44 KERATIN AND KERATINIZATION 



be expected that these devices to increase intercellular adhesion will be 

 elaborately developed. In fact, as will be described later, the cell surfaces 

 become very convoluted, deeply interdigitated and heavily studded with 

 desmosomes. Further the intercellular cement becomes modified in its 

 solubility and chemical stability and forms dense intercellular sheets 

 between the surfaces. 



The Differentiation of Surface Organelles 



We referred above to those aspects of differentiation which arise from 

 the appearance within the cell of characteristic macromolecules. An 

 equally important feature is the appearance of the specialized surface 

 organelles which have just been described. 



The factors bringing about differentiation are little understood and their 

 investigation is currently a major research preoccupation which will be 

 discussed later. Here we wish merely to refer to a certain antithesis which 

 exists between the specializations found on free surfaces and those on 

 bound surfaces. It seems adequately demonstrated experimentally, by the 

 failure of isolated cells to differentiate or to maintain differentiation 

 (Willmer, 1954) and by the appearance of differentiation when different 

 cell types are cultivated together (Moscona, 1952, 1956, 1957), that 

 differentiation results from the effect of one cell on another. For cells in 

 the interior of an organism the environment is either wholly cellular or 

 consists of solutions containing the products of other cells. On the other 

 hand, cells on the surface are uniquely situated in having at least one face 

 free from the immediate influence of other cells. Their environment on 

 this face resembles that of a free living cell and, in fact, the surface 

 differentia appearing here are identical with those found on free living 

 cells. These special responses to an external situation: the sprouting of 

 cilia, the secretion of mucins, the formation of a cuticle or intracellular 

 fibrils beneath the membrane (see Figs. 14 and 19), may be regarded as a 

 cell's free-surface " repertoire." They appear whenever the surface is 

 free and are repressed on surfaces in contact. Metazoan cells respond 

 to contact by adhering, which implies the secretion of the specialized 

 intercellular cements and the suppression of the free surface repertoire. 

 It may be argued that the property which above all others distinguishes 

 the multicellular organisms from the unicellular (see Fig. 14) is the 

 formation of intercellular adhesives, probably macromolecules among 

 which appeared to be mucopolysaccharides (p. 54) and proteins. Such 

 intercellular cements must have played their part in the evolution of the 

 metazoa and, in the life of each individual, the cells appear to pass from 

 an embryonic condition of poor adhesion to an adult in which, in many 

 organs, strong intercellular attachments are the rule (see Fig. 14). The 

 keratinized tissues carry this process to the extreme. 



