THE KERATINIZATION PROCESS 281 



Distal to the nucleus is a clear region relatively free of dense melanized 

 particles with large numbers of small, rounded vacuoles (Golgi-type 

 vacuoles) which contain variable amounts of denser material often arranged 

 in concentric shells. The vacuoles on the periphery of this region contain 

 additions of very dense material identifiable as melanin. 



On both histological and electron-microscopical grounds there is now 

 little doubt that the intracellular sites for the formation of melanin are these 



Fig. 116. Possible courses in the synthesis of melanins and melanoproteins. * 



o-diphenols > o-quinones > simple polymers 



(melanins) 

 (poorly-organized) 



+ 

 proteins (organized framework) 



bi-polymersf 

 melanoprotein 



* From Mason (1955) with modifications. 



t Morphological evidence would indicate that this is a bipolymer 

 whose components are each of a macromolecular size. 



small vesicles. An older opinion, based on similarity of size and staining 

 properties of granules and mitochondria, held that the melanogenic vesicle 

 is really a mitochondrion. Since the granules contain, in addition to 

 tyrosinase at least two other enzymes, cytochrome oxidase and succinic 

 dehydrogenase usually located in mitochondria, some support is given for 

 this view. Morphologically, however, the two organelles are quite distinct 

 although in their remote origins both may possibly be traced back to 

 similar vesicular formations. Recently mitochondria and premelanin 

 granules, characterized by containing tyrosinase and not mitochondrial 

 enzymes, have been separately isolated from melanoma homogenates 

 (Baker et al., 1960). 



