The Origin of Specific Protein- 101 



of developmental stages in the synthesis of a melanosome. These 

 fihrillar structures have heen found only in melanohlasts and pre- 

 sumably represent the response of the melanohlast to differentiating 

 stimuli. Once the melanosome has heen formed, it normally hecomes 

 pigmented by the deposition of melanin on the fibrils (Fig. 3) until 

 the protein structure of the granule is completely obscured by melanin. 

 However, the albino gene, which prevents tyrosinase synthesis (and 

 thus melanin formation), does not prevent the appearance of unpig- 

 mented melanosomes (Fig. 4). The structural details of the melano- 

 some are also under genetic control. The pink-eye gene brings about 

 a disorganization of the protein fibrils of the melanosome ( cf. Figs. 

 2 and 3) even though tyrosinase synthesis and pigmentation proceed 

 in an essentially normal fashion. These observations on the ultrastruc- 

 ture of melanosomes and the differentiation of melanocytes lead to 

 the general conclusion that the synthesis of a specific protein during 

 development requires a specific gene and the proper state of cell 

 differentiation in order for the gene to be activated. 



We have strong reasons for believing that the gene for tyrosinase 

 synthesis is present in all the cells of a mouse, not just in the melano* 

 cytes. However, the gene reveals itself only in melanocytes by the 

 production of melanin. Recently, however (Peck, 19611, it has been 

 possible to activate the gene for tyrosinase synthesis in cells that 

 normally never give any evidence of possessing this gene in an active 

 form. 



The neural retina shares a common ancestry with the pigmented 

 retina, but the cells of the neural retina differentiate into sensory and 

 nerve cells, presumably because of selective environmental stimuli 

 that do not reach the pigmented retina. However, by culturing the 

 cells of the chick neural retina in vitro in isolation from one another, 

 they can all be induced to manufacture pigment. This pigment passes 

 the tests for melanin and is presumably synthesized through the 

 catalytic activity of tyrosinase. These results obtained by Peck can 

 best be interpreted as an abnormal activation of the gene for tyro- 

 sinase in cells that normally never have this gene in an active form. 

 The word "normally" must be emphasized here, because an inherited 

 disease of human beings known as retinitis pigmentosa is character- 

 ized by the formation of melanin-like pigment in the neural retina. 



FIG. 1. Electron micrograph showing early stages in melanosome formation in the 

 pigmented retina of a pink-eyed hlack embryonic mouse. Fixed in osmic acid, em- 

 bedded in methacrylate. stained with uranyl acetate. Numbers indicate successive 

 stages in melanosome development. R indicates ribosomes. Mi mitochondrion. 

 (Courtesy of Dr. Frank H. Moyer.) 



