Chapter 5 

 MOLECULAR MECHANISMS OF DIFFERENTIATION 



AN INQUIRY INTO THE PROTEIN FORMING SYSTEM OF DEVELOPING CELLS 



Heinz Herrmann 



I. INTRODUCTION 



The basic concepts of experimental embryology such as induction, determination, 

 competence and regulation remained closed for a long time to an interpretation 

 in terms of other fields of biology, of chemistry, or of physics. Consequently the 

 bulk of experimentation in embryology appeared to be largely a verification and 

 extension of the initial observations on different organ systems and in diverse 

 species. From this work arose a body af data which was systematized according 

 to the empirically defined phases of embryonic development. Even more recent 

 comprehensive texts of embryology follow in part this type of organization e.g. 

 "Analysis of Development" (Willier, Weiss and Hamburger, 1955). 



While many phenomena of embryology have at least a well-defined empirical 

 content, cellular differentiation is lacking in any sharp delimitation. It is 

 conventionally treated as the elaboration of specific cell characteristics during later 

 embryonic or fetal development. Defined in this sense, differentiation and some 

 of its chemical aspects generally and in several organ systems specifically has been 

 covered by recent reviews (Boell, 1948; Weiss, 1950; Moog, 1952; Ten Cate, 1953; 

 Herrmann, 1953a; Gustafson, 1954; Spratt, 1954; Boell, 1955; Duspiva, 1955; 

 Ebert, Tolman, Mun and Albright, 1955; Flexner, 1955; Needham, 1955; Shen, 

 1955 and Waelsch, 1955). To add another similar survey of this topic would seem 

 unnecessary. It may be justified, however, to explore the extent to which the facts 

 of differentiation in early and in late embryogenesis can be subsumed under a 

 more general, conceptually fruitful, frame of reference. 



From the reviews of differentiation cited above, it follows that cellular differences 

 are based on the formation of new types of proteins., Even the elaboration of such 

 specific cell characteristics as the deposition of ground substances, or of a lipoidal 

 myelin sheet, initially depends on the presence of proteins with enzymatic activity 

 which synthesize these substances. As a first approach one can consider that the 

 appearance of any new structural or functional cell property is the final expression 

 of a change in the protein pattern of a cell due to the formation either of structural 

 proteins like lens proteins, collagen or keratin, or of enzymes and the products of 

 their activity. It should be stated that the mere chemical assay of a protein per se 



Literature p. ^39 



