224 KERATIN AND KERATINIZATION 



dense component was forming between the filaments and causing them to 

 pack more compactly (Birbeck and Mercer, 1957; Mercer, 1958). At the 

 level of the constriction (Fig. 90) and immediately above it, it is possible to 

 distinguish a population of fairly distinct fibrils (diameters in the range 

 005-02^) each consisting of masses of fine, light filaments in quasi- 

 hexagonal array, which thus constitute a rather definite level of organi- 

 zation of the fibrous keratin (Fig. 80 and Plate 13); then, as the cells 

 advance, these fibrils rapidly fuse laterally, to produce progressively large, 

 irregularly-shaped aggregates, and finally an almost complete fusion into a 

 solid mass in which only residual, irregularly-dispersed gaps remain as 

 evidence of the earlier existing interfibrillar spaces (Fig. 16). It should be 

 noted that whereas the filaments appear to be perfectly definite structural 

 elements, all having the same diameter, the fibrils, which are aggregates of 

 filaments (Figs. 80 and 102), are not of uniform size although they cluster 

 around an average diameter. 



In the later stages of consolidation, the hexagonal arrangement of the 

 filaments within the fibrils becomes distorted, leading to the appearance 

 in cross-section of spirals or fingerprint-like whorls. It is possible that 

 each fibril becomes slightly twisted on its long axis (Plates 14A, 15 and 

 16). 



At the histological level (light microscopy), the course of keratinization 

 in the other hard mammalian keratins seems essentially similar to that of 

 hair. The fine histology has not yet been studied, but there would seem to 

 be every reason to think it will also prove similar to that of hair. In 

 feathers the formation and condensation of fine filaments follows similar 

 lines (Mercer, 1957). There is nothing in the electron-microscopic appear- 

 ance of the feather cells to show that a /8-type rather than an a-type protein 

 is being formed. 



The Cuticle 



The presumptive cuticle cells, which form a single layer as they leave 

 the matrix of the bulb, tilt sharply as they approach the bulb constriction 

 and achieve their fully-tilted and overlapping condition at the level of the 

 constriction where Henle's layer turns birefringent. Here they still contain 

 little or no keratin and their contents are no longer symmetrically disposed. 

 Nuclei are usually found towards the basal pole of each flattened cell and a 

 well-developed system of vesicles (Golgi apparatus) lies largely apical to it. 

 The flattened cell is almost vertical and its two surfaces now face towards 

 very different environments, the internal towards the hair cortex, the 

 external towards the sheaths and follicular surroundings. In such circum- 

 stances the cell contents develop a stratification parallel to the cell mem- 

 branes (Plate 20A). 



Keratin appears as small (~300 A) rounded, dense droplets and moves 



