134 KERATIN AND KERATINIZATION 



There is no difference here between the coherent hard keratins and the 

 soft, which spontaneously exfoliate. Both varieties are subject to wear 

 and more-or-less continuous growth is required for replacement. Nails, 

 claws and epidermal horns seem, however, to be continuously produced 

 irrespective of demand and their growth in excess of needs may even 

 become a nuisance. Feathers and hairs on the other hand have a quite 

 distinct unity and grow to a defined shape. Plucking is followed by re- 

 growth, but only in a remote sense can we speak of this as a renewal in 

 response to wear. 



In the epidermis a steady state normally prevails in which cell loss is 

 balanced by cell replacement. The renewal time at any site is defined as the 

 time taken for the replacement of an amount of material equal to the 

 amount present in the layers above that site. It is also the time taken for a 

 cell to pass from the germinal layer to the surface where it is shed, and is 

 analogous to the growth period for hairs and feathers. Since growth may 

 not be continuous over short periods of time (see p. 135) the steady state 

 is only an average state maintained over a more-or-less extended period. 

 Leblond and others have determined the renewal time for a number of 

 proliferating tissues. Some examples are given in Table 6. The methods 

 employed are based on the direct counting of nuclei in division over an 

 extended period of time or on determining the number of nuclei arrested 

 in metaphase by colchicine in this time. 



The existence of a definite equilibrium thickness of the epidermis 

 differing from site to site and of the definite shapes of feathers and hairs 

 shows at once that some sort of overall control must exist throughout the 

 epidermal system. In this respect the epidermis is no different from other 

 organs whose forms and cellular composition are also strictly controlled. 

 In fact the entire cellular community constantly maintains a state of 

 homeostasis in which its numbers and composition are kept in balance 

 with each other and with the environment. 



The factors likely to affect the growth of a tissue have been sought both 

 by direct observation of normal growth and from the results of experi- 

 mental interference. In this way a large amount of information has been 

 gathered concerning the growth of whole animals and organs, which 

 although often of immediate practical value, is not easily related back to 

 the activity of the individual cells. A number of growth factors and of 

 hormones influencing growth are known, but their effects are invariably 

 complex when whole tissues are considered. For reviews, see Thomas 

 (1956). 



Undoubtedly many hormones also affect the behaviour of epidermal 

 cells, but their action is complex and far from clearly defined. Oestrogens 

 definitely stimulate cell division according to Bullough (1953). The cyclic 

 changes of the vaginal epithelium, the cells of which oscillate mucin 



