser regularity 

 of the structure. Brody (1959b) is of the opinion that a y-component is 

 derived from keratohyalin, but the evidence that the granules contain 

 cysteine is not good (see p. 230). Nevertheless it is possible to conclude 

 that in most instances the vertebrate keratins are duplex structures pro- 

 duced by embedding a primary system of filaments (usually a-type but, 

 as feathers and claws show, a /3-type is possible) in a matrix of short-chain 

 polypeptides rich in cysteine residues whose conversion into cystine resi- 

 dues stabilizes the formation. 



The concept of keratinization as a process subsequent to a primary 

 process of fibril formation harmonizes very well with the broader view- 

 point which presents the a-type proteins as the common intracellular fibre 

 type which, by secondary modifications, is adapted to a variety of functions. 



Physicochemical properties and keratinization 



A great deal has been learned concerning the stabilizing bonds produced 

 during keratinization by the study of the dependence of the mechanical 

 and dimensional properties of the hardened tissue on the physicochemical 

 environment. For the study of these " mechanochemical " properties, to 

 adopt Speakman's useful expression, the tissue chosen needs to have a 

 convenient form, and in fact the greater part of these experiments have 

 been made on hair, wool, feather and horn. Since information concerning 

 the properties of wool and hair is of value to both the textile and the 

 cosmetic industries, the amount of work carried out is enormous and it 

 would be impossible to review it here. Reference may be made to the book 

 by Alexander and Hudson (1954), the review of Ward and Lundgren 

 (1954) and papers by Speakman (q.v.). 



When stretched under well-defined conditions hairs yield characteristic 

 stress-strain curves (Fig. 70). Their dependence on temperature (Fig. 

 104) and water content (Fig. 103) shows that the effect of water and a rise 

 in temperature is to loosen those internal bonds which are opposing 

 extension and to reduce the work of extension. Various theoretical 

 attempts have been made to explain the shape of the stress-strain curve. 

 As already described in Chapter V (p. 172) the normal curve shows several 



