THE GROWTH OF EPIDERMAL STRUCTURES 137 



finding would not necessarily clash with Bullough's present views in- 

 volving control of inhibitors (p. 149). However, Gelfant's work does imply 

 that some of Bullough's experiments were carried out in conditions that 

 were " sub-optimal " for mitosis and that his conclusions may not be valid 

 under the optimal conditions which may be assumed to prevail in vivo. 



Cycles with a 24 hr period, ultimately linked to the diurnal fluctuations 

 in illumination (Reinberg and Ghata, 1957) are common (see p. 146). For 

 example in animals, the body temperature, a measure of muscular activity, 

 the glucose concentration of the blood, the concentrations of water, 

 glycogen, fat and protein of the liver all show such variations. The calci- 

 fication of teeth, a dermoepidermal function, is also cyclic. 



There is no diurnal cycle in the hair follicle of Rodents— the only case 

 examined. To account for this relative immunity of the hair follicle from 

 the fluctuations caused by alternations of rest and activity, it is assumed 

 that the follicle has its own independent source of food supply and, in fact, 

 large amounts of glycogen are found in the cells of the outer root sheath 

 (Montagna, 1956; Hardy, 1952). Glycogen is reduced in amount or is 

 absent when there is no hair growth (Montagna, 1956; Montagna et al. 

 1952). Possibly when required, the glycogen is mobilized as glucose and 

 transported by the network of blood vessels surrounding the shaft of the 

 follicle to the bulb. In support of this it may be noted that the vascular 

 network of the follicle of growing hairs is remarkably developed (Durward 

 and Rudall, 1949 and 1958; Ryder, 1956). Ryder (1958) injected 

 radioactive glucose into mice and found that in 1 hr there was isotope in 

 the bulb, and also in the outer sheath where it increased over the next 24 

 hrs. The rapid uptake in the bulb could be due to the glucose which 

 provides energy for mitosis and the slower accumulation in the sheath to 

 the storage of glycogen. 



No such detailed information exists concerning the other long-growing, 

 hard keratins, nails, claws, etc., but it is not unlikely that their continuous 

 growth is sustained in a similar way. 



The question of the mitotic rate and location of mitoses in the epidermis 

 has occasioned much discussion which has to some extent been cleared up 

 by the realization that there are diurnal variations in the rate in the skins of 

 rats, mice and humans, which provided the bulk of the material (see also 

 p. 146). Further, on hair-bearing skins subject to cyclic variations in hair 

 growth, the activity of the epidermis is linked to that of the adjacent hair 

 follicles. In the mouse and rat, the skin thickens in the early phases of hair 

 growth and relapses again before hair growth ceases. 



The mitotic rate is not the only factor which determines the thickness of 

 the total epidermis. Clearly this depends on the renewal time, the time a 

 cell takes to reach the surface and be shed. Ebling (1954) showed, for 

 example, that oestradiol while increasing the number of mitoses four 



