THE KERATINIZATION PROCESS 279 



protein in the pre-melanin granules on which the melanin later poly- 

 merizes (Fig. 115 (h) and (i)) (Birbeck and Barnicot, 1949). 



The similarity between the types of chemical resistance exhibited by 

 melanin granules and the cell membranes of the keratinized tissue has been 

 mentioned above. It is certainly possible that a similar tanning reaction 

 has cross-linked the proteins of the membranes and the granules — a 

 possibility of some phylogenic interest. Recently, tanned membranes 

 having similar solubility properties have been described by Jones (1958). 



(a) (b) <QgH^ (c) ( d) 



• • 



• •• 



• 



• £<® 



• • 



(f) 



©fa 



(e) v " (9) M 



Fig. 115. Varieties of melanin granules. Redrawn to scale from authors 



cited. The examples are chosen to show the extreme range of size and 



type. X 10,000. 



(a) Human hair white male (Birbeck, Mercer and Barnicot, 1956). 



(b) Human hair negro male (Birbeck, Cuckow and Barnicot, 1955). 



(c) Horse hair (Laxer and Whewell, 1955). 



(d) Black alpaca (Laxer and Whewell). 



(e) Retinal eye pigment (Birbeck, private communication). 



(f) Harding-Passey melanoma from section of tumour by the writer. 



(g) Squid ink (Birbeck, private communication). 



(h) and (i) Human-head hair with indications of internal structure as 

 seen in sections of melanocytes. 



The granules vary in size from 0- 1 to 3/x and for this reason their size and 

 shape has been much studied electron-microscopically. This may be done 

 either on isolated granules or in sections of fixed tissue. Fig. 115 shows 

 outline drawings of a number of types of granule and will give some idea 

 of the range of size encountered. Sections of granules often suggest a clear 

 separation between layers of melanin deposits and those of a lighter 

 material, presumably protein. 



The Chemistry of Melanization 



Although the protein moiety and its relation to the melanin polymer 

 has been little studied, much attention has been paid to the biochemical 

 steps in the formation of the melanin itself. Melanins, quite apart from 



