THE KERATINIZATION PROCESS 275 



are only partly understood and are more properly viewed in the wider 

 context of the general factors controlling the differentiation of the many 

 cell subtypes produced from the epidermal germinal layer. 



The problem of crimp formation has already been discussed in Chapter 

 IV and it is obvious that the factors governing keratinization must be related 

 to the other morphogenetic factors involved. Auber (1950) and Rudall 

 (1936) discovered that keratinization is bilateral in follicles producing 

 crimpy fibres, i.e. the hardening of the fibre begins at a lower level on the 

 side which will emerge as the more keratinized (Fig. 114). Auber showed 

 that in such follicles the hair shaft was asymmetrically placed within the 

 cylinder of the outer root sheath and that hardening commenced on the 

 side of the hair nearest the thinner part of the sheath. This immediately 

 suggested that something supplied from beyond the sheath was required 

 for keratinization and could penetrate (or escape) most readily on the thin 

 side. There is ample other evidence implying a close dermo-epidermal 

 co-operation in controlling epidermal differentiation (p. 61). By partly 

 dissolving plucked follicles it has been shown that the difference between 

 the two sides extends below the keratinization zone, i.e. that the cell 

 types are committed early in their course. This again illustrates the 

 interlocking of the events at various depths in the follicle (p. 156). 



The chemical basis of the difference between these variants of keratin is 

 not yet clear even in a case as well studied as wool. There is evidence from 

 histochemistry (Dusenbury and Menkart, 1956 (Plate 24B) : Menkart and 

 Coe, 1958) and from the analysis of resistant residues to suggest (but not 

 prove) that in wool the resistant fraction (para-type) has a higher cystine 

 content and perhaps differs in other respects. Simmonds (1958) could, 

 however, find no difference between high- and low-crimped material. 

 Rogers (1959b) found that the packing of the a-filaments in the ^>ara-cells 

 was hexagonal (Plate 16) and that whorls occurred more often on the 

 ortho-side (less resistant). Nevertheless whorls are no less common in 

 human hair which on a basis of its resistance and sulphur content is para- 

 type (Birbeck and Mercer, 1957). 



Keratinized cysts and epidermal tumours 



Tumours and cysts arising from epidermal cells are not uncommon and 

 may exhibit interesting, if abnormal aspects of keratinization that deserve 

 notice here. The skin of the mouse is also one of the commonest test 

 objects of experimental carcinogenesis, and figured largely in the classical 

 work of Kennaway and his colleagues which led to the isolation of definite 

 carcinogenic compounds from tar (Kennaway, 1955; Ludford, 1925). 



It seems now established that the tumours produced by benzpyrene and 

 other carcinogenic hydrocarbons take their origin from the rather undiffer- 

 entiated basal cells of the upper outer root sheath of the hair follicle 



