74 KERATIN AND KERATINIZATION 



sense organs, and others again in specialized scales. If the problem is 

 less discussed today, this is not so much through lack of intrinsic interest 

 as in the difficulty of obtaining further evidence which might bear upon 

 the question. Because of its importance some aspects of these discussions 

 will be summarized here. 



The lining of the mouth is epidermal in origin and character, and the 

 formation of teeth, like that of scales, hairs and feathers, is another example 

 of dermoepidermal co-operation for the production of a superficial organ. 



Fig. 32. Diagram of the early development of a tooth to show the 

 dermal and epidermal contribution to its structure. The tooth consists 

 of dentine, secreted by dermal cells of the dental papilla, capped by 

 enamel formed by ameloblasts of the enamel organ derived from the 

 basal layers of the epidermis. The keratinous constituent is found in 

 the enamel. Redrawn from Hyman (1947). 



The earliest sign (Fig. 32) of an impending tooth is an epidermal pro- 

 liferation leading to an infolding of the epidermis, cf. the hair primordium, 

 to form the enamel organ. Beneath the enamel organ a mesodermal 

 papilla now forms and presses into the enamel organ to form a double- 

 walled cap. The cells of the enamel organ, the ameloblasts, are of epidermal 

 origin and secrete the hard enamel which caps the tooth; the cells of 

 the dermal papilla, or odontoblasts, secrete the dentine, the bulk of the 

 tooth. Thus the tooth, like the hair bulb and papilla, is in origin partly 

 epidermal and partly mesodermal. 



