98 The Nature of Biological Diversity 



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Thus cell differentiation becomes simply an expression of an ante- 

 cedent pattern of gene activation or inhibition. The central problem 

 of developmental biology then becomes the problem of the control 

 of gene function. 



Some investigators would state the problem differently; they favor 

 the alternative hypothesis that pictures all the genes as functioning 

 continuously. The effect, or consequences, of gene activity in this view 

 would be modified by the specific environment in which the gene was 

 functioning. Thus different cells with identical arrays of functioning 

 genes could be produced. Although this view is logically adequate, it 

 confronts the cell with particularly difficult and complex probems of 

 regulating metabolism. The simplest and most efficient control mecha- 

 nism should, it seems, be directed toward turning the gene on and off 

 rather than toward controlling the products of gene activity, for these 

 products would be far more numerous and prone to migrate to many 

 different positions in the cell. 



Metanogenesis 



Whatever the correct hypothesis of cellular differentiation may be, 

 the various developmental and genetic aspects of the problem of cell 

 differentiation are well illustrated during the development of the 

 mammalian melanocyte (Markert and Silvers, 1955). The differen- 

 tiation of an embryonic cell into a melanocyte requires a highly 

 organized sequence of environmental stimuli. These successive stimuli 

 transform the inelanoblast into a cell with distinctive biochemical 

 and morphological characteristics. According to the preferred view, 

 these characteristics soon result in activation of the gene for tyrosinase 

 synthesis. The enzyme tyrosinase is not only characteristic of melano- 

 cytes but is indispensable for the terminal stage of differentiation 

 which results in the synthesis of melanin. Thus the origin of this 

 specific protein (tyrosinase) is dependent upon two distinct but re- 

 lated processes — the transformation of an embryonic cell into a ma- 

 ture melanocyte and then the activation of the specific gene for 

 tyrosinase synthesis. These sequential steps in melanocyte differentia- 

 tion are normally controlled by many different genes. We can recog- 

 nize several of these steps because mutant genes affecting them have 

 been discovered in a variety of animals, particularly mice. The ab- 

 normal effects of these mutant genes enable us to construct a partial 

 picture of the normal activity of several genes involved in the differ- 

 entiation of mouse melanocytes. 



Two quite different kinds of melanocytes — epithelial and dendritic 



