2 KERATIN AND KERATINIZATION 



on the answers arrived at (Perutz, 1959 and Schmitt, 1960). Even today, 

 when only partial and tentative answers can be given, the effort is made to 

 pose the problem in a form which envisages an answer in terms of the 

 physical chemistry of the molecular constituents of the system. Thus, 

 while it may be admitted at the outset that the treatment is foredoomed to 

 be incomplete, it is in this spirit that we shall approach the study of keratin 

 and keratinization. The discussion of keratin is not in itself an unfavour- 

 able theme for the purpose of systematizing parts of the already consider- 

 able amount of information on macromolecular biology since, largely as a 

 result of the work of Astbury, this protein and others closely related to it 

 have been shown to occupy a central position in any such discussion. 



The keratins form a class of resistant, insoluble proteins found in the 

 vertebrate epidermis and its appendages: hairs, feathers, claws, horns, 

 etc., and in small amounts in certain of the internal epithelia. The name 

 keratinization is given to the process by which these tissues are rendered 

 tough and insoluble. Together with the dermis, which is in effect a closely- 

 knit fibrous meshwork of the protein collagen, the epidermis and its 

 derivatives constitute the protective integument. Everyone is familiar in a 

 general way with the properties of the keratins: their insolubility, their 

 toughness combined with elasticity, the enormous variety of forms 

 assumed. Our object will be to try to correlate these properties with their 

 molecular basis and to sketch (for that is all we can do at present) their 

 development. Attention will be mainly directed towards cellular and 

 subcellular structures for, although the macroscopic anatomy of these 

 tissues is of great variety and interest, it would carry us beyond the present 

 intention to attempt a detailed description. For convenience, the salient 

 facts of their anatomy will be recalled ; for the details reference may be 

 made to the specialized texts referred to on p. 80. 



Current research emphasizes the role of two classes of macromolecules 

 in biology: the nucleic acids and the proteins. The genetic functions of 

 cells devolve on the nucleic acids which, it is currently believed, alone 

 exercise the power of self-replication and of controlling the replication 

 of other vital molecules (Crick, 1958); the proteins are components of 

 most of the other working parts of cells and of many extracellular deposits. 

 On the basis of their degree of aggregation two classes of proteins are 

 distinguished : the corpuscular proteins, that is proteins normally carrying 

 out their function in a particulate form in solution or adsorbed on surfaces. 

 Examples of this class are the respiratory proteins and most enzymes. 

 The other class is that of the structural proteins, which carry out their 

 function, often in part mechanical, by virtue of their property of forming 

 large aggregated, often fibrous masses. Among these we place, for example, 

 the contractile muscle proteins and the various reinforcing or protective 

 proteins including the keratins. 



