30 KERATIN AND KERATINIZATION 



According to this definition various fibrous proteins of the internal 

 epithelia of vertebrates (oesophagous, vagina, etc.) would be keratins and 

 are generally so called. A purely epidermal location cannot therefore be 

 insisted on. Certain extracellular exudates in birds, such as the horny 

 linings of the gizzard and proventriculus, are hardened by the presence 

 of cystine bonds and are therefore also keratins (Broussy, 1932). The 

 so-called " ovokeratin," which forms, along with mucin, the fibrous 

 membranes of birds' eggs, is a more ambiguous case. Some analyses 

 (Calvery, 1933) stress a resemblance to wool keratin in composition (cystine 

 content 3-7 per cent); on the other hand X-ray photographs suggest a 

 protein of the collagen type (Champetier and Faure-Fremiet, 1938). The 

 fibrils are quite unlike those of other keratins; they consist (as seen 

 electron microscopically) of a core of osmiophilic material enclosed 

 in a sheath of less-stained material (Mercer, unpublished. Plate 18B). 

 They may thus consist of two distinct substances of which one could 

 be cystine-stabilized. 



With such a broad definition the keratins cannot be considered as 

 exclusively limited to the vertebrates since there are instances reported 

 of cystine-stabilized proteins of invertebrate origin (Brown, 1949). 

 Krishnam (1953, 1954) has shown in a scorpion that the proteins of the 

 epicuticle (analogous morphologically to the tanned structures of other 

 arthropods) are stabilized by disulphide bonds and give an X-ray pattern 

 not identical with either the a- or jS-patterns of keratins or of the /S-pattern 

 of arthropodin. The cuticle of Limulus is reported by Lafon (1943) to 

 be similar. Brown (1949, 1950) has cited other instances of invertebrate 

 proteins stabilized by disulphide bonding although she is not inclined to 

 call all these keratins. There is even some suggestion that quinone cross- 

 linking may occur along with cystine cross-linking in some instances, or 

 perhaps the quinone may link directly to the thiol of a cysteine residue 

 (Hughes, 1959). Thus tanning and keratinization may become inter- 

 changeable or even mixed as a means for the insolubilization and stabiliza- 

 tion of proteins. 



It may be claimed also that the spindle fibre protein, which plays an 

 important role in the mitotic cycle in animal cells, is a keratin. Mazia and 

 Dan (1952) have succeeded in isolating the mitotic apparatus from sea- 

 urchin eggs and found that to redissolve the isolated fibres it was necessary 

 to reduce them with thioglycollic acid (see p. 240) or to use strongly- 

 alkaline or strong solutions of urea. The mitotic cycle, involving an 

 appearance and disappearance of the fibrils, could thus be based on a 

 " reversible keratinization " effected perhaps by an oxidation-reduction 

 cycle between the protein-bound SH groups and cellular glutathione. 

 The spindle fibres might be formed by an aggregation of particles effected 

 by disulphide bonds. 



