THE KERATINIZED TISSUES 63 



latent potentialities of the cells may find expression. Small nests of cells 

 with a well-defined, but aberrant histology, e.g. ciliated borders, mucin- 

 secreting surfaces, etc., may often be observed. That the potentiality of 

 producing both keratin and mucin can exist even in the same cell is 

 indicated by some observations of Gliicksmann and Cherry (1956) on 

 mixed carcinomas. 



It would seem that the several varieties of epidermal cells are examples 

 of what Weiss (1950) has preferred to term cell modulations which are at 

 first reversible and which require for their maintenance the piesence of 

 other elements of the cellular community. Modulations are to be con- 

 trasted with the perhaps irreversible differentiations which accompany 

 embryogenesis and which divide the total cell community into several 

 major families of common descent. Support for such a view is given by 

 direct experimental evidence of cell metaplasia produced by relatively 

 simple chemicals. The work of Fell and Mellanby (1953) (Fell, 1957) and 

 their associates has established that vitamin A disposes the epidermis 

 towards mucin formation. Seven-day old chick embryo ecoderm cultivated 

 in vitro in a normal culture medium undergoes precocious keratinization, 

 the two-layered epithelium being replaced by a stratified layer. When 

 vitamin A is added to the medium (2000-3000 i.u. per 100 ml) keratiniza- 

 tion is prevented and a mucous secreting, often ciliated, epithelium appears. 

 The change is not stable, for when the layered epithelium was trans- 

 ferred back to a normal medium (i.e. lacking vitamin A) a typical mucous 

 membrane containing ciliated cells and mucous cells at first appeared, but 

 after a time this was replaced by a squamous keratinizing epithelium 

 forming beneath it. 



Lasnitzki (1956) showed that the effect on mammalian skin (human 

 embryo) was essentially the same. Embryonic epidermis (3-4 months 

 foetus) in normal medium formed a typical squamous keratinizing 

 epithelium including a keratohyalin layer. In a medium containing 

 vitamin A several layers of large cuboidal cells appeared which contained 

 mucin-like materials. Older skin is less responsive but vitamin A sup- 

 pressed keratinization. 



The vitamin A induced metaplasia was correlated with changes in the 

 uptake of sulphur detected by using radio-active sulphate. In explants of 

 skin treated with S 35 (as sulphate), mucous secreting material was intensely 

 active; the keratinizing layers much less so. On the other hand, the 

 uptake of radioactive cystine was greater in the keratinizing epidermis 

 (Fell et al, 1954 and 1956. See also p. 264). 



Weiss and James (1955) found that a brief exposure to vitamin A in 

 higher concentration produced the same effect as the continuous administra- 

 tion of lower doses used by Fell and co-workers, and concluded from this 

 that vitamin A acted as an inductive agent which switched the development 



