58 



KERATIN AND KERATINIZATION 



The secreted mucins are viscous, shiny substances providing a protective, 

 slippery, extracellular sheath possibly related chemically to the more 

 permanent and condensed cuticles. They are ill-defined chemically, 

 containing a protein and a carbohydrate moiety, but the nature of the 

 association of the two is obscure (Meyer, 1957 and p. 54). The relation 

 between the mucoprotein and other types of protein which may be 

 synthesized by cells arising from the same germinal layer does not seem to 



epg 



ectoderm neural crest 



melanocytes! 



mesoderm 



endoderm 



internal 



epithelia 



(some keratin) 



mucous 

 glands 



sweat 

 glands 



sebaceous 

 glands — 

 hairs* 



horns 



feathers* 



adult 

 epidermis 

 digital 



appendages — 

 claws, nails, 

 hoofs 



dermis 



Fig. 26. Genealogical tree of the epidermal family and related tissues. 



* It is not implied that all types of appendage (including hair and 

 feathers) appear on a single skin. 



t The pigment-forming melanocytes migrate into the dermis and 

 epidermis where, after attaching to the dermo-epidermal basement mem- 

 brane, they may pigment the growing epidermal-type cells (p. 276). 



I This symbol is meant to indicate the special anatomical union 

 between dermis and epidermis. 



have been explored. Owing to the extremely elongated nature of their 

 molecules and their high negative charge, they raise the viscosity of the 

 secretions and in this way lubricate the surfaces and protect them against 

 mechanical, chemical and perhaps bacteriological injury. The lipids 

 comprise a multitude of compounds having in common a solubility in 

 non-aqueous solvents. They may exhibit species specificity (Hilditch, 

 1949). The phospholipids are an essential constituent of most biological 

 membranes (p. 37 et seq.). 



Mucin secretion is common in aqueous forms or on moist internal 

 surfaces of land forms; lipid secretion is found more among the dry 



