148 



KERATIN AND KERATINIZATION 



layer. The time elapsing between divisions will depend on the rate of 

 synthesis (i.e. also the rate of formation of inhibitor) in the differentiated 

 layers and the rate of loss or decay of inhibitor at the germinal level. Since 

 a certain lateral diffusion from any cell is possible, a synchrony could 

 develop in the germinal layers owing to the overlapping of the effects of 

 several adjacent cells. This synchronization may be favoured also by the 



■ • • ' ' ' ••' ■ "/ wound V ' ' ' -^ 



hummi!i^iii\iiiiL 



THUD 



rmnnm 





wound 





Fig. 61. Diagrams to show the regions of high epidermal mitotic activity 

 expected in undamaged ear epidermis opposite to an area 3 mm square 

 from which the epidermis and superficial dermis have been removed on 

 the assumption (a) that a stimulating " wound hormone " is produced by 

 damaged epidermis, and (b) that the concentration of epidermal inhibitor 

 is reduced in the neighbourhood of a wound (from Bullough and Laurence, 

 1960). 



possibility that a general systemic stimulation reaching a group of cells, in 

 which the inhibitor concentration is already low, may cause them to enter 

 division together. The diurnal variations in mitotic activity noted by 

 Bullough, may be an instance of this type of control (p. 136). 



(c) Stimulation of growth may be caused by activities which facilitate 

 the fall in inhibitor concentration. Experimentally, Pinkus (1951) found an 

 increased mitotic rate following the stripping off of the upper layers of the 

 skin with adhesive tape. Mechanical trauma, such as piercing or cutting 



