214 KERATIN AND KERATIN IZATION 



The appearance of an oriented structure at the molecular level can be 

 demonstrated readily by X-ray diffraction (see Chapters I and V), and the 

 method has the advantage that the type of structure (in the molecular 

 sense) is also demonstrated. From the bulb alone only a diffuse, uncharac- 

 teristic diffraction pattern of two rings (Fig. 91) can be obtained. 

 Immediately above the bulb, however, an oriented a-type pattern, apparently 

 identical to that of the final hair, is obtained. This finding shows beyond 

 question that at this level there is a considerable amount of oriented 

 protein present with essentially the same crystalline molecular organization 

 (a-type) as in mature hair. 



This important point, that the appearance of the typical a-structure 

 precedes keratinization was demonstrated originally by Derksen, Heringa 

 and Weidinger (1937), using the thickened epidermis of cow's lip, a more 

 amenable material than hair follicles, and by Giroud and Champetier 

 (1936) using the " chestnut " of the horse. Sections cut at different levels 

 up to the fully-hardened layers gave the same a-pattern. Similar experi- 

 ments on a cow's nose were later carried out by Rudall (1946) and by the 

 present writer using the hair root (1949b). After heating in water the 

 lower unstabilized layers become disoriented. 



In their totality these experiments prove that the filaments, which can 

 be seen to form in the cells below the keratinizing zone, already possess 

 the typical a-structure, and further that, whatever chemical reactions may 

 go on in the later stages of keratinization, they in no way affect the arrange- 

 ment of molecules in the crystalline regions. 



The older histologists, whose work is summarized by Biedermann 

 (1926), recognized that the oriented structures and birefringence arose 

 from fibrils and deduced the existence of smaller invisible anisotropic units 

 from their polarization studies. Various schemes of fibrillar architecture 

 (see Biedermann) were developed which were substantially correct. For 

 this type of research the modern electron microscope is now more con- 

 venient (see p. 223) but the polarizing microscope is still much used. 



The Development of Stability 



Fully-hardened hair has a high stability towards many chemical reagents 

 and physical conditions and, accordingly, the development of keratinization 

 may be assessed in terms of the action of any of these influences. Owing 

 to its linear arrangement the plucked hair root is a very convenient object 

 on which to make such tests, and the results of several are depicted in Figs. 

 91 and 92. 



All these tests agree in showing that the cortex of human head hair is 

 fully stabilized at a level about one-third of the total length above the bulb. 

 Further, it shows that the keratinization zone itself which extends from the 



