THE KERATINIZATION PROCESS 231 



dissolve and reform as the final keratin but this is not provable by 

 microscopy. There is some evidence in electron micrographs that the 

 early-formed tononbrils partly disappear in the granular layer. 



A property of the fibrous form of trichohyalin of the hair follicle, 

 probably connected with the fact that it is not stabilized by disulphide 

 cross-linking, is its tendency to " fall apart " during the disintegration of 

 the inner root sheath. This same property plays a similar role in the des- 

 quamation of the epidermis. 



One further puzzling feature of epidermal keratinization is that kerato- 

 hyalin is not invariably present. Many thin skins (e.g. birds) keratinize 

 normally without a granular layer and it is found in a variable degree 

 elsewhere. The view that it represents an alternative pathway of synthesis 

 and may make a contribution to an independently-formed system of fibrils 

 does something to explain these facts. The scaly (hard) keratin of rat tails 

 forms without a granular layer, whereas adjacent perifollicular skin is 

 softer, more flexible and has a granular layer. According to Jarrett and 

 Spearman (1961) treatment (externally) with vitamin A causes the appear- 

 ance of a granular layer and a softer keratin in the scale regions. Epidermal 

 cells differ evidently in the effect on their keratinization of vitamin A with 

 a range of responses included in the sequence : 



... . Vit A f , Vit A 



hard keratin > softer keratin -> mucin 



(no keratohyalin) (keratohyalin) 



See also p. 63 et seq. 



X-ray photographs, optical methods (Matoltsy, 1957) and electron 

 micrographs (Plate 17) all show that the arrangement of the filaments in 

 the stratum corneum is less perfect in soft keratins than in hard. It is also 

 likely then that this imperfect organization contributes to lowering the 

 stability of the formation. It is known in wool (Rogers, 1959b) that in cells 

 where the filaments are less perfectly aligned, the keratin is less stabilized. 

 In skin no fibrils analogous to those of the hair cortex form, the filaments 

 appearing to form simply loose inter-connecting bundles (see Fig. 99). 



Recently Swanbeck (1959), on the basis of the scattering of X-rays at 

 low angles, has concluded the existence of scattering unit of diameter of 

 260 A, which, assuming that the scattering phenomena have been correctly 

 interpreted, would seem to imply a close association of the 100 A filaments 

 in small groups. This is not immediately apparent in micrographs. 



Keratinization of Horn 



The special interest of horn lies in the fact that from it one can obtain 

 massive samples particularly suitable for some kinds of experiment. The 

 SH reactivity was examined by Giroud and Bulliard (1930) (Fig. 95) and 

 in further detail by Rudall (1956) who established clearly that there was an 



