228 KERATIN AND KERATINIZATION 



Soft keratinization 



The Epidermis 



Matoltsy (1958) gives as constituents of stratum corneum of human 

 epidermis: 65% insoluble keratin, 10% soluble protein, 10% dialysable 

 material (largely amino acids), 7-9 % lipid and 5 % cell membranes. The 

 tissue is clearly more heterogeneous than a hard keratin and gives the 

 impression, from its content of soluble protein and low molecular weight 

 substances, that the hardening process is incomplete. On the other hand, 

 a comparison of Plates 16 and 17 shows that, apart from the orientation 

 of the filaments, the fine structure of soft keratin is very similar to that of 

 hard keratin. 



There is a large amount of histochemical work on the epidermis. Like 

 that on hair its significance is marred by the lack of specificity of the means 

 employed. The significant tests (summarized in Montagna (1956)) are 

 essentially those already described for hair and their meaning for the 

 understanding of keratinization is much the same. A good deal of attention 

 has been paid to elucidating the nature of keratohyalin granules but 

 without complete success (see p. 226). Staining with fluorescent dyes 

 (Jarrett et al., 1959) demonstrates vividly several phases of keratinization, 

 but the interpretation of the effects is not obvious. 



The distinct histological feature of epidermal (soft) keratinization, when 

 several layers are fully developed, is indeed the occurrence of a granular 

 layer due to the deposition of granules of keratohyalin. This substance is 

 in essentials identical with trichohyalin of the inner root sheath of the hair 

 follicle but whereas in the i.r.s. trichohyalin is apparently produced alone 

 and its transformation provides the entire fibrous contents of the hardened 

 cells, the course of formation of the fibrous component of the epidermal 

 cells is more complex and not fully understood. Filaments, as far as can be 

 ascertained, of the same type as those of the cortex of hair follicle, are 

 already to be found in small amounts in the basal layer cells (Plates 7 and 

 9). Here these are directed predominantly at right angles to the basal 

 layer and a weak birefringence in this direction is detectable (Montagna, 

 1956; Biedermann, 1926). In the stratum granulosum keratohyalin 

 granules appear as dense amorphous bodies apparently quite independently 

 of the earlier-formed filaments (Plates 22 and 23). 



The relation between keratohyalin granules and the fibrous keratin of 

 the stratum corneum has for long been a subject for dispute and it cannot 

 be said that electron microscopy has finally settled the points at issue. 

 There are two main points of view current, both of long standing although 

 it is not always easy to interpret the views of the classical microscopists 

 immediately in modern terms. The first view is that keratohyalin " mixes 

 with " or " spreads over " the fibrils (tonofibrils) already present to yield 



