260 KERATIN AND KERATINIZATION 



solutions of high or low pH. Evidently this set is being maintained only 

 by salt-bonds and hydrogen bonds (Speakman, 1934; Woods, 1933). A 

 more permanent set is induced by relaxation at higher temperatures under 

 conditions in which the crystallites are transformed into the /3-modification 

 (Astbury and Woods, 1933; Woods, 1933). This type of set can be 

 released by strong solutions of urea (Rudall, 1946), and here therefore 

 H-bonds are obviously the factors stabilizing the jS-crystallites. Permanent 

 set is defined as a set which is not relaxed by prolonged steaming or by 

 solutions which rupture hydrogen bonds, i.e. it is a set which is sustained 

 by covalent cross-linkages analogous to the disulphide bonds which are 

 effective at the original length. These cross-linkages may be reformed 

 disulphide bonds (Speakman, 1933) or bridges introduced between 

 reduced disulphide bonds by dihalides (see p. 252) or linkages apparently 

 formed between COOH groups and amino groups in steamed fibres 

 (Speakman, 1933). 



Taken together all these physicochemical methods provide semi- 

 quantitative measures of the contribution of the several cross-links: 

 salt-linkages, disulphide and hydrogen bonds, to the stabilization of the 

 keratinized fibres, which are in good agreement with conclusions reached 

 on other grounds. 



Cell membranes in keratinized tissues 



The Membranes and Cellular Adhesion 



Up to this point we have concentrated attention on what happens to the 

 intracellular proteins during keratinization. Other constituents of the cells 

 also undergo changes during keratinization. The cell membranes, in 

 particular, play an important role in maintaining the hardened structure. 

 These membranes and their behaviour during the establishment of tissues 

 have been discussed already in Chapter III; that they are also important 

 after keratinization is proved by many experiments. 



Electron microscopy has shown that the fibrils of keratinizing cells are 

 wholly intracellular and that no intercellular connexions composed of 

 fibrils cross from cell to cell binding the mass together. Such " bridges " 

 were often described in earlier works, but their true nature is now better 

 understood. For many histologists who feel that cells must be held to- 

 gether by " string " rather than "sealing wax", they had a strong fascin- 

 ation. In fact cells are stuck together, and for this reason the properties of 

 the adhering surfaces, and of the adhesive, are as important in maintaining 

 the whole formation as the hardened cell contents themselves. This is 

 shown very simply by digesting a tough keratin, such as hair, by means of an 

 enzyme (trypsin) which removes the membranes and cement. The 

 tensile strength falls rapidly (Elod and Zahn, 1946), and in a few days the 

 fibres drop apart. Examination of the residue by a variety of means 



