152 KERATIN AND KERATINIZATION 



in most animals hair of a well-defined species-characteristic, site-character- 

 istic length is produced during a growth phase and a more-or-less lengthy 

 resting phase follows (Chase, 1954 and 1955). Shedding may be continuous 

 or exhibit a seasonal dependence (Fig. 40, p. 89). 



The pattern is essentially similar in the mouse and the rat. Important 

 dermal changes occurring (Durward and Rudall, 1949; Montagna, 1956, 

 Review) in correlation with the growth cycle are (a) an increased vascu- 

 larity beneath the growing area and (b) increased deposits of hypodermal 

 fat. It has been debated whether these changes cause or are caused by the 

 epidermal activity. Durward and Rudall proved the absence of nervous 

 control of the growth wave in the rat by severing all nervous connexions. 

 They demonstrated that the growth proceeded largely under local control 

 and independent of systemic control by exercising portions of skin 

 rotating them and grafting. The pattern of growth (and also that of 

 pigmentation) is quite unrelated to the distribution of nerves or blood 

 vessels. Butcher earlier had reached different conclusions but Durward 

 and Rudall believe that, by choosing to study young animals, he may have 

 failed to distinguish the characteristic behaviour of the mature skin. 



Durward and Rudall conclude that, in very general terms, the growth 

 wave is a consequence of a " resting stage inertia and a stimulus provided 

 by neighbouring vascular activity." The vascular activity probably arises 

 in response to the demand of the cells in follicles already actively growing 

 and synthesizing keratin. Special modifying systemic factors such as 

 hormones or food supply (p. 135) certainly exist but they are assumed to 

 remain constant in these experiments. 



Montagna and Chase, using the mouse, have reached the same con- 

 clusions. Chase (1954) in particular has tried to trace out in greater detail 

 the interconnexion of the various elements of the skin. He emphasizes the 

 morphological continuity of the basal layer of the epidermis, the external 

 root sheath and bulb of the hair follicle and the peripheral cells of the 

 sebaceous gland and their functional interdependence. These elements 

 act as a unit (the pilosebaceous unit) sometimes centred on a single follicle, 

 sometimes on a group of follicles and their associated glands as described 

 earlier (Fig. 63). 



The control diagram, Fig. 63 taken from Chase and simplified, is an 

 attempt to bring out the morphological continuity and to indicate the 

 possible lines of communication which transmit the control from one unit 

 to another in the skin, and effect the integration of the whole. 



Essential conditions required for prolonged growth are themselves 

 probably provided by the geometry of the follicle, a long thin cylinder 

 penetrating deeply into a potentially well-vasculated region, the dermis 

 and hypodermis, where growing cells may satisfy their demands for food 

 and where, at the same time the rate of accumulation of inhibiting 



