THE KERATINIZATION PROCESS 



211 



diagrammatic form in one place. The hair follicle, the best-studied and 

 most "diagrammatic" tissue, will be used for this purpose and is shown in 

 the series of Figs. 90, 91 and 97. The illustrations in most cases refer to 

 events in the presumptive cortical cells and each diagram summarizes the 

 findings relative to some property or particular activity. An inspection of 

 these diagrams shows clearly that in the hair follicle several distinct stages 

 in keratin formation, separated in time and in space, may be distinguished. 

 These are indicated in Fig. 90 : 



A, zone of cell division (germinal matrix). 



B, zone of differentiation and cell growth. 



C, zone of fibril formation. 



Dl , ... f(i) 



„ }, keratimzation zone {;...• 

 e y l(u) 



F, keratinized zone. 



Analogous levels can usually be distinguished in the other tissues. 



The diagrams have been devised in terms of the several interrelated 

 themes which can be distinguished in the process of keratin formation: 



( 1 ) General cellular phenomena : (a) nucleic acid metabolism and protein 

 synthesis (Fig. 97b); (b) cell metabolism (Figs. 97a and d). 



(2) Phenomena peculiar to Keratinisation: (a) orientation (Fig. 91); 

 (b) stabilization (Figs. 91, 92 and 97c). 



The Development of Orientation (Fig. 91) 



The fibrous contents of the cells of the hard keratins are usually well 

 oriented and, since this orientation is related to the mechanical function, 

 its development is of special interest. In the upper regions of the bulb 

 (Chapter III) the cells of the presumptive cortex elongate and there is a 

 marked increase in the number of oriented fibrils within the cells. The 

 orientation is most conveniently observed by means of the polarizing 

 microscope (Schmidt, 1924) and its quantitative development may be 

 measured by means of a compensator (Mercer, 1949b). Figure 91 (r.h.s.) 

 shows the growth of birefringence in a follicle plucked from the human 

 head and the l.h.s. shows the development of birefringence in relation to 

 the anatomy of the follicle. The important feature is the rapid rise at the 

 constriction of the bulb to a value which is almost equal to that of the final 

 hair. 



While polarization microscopy provides the simplest method of de- 

 tecting the existence of an oriented structure, the interpretation of the 

 results is not without ambiguity. It is useful to be able to distinguish 

 between intrinsic birefringence, i.e. birefringence due to an oriented 

 molecular structure, and form birefringence, which arises simply from a 



