THE KERATINIZED TISSUES 57 



Nevertheless keratinization in vitro does not take place in all types of 

 cell regardless of origin ; nor, as was shown very definitely by Miszurski 

 (1937), in epithelial cells themselves is it initiated or promoted by poor 

 nutritive conditions or low oxygen tension or lower temperature. The 

 phenomenon is properly to be regarded as the final stage of an intrinsic 

 differentiation of epithelial cells, as well adapted to the function of the 

 tissue as in, for example, the production of collagen by fibrocytes. This 

 is not to say, of course, that it cannot assume an abnormal, perhaps 

 degenerative form. 



That the epithelial habit with the potentiality of keratinization is a 

 fundamental type of cell behaviour is shown by the fact that it is one of the 

 forms to which cells revert when cultured in vitro for some time. Willmer 

 (1954) describes three such cells forms: epitheliocyte, mechanocyte and 

 amoebocyte. When conditions are appropriate epitheliocytes adhere 

 laterally and grow to form flat unicellular sheets. The intercellular contacts 

 are of the types described above (p. 40); the cells may secrete muco- 

 polysaccharides (mucins) and may keratinize. Mechanocytes form open 

 meshworks, secrete collagen and a distinct type of mucopolysaccharide. 

 The mobile amoebocytes form no permanent associations with other cells. 

 Recent work (e.g. Puck, 1957) shows that the morphological appearances 

 may be deceptive but the biochemical differences are more persistent and 

 significant. 



The epidermal family of cells 



From the ectcderm of the embryo is developed not only the adult 

 epidermis but an entire family of cells which includes the keratinized 

 appendages and numerous glands. The " genealogical tree " of the 

 epidermal family is shown in Fig. 26. The entire population is produced 

 and maintained by the proliferation of the undifferentiated cells of the 

 germinal or Malpighian layer, which everywhere covers the outer surface 

 of the dermoepidermal membrane. These cells not only have the same 

 embryonic origin but, according to some authors (Montagna, 1956), retain 

 everywhere, even in the adult, the potentiality to differentiate into any of 

 the cell types found among their descendants and thus on the biochemical 

 level to produce any of a number of distinct chemical substances, such as 

 keratin (in several forms), sebum, mucin, etc. 



The development of glands capable of secreting mucin or lipids is a 

 character of the epidermis as typical as and, phylogenetically speaking, of 

 earlier development than, its keratinizing potentialities. The glands may 

 take the form either of unicellular " glands," i.e. single cells discharging 

 their contents directly on to the surface, or of more elaborate multicellular 

 formations sunk into the dermis and communicating by a duct with the 

 surface. 



