100 KERATIN AND KERATINIZATION 



the papilla, they are always the fourth or fifth cell layer from the surround- 

 ing basement membrane. Their cells acquire a cuboidal shape, which is 

 easily apparent, since their membranes have drawn together zipper- wise, 

 effacing surface irregularities and becoming somewhat denser. The same 

 events follow in the layers of Huxley and Henle; they occur several cell 

 diameters higher in the central mass of cells destined to become cortical 

 cells. Between these latter cells, openings are observed as high as the 

 papillary tip where they are occupied by processes of the melanocytes 

 (see p. 276). 



While it must remain purely speculative in our present state of know- 

 ledge, it is possible to suppose that the cuticles lead the way in differen- 

 tiation because they, in some way dependent on their position relative to 

 the dermis, first begin to form and secrete their specific intercellular 

 cement. The possibility of developing a columnar epithelium by a zipper- 

 like spread of adhesive cell contacts was envisaged by Schmitt (1941) 

 (Fig. 44), and the change in shape of the cuticle cells looks very like an 

 illustration of Schmitt's hypothesis. 



Intracellular differentiation in the hair bulb 



The first visible signs of synthesis of intracellular products are observed 

 in the cells immediately in contact with the external root sheath layer. 

 These cells become Henle's layer, the most peripheral layer of the inner 

 root sheath (Fig. 42). Small trichohyalin droplets appear in the cyto- 

 plasm of cells at about the middle line of the bulb and rapidly grow in size. 

 Henle's layer hardens, becomes birefringent and clear suddenly at the 

 level of the constriction of the bulb. Its function at this level appears to 

 be largely mechanical; it provides a solid cylindrical support to carry 

 upwards the soft cells within. Its outer surface is said to slide over the 

 surface of the outer root sheath, which in this respect is a static structure 

 and does not move out with the rest of the follicle. The mechanics of 

 these movements are far from being clearly understood (Montagna, 1956; 

 Auber, 1950). 



When first formed the trichohyalin droplets are isotropic ; at the level 

 where Henle's layer hardens, the cells suddenly become birefrin- 

 gent and the layer itself becomes clear and more difficult to stain. 

 Electron-microscopically fine filaments or ribbons can be seen extending 

 from the tips of the lenticular-shaped droplets of the amorphous precursor 

 (Plate 21). These condense to give a compact highly-oriented mass 

 which is the clear, glassy, birefringent layer visible in the light microscope. 



The course of events in Huxley's layer and in the cuticle of the inner 

 root sheath is similar, but the tempo is slower, the cells remaining full. 

 Following Auber (1950) we may suppose that these cells form a firm but 

 tenacious vice to grip and support the softer tissues of the hair itself. 



