144 



KERATIN AND KERATINIZATION 



linked equally-marked changes in the entire endocrine system (Burrows, 

 1949; Bullough, 1951). The often striking periodic changes in the plum- 

 age of birds and the hairy covering of mammals are clearly linked both to 

 the sexual and seasonal cycles. In exhibiting these changes it seems likely 

 that the integument responds to the general endocrine situation, that is to 

 say, its periodicity arises indirectly from periodic changes in the concen- 

 tration of circulating hormones. 



cm* 





Fig. 59. The production of two cells of contrasted type from the same 



germinal layer: upper l.h.s. a keratinizing cell, upper r.h.s. a mucin 



forming cell. The metaplastic change may be effected by hormonic means 



or by such additions as vitamin A (p. 63). 



Obviously cyclic activity, which ranges from short period oscillations, 

 such as the heart beat, to the slow oscillations, which gear organisms to 

 the daily and annual changes in their physical environment, must be 

 regarded as one of the most important biological phenomena. The cyclic 

 changes may be reflected not only in size, but in cell function which is 

 revealed in cyclic histological and cytological changes (metaplasia). For 

 example, in the not uncommon metaplastic cycle between a mucin- 

 producing and a keratinizing epithelium, the same germinal layer gives 

 rise to two types of cells of contrasted cytology: (a) keratinizing cells 

 (l.h.s. Fig. 59) with many RNP granules and a poorly-developed system 



