106 KERATIN AND KERATINIZATION 



by the type of protein fibril contained, may be regarded as a modulation 

 although little is known about the stability of cell-type in the basal cells 

 supplying the different streams of cells. In the feather follicle the intermed- 

 iate cells (Fig. 48) produce feather keratin exclusively; the outermost layers 

 of the stratum comeum and the stratum cylindricum produce a-keratin with 

 little or no feather keratin. The several streams of cells advance in parallel 

 and the situation is not unlike that found in the hair follicle in which 

 several parallel streams also occur. In the hair follicle the weight of 

 evidence seemed to support the idea that the cells of the germinal matrix 



Fig. 48. The germinal collar at the base of the feather follicle showing 

 the several cell streams (1, 2, 3, 4) arising from it : (1) is the layer adjacent 

 to the dermal papilla which gives rise to the medullary caps; (2) is the 

 feather proper; (3) the feather sheath; and (4) the epidermal lining of 

 the papilla. The distribution of protein types is shown on the left-hand 

 side. 



were all similar and that their position determined their subsequent 

 development. In the feather system a more fundamental difference de- 

 velops: a difference in basic chain-type between the proteins produced 

 in the contiguous streams. No histochemical or electron-microscopical 

 feature distinguishes two classes of germinal cells. On his evidence Rudall 

 is unable to decide whether the factors for synthesizing the two kinds of 

 keratin are segregated into two classes of cells but considers the possibility 

 of cells producing both kinds of keratin in a mixed form. 



In snake scales the hard horny outer layer is feather keratin and the less 

 compact inner layers give an a-pattern (Rudall, 1947). Here it would seem 

 that the same germinal matrix produces both types of protein in series. 

 Unfortunately, again a certain ambiguity remains as to whether precisely 

 the same cells in the matrix contribute to both layers. There seems good 

 reason to believe that growth rhythms (perhaps diurnal) occur in the 

 feather follicle. The regularity of the barb structure suggests this clearly, 

 the presence of growth bars and of the succession of bands of radio- 

 active sulphur deposited in the calamus (Liidicke, 1959) following injection 

 of radioactive sulphur compounds are further indication. The relation of 

 rhythmic growth to the whole phenomenon of feather formation has not 

 yet been explored in detail. 



