414 THE BIOLOGY OF HYDRA : 1961 



such as the temperature at which the hydra are tested, the 

 temperature at which they are maintained prior to severance, the 

 site at which they are cut, their state of nutrition, and intracellular 

 parasitism (4, 11). 



A unique effect upon the regeneration of hypostomes and 

 tentacles was demonstrated by Ham and Eakin (5) using lipoic 

 acid, a cyclic disulfide which functions normally as a coenzymatic 

 unit in the decarboxylation of «-keto acids. By exposing "decapi- 

 tated" hydra to extremely dilute concentrations of this compound 

 they were able to "permanently" inhibit regeneration of severed 

 structures in Htjcha lifforolis. However, the "vitamin" was not 

 otherwise toxic to the organisms, and such inhibited hydra re- 

 generated normally if the non-regenerated tip was cut away after 

 several days. Subsequent studies showed that alterations in the 

 normal pattern of development induced by exposure to lipoic acid 

 were not related to the compound's enzymatic functions but rather 

 to a specific effect of the cyclic disulfide structure of the oxidized 

 form of this vitamin. In Chlorohijdra exposed to lipoic acid, the 

 regeneration of tentacles and hypostomes is likewise retarded early 

 in the regeneration period, l)ut in this species abnormal regenera- 

 tion subsequently occurs wherein large numbers of tentacles and 

 extreme body deformity results ( 13 ) . With the latter, higher 

 amounts were needed. 



Early studies on the action of chemical agents which counter- 

 acted the lipoic acid effect in H. littoralis (5) indicated that the 

 disulfide might be affecting the nervous system of hydra. However, 

 cytological studies of the nervous system of normally regenerating 

 and inhibited hydra failed to reveal morphological evidence that 

 the nervous system was altered in these treated hydra. Therefore, 

 detailed studies of other processes either known to be or suspected 

 to be involved in normal regeneration were carried out using 

 both normal and lipoic acid treated hydra. The study of these 

 processes ( interstitial cell differentiation, cell mitosis, cell migration 

 and the interaction of cell layers) led to the development of 

 postulates concerning mechanisms involved in normal regenera- 

 tion and to postulates explaining, at least in part, the effect of 

 lipoic acid on hydra regeneration. 



