KERATIN AND MOLECULAR BIOLOGY 5 



to the various texts available (Bunn, 1946; Hall, 1953; Schmidt, 1924; 

 and Oster and Pollister, 1955-56). 



It will be seen from Fig. 1 that with present-day instrumentation we 

 possess the means to investigate all levels of biological significance. Thus 

 it is now feasible to hope to obtain the information necessary to permit of a 

 stepwise reconstruction of a tissue beginning with elements of a 

 molecular size. Already the results of comparative fine-structure studies 

 have indicated the nature of the basic structural elements found in all 

 cells and developed to different degrees in different types of cells. Its 

 particular endowment of these structural elements gives a cell its 

 characteristic cytology and, in turn, from the special association of these 

 cells arises the histology of the tissue. For example the keratinized tissues 

 themselves achieve their primary function as the toughened outposts of 

 the protective integument by virtue of an enhancement principally of two 

 structural features common to all Metazoan cells. In the first place all 

 cells contain greater or lesser amounts of structural fibrous proteins; in 

 keratinized cells there is a great increase in these proteins which are 

 subsequently subjected to chemical changes which stabilize them. 

 Secondly, the cells of most tissues must adhere and in the purely cellular 

 tissues, such as the epithelia, this means that the surfaces of the cells 

 themselves must stick together. In the specialized protective epithelia 

 including the keratinized tissues, clearly this intercellular adhesion must 

 be enhanced. Thus our discussion of keratinization will be found to be 

 concerned largely with these two aspects of the tissues : the nature of the 

 stabilized proteins within the cells and the manner in which the cells 

 themselves are held together. We shall accordingly begin by giving a 

 general survey of the types of fibrous proteins found in tissues and follow 

 this with an account of the fine structure of cells with emphasis on the 

 structural devices associated with their surfaces which may be concerned 

 with intercellular adhesion. 



Types of fibrous proteins and their classification 



The classification of the proteins has been made on the basis of several 

 grounds all more or less arbitrary. We have mentioned the convenience 

 of the division into structural and particulate proteins, which is a useful 

 distinction when one is concerned with insoluble proteins forming 

 structures extending far beyond the molecular level. Traditionally the 

 soluble, particulate proteins are classified on the basis of their solubilities 

 in various aqueous media, a means of distinction which dates back to a 

 time when the principal preoccupation of the protein chemist was the 

 separation of definite individuals from mixtures. 



The basic chemical character of a macromolecule (protein, poly- 

 saccharide, nucleic acid) may be established by the chemical analysis of a 



