The keratinization process 227 



regarded as containing RNA although the adjacent cytoplasm itself is 

 densely packed with these. 



All the cells of the i.r.s. produce trichohyalin, but its appearance is the 

 more delayed the further the cells are from the outer root sheath, i.e. it 

 appears last and least abundantly in the cuticle of the i.r.s. 



The most characteristic property of trichohyalin (as also of keratohy- 

 alin) is its ability to be transformed into a compact fibrous modification. 

 This change is shown vividly in the light microscopy by sudden appearance 

 of birefringence in the cells of Henle at the level of the constriction in the 

 follicle. At the same time these cells become more difficult to stain and 

 the tubular sheath formed by Henle's layer becomes relatively insoluble in 

 strong solutions of urea. 



Electron-microscopically the change is equally sudden and remarkable. 

 It is associated with the appearance of bundles of fine filaments which seem 

 to " stream " out of the tips of the now lenticular-shaped droplets of 

 trichohyalin (Plate 21). Birbeck and the writer (Birbeck and Mercer, 1957 ; 

 Mercer, 1958) consider that this is evidence of a transformation of the 

 accumulated reserves of amorphous trichohyalin and is in some ways 

 analogous to other instances, e.g. that of actin, in which a fibrous modifica- 

 tion develops from a non-fibrous precursor (see Chapter III, p. 127). The 

 filaments appear in well-ordered bundles strictly parallel to the axis of the 

 follicle, i.e. to the long axis of the extended cells. Electron micrographs, 

 such as that shown in Plate 21, suggest a likeness to crystallization, the 

 long thin filaments seeming to move out from the droplets as though they 

 were being continuously formed at their surface. 



In cross-sections the bundles of trichohyalin are ambiguous in appear- 

 ance; both dots, i.e. sections of filaments, and wavy lines, sections of 

 sheets or ribbons, are found (Plate 20B). 



After the transformation, Henle's layer forms a strongly-coherent 

 sheath not dispersed in urea solutions. There is, however, no sign that the 

 filaments of trichohyalin cross from cell to cell. They continue to the cell 

 membrane and seem to end in close contact with this structure. A very 

 similar picture is found in muscle cells where the filaments also end on 

 extensive desmosomes (p. 42). The actual seat of intercellular adhesion 

 must still be sought in the intercellular cement between the cells. This 

 question has been discussed earlier (p. 84). 



The function of the transformation of the trichohyalin into a hard 

 fibrous tube at the level of the follicular constriction seems to be to provide 

 a solid retaining support to carry the still soft tissues contained within it 

 towards the surface of the skin. The somewhat slower transformation in 

 the inner layers of the sheath may permit the layers to remain plastic and 

 to form a more snug attachment between the hair and its sheaths (Auber, 

 1950). 



