CHAPTER VI 



The Keratinization Process 



In this chapter we shall discuss in detail the changes by which the 

 living and growing epithelial tissue is converted into a lifeless, tough, 

 insoluble, translucent, fibrous substance. These changes, occurring in a 

 more-or-less clearly-defined zone which follows the zone of differentiation 

 and growth (Fig. 25, p. 55) are characteristic and broadly similar in all 

 instances; by them we recognize and define a keratinized or cornified 

 tissue. In thin or quiescent skins it may not be possible to distinguish a 

 distinct zone of keratinization ; in many thicker skins, or in the matrix of 

 the hard keratins, even more subdivisions suggest themselves. 



The literature concerning the staining of keratinized tissues is large and 

 many writers have described progressive changes in staining properties of 

 cells during their keratinization. A good historical account with particular 

 reference to wool and hair will be found in Auber's monograph (1950). 

 Recent reviews have been given by Montagna (1956) and Braun-Falco 

 (1958). There is no point in further summarizing these accounts; rather, 

 we shall by selection and omission attempt to co-ordinate the findings into 

 a coherent view of the process. 



For all its usefulness in making evident histological detail, much of this 

 work is disappointing in contributing little towards elucidating the process 

 itself because most dyes have little specificity and their reactivity is poorly 

 understood. Histochemical demonstrations of definite substances (or 

 groups) are more rare and more useful. It is fortunate that reliable histo- 

 chemical methods exist for demonstrating thiol groups (SH). 



Since there is some difference between the sequence of events in the 

 formation of a hard keratin and a soft keratin, it will be better to describe 

 these separately, commencing with a typical hard keratin, hair. 



The hard keratins 



These include all the coherent appendages: hairs, feathers, nails, 

 claws, horns, etc. Most of the data relating to the hardening of this type of 

 keratin comes again from the hair follicle, but the fewer observations made 

 on other examples (horn, nails, and feathers) are sufficient to show that 

 the changes seen in the hair follicle are general and justify the use we 

 propose to make of it for purposes of illustration. For economy of pre- 

 sentation it will be an advantage to summarize the data in a uniform 



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