THE GROWTH OF EPIDERMAL STRUCTURES 



153 



molecules is reduced. In isolation from other follicles, a follicle might be 

 expected to continue in production until the accumulation of inhibiting 

 molecules reaches a critical concentration, when growth would cease. It 

 could recommence when the concentration fell again to a lower critical 

 threshold. Local histological pecularities probably control the rate of 



Yield 



Yield sloughed or outside 

 Periodic, not continuous, loss 

 Reversible process 

 Inhibition or neg feedback 

 Draining effect 

 Morphological continuity 



fluence 

 Induction 



Causing degeneration of 

 enclosed elements 



Fig. 63. Chase's attempt to illustrate the factors which operate to 

 integrate the pilosebaceous unit (hair follicle plus sebaceous gland). 



BE basal layer of epidermis. 



PSC peripheral cells of sebaceous gland. 



UEC upper, permanent outer root sheath. 



DP dermal papilla. 



CTS connective tissue sheath. 



C corium. 



A adipose layer. 



accumulation and dissipation of the inhibitors ; and, as an important dis- 

 tinction from the general systemic control mechanism described above, it is 

 supposed that the inhibitor molecules which enter general circulation have 

 a short life and that, therefore, little control over distant sites exists. Since 

 there are no diurnal variations in growth rate, as is found in skin (p. 136) 

 we must assume an adequate supply of nutrition (p. 137). 



Co-operative behaviour becomes possible according to Chase in a 

 population of follicles when the individuals are close enough {ca. 1 mm 



