276 



KERATIN AND KERATINIZATION 



(Wolbach, 1951; Borum, 1954) and that the effectiveness of the carci- 

 nogen depends on the phase of the hair cycle. When applied to a skin 

 area containing resting follicles the effect is small ; however, when the hairs 

 are growing the carcinogen seems able to penetrate the skin via the hair 

 follicles and produces a more profound effect. If the hair papilla cells are 

 killed by the applied chemical, normal follicle reformation becomes 

 impossible and the cyclic regenerative changes associated with growth 

 waves (p. 150) leads to proliferation of the outer root sheath cells with the 

 production of deeply-seated keratinizing cysts or tubes at the centres of 

 which the keratinizing cells, unable to exfoliate in the normal way, build up 

 to form keratin pearls (see Plate 24A) (Gliicksmann, 1945). The continued 

 growth forms at first a benign papilloma. According to Wolbach such 

 cells are still responsive to the stimulus of the growth waves passing over 

 the neighbouring normal skin. Genuinely malignant tumours may 

 ultimately develop from such papillomas. 



In the so-called " hairless " mouse mutant after the first wave of hair 

 growth, follicles fail to reform normally and very similar keratinizing cysts 

 may form beneath the skin from the upper portion of the root sheath 

 (Gnineberg, 1952). 



Pigmentation 



Most epidermal derivatives are pigmented, and the great variety of 

 integumental colours and pattern which can be produced is of immense 

 importance in the life of animals. The various colours (" structural 

 colours", which result from the diffraction of light by regular structures, 



excepted) are derived solely from combinations of black, brown or yellow 

 pigment in the form of granules combined possibly with a red non-granular 

 pigment. Table 21, adapted from Fitzpatrick, Brunet and Kukita (1958) 

 summarizes the main facts relating to granular pigments. 



Pigment granules are the exclusive product of pigment-forming cells 

 called melanocytes which in the adult animal are found among the basal 



