34 KERATIN AND KERATINIZATION 



(Ingram, 1957) may seem a serious objection to the universal application 

 of this proposal. On the other hand residues not involved in forming the 

 precisely-patterned topography of the active patches of a protein molecule 

 may perhaps be exchanged with greater impunity (Tristram, 1953). The 

 probability that the function of a keratin does not demand the same 

 detailed specificity of structure, as for example an enzyme, may be relevant 

 here. Keratinized tissues have a mechanical function and a role as water 

 barriers. The properties required are a certain insolubility and toughness 

 combined with elasticity. For this a precise sequence of amino acids does 

 not seem immediately in demand ; numbers of polymeric networks having 

 an appropriate balance of hydrophobic and hydrophilic side chains might 

 be envisaged with similar properties. Further, a case could be made out 

 for supposing that the ability of the germinal cells to differentiate into 

 cells producing proteins of a variable composition could be the basis of 

 adaptation. Normally, we could suppose, the pattern of synthesis is 

 dominated by the site, e.g. producing hair keratin, horn keratin, etc., in 

 special sites, but, if the synthetic mechanism were also capable of con- 

 tinuous adjustment to (say) mechanical demands, the system would be 

 adaptive as in the epidermis it seems to be. 



The biochemical mechanism of such an " adaptive synthesis " would 

 have to be sought in a selective pressure brought to bear on the population 

 of RNA molecules which emerges from the nucleus during the course of 

 synthesis (see p. 1 10). Since the mechanism by which external influences are 

 fed back through the cytoplasm to influence nuclear activity is one of those 

 phenomena most in need of experimental elucidation, we can carry this 

 speculation no further. 



The fine structure of cells 



Electron Microscopy and Cytology 



X-ray diffraction methods are rarely applicable to cell inclusions or to 

 surface structures except when components can be isolated in a suitable 

 form. Chemical analyses and indirect physicochemical methods have 

 proved of more value but at the present time most of our knowledge is 

 coming from electron microscopy. In many cases, as may be appreciated 

 from Fig. 1, this form of microscopy appears the only approach to such 

 minute and irregular detail. 



In the last few years following the perfection in the early 1950's of 

 methods of fixing, embedding and sectioning of biological material for 

 the electron microscope, cytology has undergone a veritable revolution. 

 Today we possess a wealth of morphological material covering most cell 

 types expressed largely in terms of the membranes, particles and filaments 

 whose images appear in electron micrographs. The work of recognizing 



