PERIODICITY IN HUMAN BEINGS AND MICE 823 



Also of great physiologic interest is the study of other rhythms, 

 with their time relations to the adrenal cycle, e.g., the onset of blood 

 corticosterone rise and the initiation of motor activity. With this in 

 mind, we may discuss the time relations of body activity and serum 

 corticosterone in data from two experiments in which the serum pools 

 were obtained for corticosterone determination, while motor activity 

 was evaluated during the same period in an electronic actometer on 

 comparable mice. It would appear that the corticoid curve rises prior 

 to the major daily bursts in motor activity, and almost certainly the 

 corticoid curve does not lag behind that of activity. The data thus 

 agree with the suggestion (Halberg, 1953) that the adrenal cycle 

 describes processes occurring, at least in part, in preparation for our 

 daily activities, rather than solely as immediate reactions to them. 



Cellular Mechanisms 



The mere fact that periodicity exists in lower forms of life, devoid 

 of conventional nerves and hormones, suggests that periodicity may 

 characterize cellular functions even in the absence of a superimposed 

 control such as the adrenal cycle. It is therefore hardly surprising to 

 find in the mammal as well, that 24-hr periodicity is a characteristic 

 of the intracellular level of organization. As in the case of unicellular 

 forms, one of the most obvious periodicities of the cell is that in its 

 mitotic activity. Moreover, in the mammal, with its cells grouped to 

 form tissues or organs, the rhythms of mitoses from one organ to the 

 next, or, in a given tissue, from one region to the next, pose problems 

 of timing that are of physiologic interest. In the mouse, for instance, 

 there is a remarkable degree of synchrony of the epidermal mitotic 

 rhythms in ear pinna and in the interscapular area of back skin 

 (Chaudhry, 1956). In the same species, the timing of mitotic rhythms 

 in liver parenchyma (Halberg, 1957) and in skin epidermis shows a 

 surprising degree of synchronization, as far as data obtained at 4-hr 

 intervals over 24-hr periods are concerned, i.e., at a "magnification" 

 (At) = 4 hr (Halberg, Barnum, Silber, and Bittner, 1958). At a much 

 lower "magnification" it was found in the rat that the mitotic rhythms 

 of pinnal epidermis, of oral mucosa, and of connective tissue in the 

 periodontal membrane also are roughly synchronized (Halberg, Zan- 

 der, Houolum, and Miihlemann, 1954). The same applies to mitoses 



