262 KERATIN AND KERATINIZATION 



/3-keratose), which consists largely of membranes (p. 270) and found it to 

 resemble whole wool. The few small differences observed are not suffi- 

 cient to explain the insolubility. All these analyses must, however, be 

 treated with some reserve unless the absence of unremoved keratin in the 

 membrane preparations is demonstrated. 



Since disulphide bonds and hydrogen bonds seem inadequate to explain 

 the insolubility (see above), suggestions have been made that some new 

 bond is present, e.g. a type of tanning linkage as found in melanin or the 

 insect cuticle. It may be significant that melanin granules have a similar 

 chemical resistance to the hardened membranes. Residues consisting 

 largely of membranes have been examined by X-ray diffraction and were 

 found to give only an unidentifiable pattern with some /^-characteristics 

 which, after such a chemical treatment, is of little significance. 



The Morphology of the Membranes of Keratinized Tissues 



The special nature of these membranes is made clear by examination of 

 their fine structure. 



The intercellular spacing commonly found between cell membranes in 

 normal tissues is about 150 A wide (p. 41) and a spacing of this order is 

 found between cells in the upper bulb of the hair follicle and in all other 

 germinal tissues. As the cells of the presumptive cortex fill with fibrous 

 keratin, this spacing widens and may reach 400 A. At the same time the 

 outlines of the cells become more wavy, so that the surfaces themselves are 

 more interlocked (Birbeck and Mercer, 1957; Rogers, 1959a and b). In 

 material fixed in osmium tetroxide the membranes are dense and the 

 intermembrane material contains light and dark bands. In the final 

 keratinized cortex these relative differences in density still persist. These 

 observations prove that the amount of intercellular cement is increased 

 during keratinization and that it undergoes some chemical modification. 



More elaborate changes associated with membranes are visible in the 

 cuticle and the inner-root sheath (Birbeck and Mercer, 1957a and b; 

 Rogers, 1959a and b). The intercellular space between the cuticle cells also 

 widens and in this instance several dense layers, separated by lighter layers, 

 appear transiently and later fuse to give a single broad dense layer. Be- 

 tween the various cells of the sheath, single broad sheets are deposited 

 (Plate 6B). Again while we have no idea of the chemistry behind these 

 appearances, they show that changes are going on in the intercellular 

 spaces and we may assume they they are related to strengthening the 

 adhesion. 



Somewhat similar events occur in skin. The localized " studs " or 

 desmosomes have been described (p. 83). In the granular layers and 

 higher, the contacting cell surfaces may become extremely wave-like 

 producing a very considerable degree of quasi-regular interlocking. The 



