268 KERATIN AND KERATTNIZATTON 



The surprisingly hydrophobic character of the intact surfaces of hairs and 

 feathers, even after thorough removal of lipids, is presumably due to the 

 chemical inertness of the modified external cell membrane. When this is 

 mechanically or chemically damaged the material becomes wetable and 

 dyes and other large molecules more readily penetrate (Mercer et al., 1949). 



Fig. 112. Illustrating the resistant residues which remain after a wool 

 fibre, oxidized by means of peracetic acid, is extracted with ammonia. 

 The cell membrances and nuclear residues of the cortical cells are at M 

 and R, respectively. The external membrane E of the cuticle cells 

 encloses the swollen fibre. In the paracortical segment P some keratin 

 remnants may persist; O is the orthocortex (see text). 



The medulla 



As is apparant from the description given by Auber (1950) (Fig. 113), the 

 changes taking place in the medulla of hairs are complex. Keratohyalin 

 granules appear in the differentiated cells and electron micrographs of 

 rodent hairs show that they change into a fibrous form as elsewhere. The 

 total amount synthesized is inadequate to fill the cell cavities and much of 

 the transformed material simply condenses against the cell membranes. 

 During desiccation intracellular gaps appear (Fig. 113c) and the final result 

 may be a rather open girder-like structure, light but stiff. 



Chemically the medulla resists alkalis and keratinolytic reagents and 

 may be isolated (undoubtedly altered) by digesting away the keratinized 

 cortex and cuticle. It thus resembles the altered membranes of these 

 structures rather than their keratinized contents and morphologically this 

 seems to arise from a close fusion of fibrous keratohyalin and membranes. 

 That much of the protein of the medulla is of the trichohyalin type is 

 supported by analyses, reported by Rogers (1959a), of the medullary cells 



