64 KERATIN AND KERATINIZATION 



of the cells along an alternate pathway. A demonstration of induction by 

 " a crucial event of relatively short duration " is of some theoretical 

 importance, but Lasnitzki and Greenberg (Lasnitzki, 1958) have cast doubt 

 on the conditions of the experiment by demonstrating the persistence 

 of vitamin A in cultures treated as were those of Weiss and James. 

 They conclude that the action of vitamin A is due to its continuous 

 presence. 



The effect of vitamin A is most apparent on the germinal layer cells as 

 might be expected, since these are " uncommitted," but cells in the 

 process of keratinization can still be deflected in their course by the 

 vitamin. The mucin forming cells once formed cannot, however, revert 

 when returned to a normal medium but are shed. 



The variety of differentiations of the epidermis, its simplicity and its 

 experimental accessibility assure that in the future it will continue to 

 play a part in the investigation of the general problems of differentiation. 



Hard and Soft Keratins 



It is customary to distinguish between " soft " keratins (the epidermis 

 itself) and the " hard " keratins, hair, feather, horn, etc. The classification 

 was put on a well-defined basis by Giroud, Bulliard and Leblond (1934). 

 Primarily the distinction is based on the immediate sensation of hardness 

 or softness, and the fact that soft keratins (epidermis) desquamate while 

 the hard keratins (hair, nails, etc.) persist. These properties were shown 

 to be linked with other differences appearing in the course of keratinization 

 and with the chemical composition of the final product. These are 

 tabulated in Table 5. 



Some of these distinctions, which seem obvious enough at first sight, 

 become less obvious on closer analysis. The difference between the two 

 types is, in fact, only relative and, if pathological material is admitted, a 

 continuous spectrum of tissues between hard and soft exists. Moreover, 

 the same germinal matrix can be made to produce a graded series of 

 tissues of various textures, as is shown by the development of callosities, 

 warts and corns, the thickened skin of Ichthyosis vulgaris and by some 

 effects following radiation (Chase, 1954). However, the classification is 

 useful as representing two extremes of the synthetic potentialities of 

 epidermal cells with obvious adaptive potentialities. Typically where the 

 site and function demand a squamous tissue, soft keratin develops; 

 where a persistent growth is required, we find hard keratins. We know as 

 yet, little of the underlying causes determining the type of keratin produced 

 at any site. 



In general terms a structure is hard and coherent when its units are 

 hard and they do not fall apart. Translated into histological terms, this 

 means that the factors underlying the differences between hard and soft 



