THF KERATINIZATION PROCESS 223 



The fine histology of the hair follicle in the keratinization 

 zone 



In the hair follicle may be found examples of the formation of both hard 

 and soft keratins and for this reason the detailed study of the fine structure 

 of its various layers provides an opportunity to compare and contrast the 

 two modes of development. 



As indicated in Chapter III, the separate cell streams, which form the 

 inner root sheath, the cuticle, cortex and medulla of the hair, are clearly 

 differentiated above the middle of the bulb ; synthesis and stabilization 

 follow distinct courses in each. They will be described separately. 



The Cortex 



Interest here chiefly concerns the further development and consolidation 

 of fibrous keratin. The fine parallel filaments which first appeared as 

 wispy bundles in the cells of the presumptive cortex at a level a little below 

 the tip of the papilla (Fig. 90, zone B) rapidly accumulate in the upper 

 regions of the bulb until, at the level of the constriction of the follicle, the 

 cells appear almost full. The length of the follicle from its constriction to 

 the level where the cortex achieves its definitive form has been called the 

 zone of keratinization (D and E, Fig. 90). 



The fine filaments (diameter ~ 60 A) (Plate 12B) also described as 

 the protofibrils (Chapter III) are at first individually distinct and form 

 small clusters which rapidly grow in size. When in the upper bulb, these 

 aggregates become a few tenths of a micron in width, they can be seen in the 

 light microscope and are then described as fibrils (Plates 13, 15 and 

 16). It is at this same level, a point immediately following the follicular 

 constriction, that birefringence rapidly rises and reaches almost the 

 value of the final fibre and an oriented a-type X-ray pattern (Fig. 91), 

 indistinguishable from that of the fibre, can be obtained. These obser- 

 vations show again that the synthesis of the basic fibrous structure is 

 virtually complete at this level and that the chemical changes, which follow 

 and produce hardening and stabilization of the formation, must take place 

 outside the crystalline regions, and probably outside the filaments them- 

 selves if we take the further step of identifying the filaments with the 

 " X-ray crystallite". 



Some support for the idea that changes external to the filament occur 

 comes from changes in the electron-microscopic appearance of the bundles 

 of filaments as they advance through the zone of keratinization. At first 

 separate filaments are seen in cross-section as clusters of dense dots and 

 their arrangement is rather irregular. Then areas of better order (quasi- 

 hexagonal packing) appear in which the filaments now appear relatively 

 light against a darker matrix (Plates 15 and 16). It is as if a new and more 



