282 KERATIN AND KERATINIZATION 



The appearance of granules in early stages of formation suggests that a 

 rather regular protein framework (Fig. 115 (h)) is laid down first and is 

 subsequently melanized by a polymerization occurring in its interstices 

 (Birbeck and Barnicot, 1959). The melanoprotein thus seems to be a well- 

 ordered particle although little of this is apparent after melanization is 

 complete. Melanocytes in albino hair possess large numbers of small 

 vacuoles containing the protein framework, but no deposition of dense 

 material occurs. In these cases, in the absence of tyrosinase, the melanin 

 polymer is not formed. 



Some interest attaches to the control of melanocyte activity in the hair 

 root which is correlated with the hair growth cycle (Montagna, 1956) and 

 with the development of various pigmentary patterns both in hair and in 

 feathers. Montagna's observation that melanocyte activity ceases shortly 

 before hair growth ceases is cited as evidence of the accumulation of a 

 general growth inhibitor (see Chapter IV) affecting first the more sensitive 

 melanocytes. 



The yellow-red pigment (pheomelanin) also occurs in granular form, 

 but less is known about the metabolic pathways leading to its formation. 

 The ultimate precursor appears to be the amino acid tryptophane. 

 Reference may be made to the review by Fitzpatrick et al. (1958). 



