THE GROWTH OF EPIDERMAL STRUCTURES 149 



the skin, are known to be sufficient to provoke regrowth in quiescent areas 

 of skin in rabbits and rodents (Slen, 1958). In animals such as sheep with 

 constantly-growing follicles, an increased blood supply has been thought 

 to increase the rate of growth of wool (Ferguson et al., 1949). Perhaps one 

 of the most characteristic properties of the epidermis, its adaptive response 

 to the effects of hard work, may be produced by the dissipation of inhibitor 

 resulting from friction and pressure. Thorium-X plaster acts similarly. 



The epidermis in health fits its bearer snugly. Obviously some control 

 adjusts lateral growth so that the area of the covering increases or de- 

 creases with the volume contained. Possibly the factor here is mechanical, 

 a tension or compression arising from the expansion or contraction in 

 volume of the tissues beneath. Stretching would thin the covering layers 

 and reduce the amount of inhibitor diffusing back to the germinal cells 

 which would then divide and replace the overlying layers. 



Recently by means of an ingenious experiment, Bullough and Laurence 

 (1960) claim to have shown that inhibition rather than stimulation is 

 the real growth controlling factor. Following a wound, mitosis and 

 growth are initiated in a limited area (approx. 1 mm wide) of the epidermis 

 surrounding the wound. This could be described, and usually was, 

 as the result of the liberation of a " wound hormone " stimulating 

 growth. Using the fact that the skin on a mouse's ear is less than 1 mm 

 thick, Bullough removed the epidermis on one face making a wound more 

 than 1 mm wide (Fig. 61) and observed the effect on the epidermal layer 

 on the other side of the ear. If the diffusion of a stimulant to a radius of the 

 order of 1 mm was the stimulating factor, a limited area of mitoses opposite 

 the edges of the wounds should be seen. If the removal of inhibitor was 

 the cause, mitoses should be seen over the whole area lying beneath the 

 wound, which was the condition actually observed (Fig. 61). 



Competition 



Another factor, which almost certainly plays a part in controlling 

 growth rates and through them morphogenesis, is competition between 

 cells and organs and parts of organs for some essential requirement for cell 

 growth. Here again data obtained from the observation of epidermal 

 growth have been extensively used to demonstrate the actual operation of 

 competition. We have mentioned that a competition between the pri- 

 mordia of follicles may account for their appearance in a hexagonal 

 pattern on certain skin areas (p. 77) and that inter-organ competition 

 within a " scale-hair-gland unit " may be appealed to for an explanation of 

 the suppression of some structures in favour of others. 



The very extensive quantitative data concerning the growth rate and 

 dimensional properties of wool fibres available from Australian sources 

 have been used particularly by Fraser (Fraser and Short, 1960) in an 



