24 KERATIN AND KERATINIZATION 



The very convenient X-ray method, supplemented by chemical and 

 histological data, has permitted a fairly extensive survey of the types of 

 structural proteins and of their distribution among the various groups of 

 animals and plants. The results of this survey show that in structural 

 matters, as in biochemical matters in general, organisms are conservative 

 for, in the whole course of evolution, only the very limited number of 

 basic macromolecular structures described above have appeared and have 

 been adapted to the necessary variety of uses by introducing variations in 

 the side-chain composition. Given the limited range of biological polymers : 

 proteins, polysaccharides and nucleic acids, the number of solutions at 

 the molecular level to the problem of forming a protective integument is 

 seen to be severely limited. In a similar way the chemical devices available 

 to render proteins and polysaccharides more stable and tough are also 

 limited by the chemical possibilities of these polymers. 



There has, nevertheless, been a degree of biochemical evolution in the 

 usage of fibre-types and the several great branches of living organisms are 

 endowed to different degrees with the various possibilities. Cellulose is, 

 for example, the typical structural support in plants, although a material 

 morphologically very similar (but chemically distinct) is found in the tunics 

 of the tunicates— creatures (protochordates) a long way from plants 

 and related to our vertebrate ancestors, and perhaps even in vertebrates 

 (Jeffery and Cruise and Keech, 1959; Cruise and Jeffery, 1959). Higher 

 plants lack the fibrous proteins associated with movement in animals. 

 Insects and crustaceans typically contain chitin in their hard parts 

 (Rudall, 1949 and Lotmar and Picken, 1950). They possess contractile 

 muscle proteins, but apparently little collagen (see Fig. 13). 



The vertebrates, a comparatively uniform group, have most of their 

 protein fibres in common: all the land-dwelling forms harden their 

 epidermis with a keratinized protein and support the epidermis with a 

 dermis containing collagen. One remarkable difference distinguishes the 

 reptiles and birds on the one hand from the mammals on the other. The 

 mammals, in this sense at least the more conservative, have taken over 

 and keratinized an a-type protein probably similar to that occurring 

 primitively in cells and still found in the partly-keratinized skins of lower 

 vertebrates; in birds and reptiles a similar a-keratin is found in the softer 

 regions of the skin, but in their more characteristic hard parts, feathers, 

 scales and claws, an entirely different keratin of the 0-type, referred to as 

 feather keratin, is found. This discovery by Astbury and Marwick (1932) 

 was one of the early triumphs of the X-ray method of detecting and 

 classifying proteins. 



The production of proteins having a particular type of polypeptide 

 configuration is undoubtedly a consequence of a genetically-controlled 



