60 KERATIN AND KERATINIZATION 



and whose persistent habits of licking and grooming must introduce many 

 substances of epidermal origin into the alimentary canal. Vitamin I) 

 production is an epidermal function. Among birds the preen gland has 

 been shown to be essential for the well-being of its possessors. An extra- 

 ordinary example is provided by the aquarium fish (Symphysodon discus) 

 whose young are nourished by the mucous secretion of the parents which 

 covers large areas of their bodies (Hildemann, 1959). Probably much 

 remains to be discovered in this field. 



An important anatomical aspect of mammalian skin is the close 

 association of glands and hair follicles. Except in rodents, the central 

 primary follicles and most, but not all secondaries are associated with a 

 sebaceous gland which usually opens into its lumen. This rather constant 

 association of hair follicles and sebaceous glands suggests that the natural 

 functional unit of the mammalian skin is the follicle group and its glands, 

 the " pilosebaceous unit " (Montagna, 1956) (Figs. 34 and 35), which 

 together produce keratinized hairs and the means to lubricate and condition 

 them. In the dry scaly skins of reptiles or the glandless (with the exception 

 of the single preen gland near the tail) skin of birds, it is hard to find an 

 analogue of such a unit. 



The Differentiation of Epidermal Cells 



It is a problem of wide interest to determine what are the factors which 

 are responsible for the appearance of the variety of cell-types which we 

 have included in the " epidermal family." Information has been sought 

 principally on the factors determining early embryonic differentiation and 

 on those maintaining the stability of the various cell-types found at 

 different sites on the adult. 



The epithelial character of the external cells of an embryo appears very 

 early in life — in effect already in the blastula — and we have mentioned the 

 hypothesis that this is due to the appearance of intercellular adhesion 

 which continues thereafter to play an important morphogenetic role. 

 A further discussion of the role of cellular adhesion in controlling cell 

 differentiation among the persistently embryonic cells of the germinal 

 layer of the epidermis will be given in Chapter 3. Granting that the 

 primary step in the differentiation of epidermal cells is the production of 

 the epithelial habit by intercellular adhesion, there is direct experimental 

 evidence provided by grafting to suggest that in the next phase the factors 

 responsible for localized specializations arise in the underlying mesoderm. 

 For example, using the embryonic chick, by grafting mesoderm from a 

 presumptive foot bud beneath wing ectoderm, one can cause the formation 

 of a claw instead of a wing feather (Cairns and Saunders, 1954). The 

 epidermal cells at this stage may be described as being sandwiched 

 between two environments: the external relatively-free space and the 



