DOROTHY B. SPANGENBERG 415 



INTERSTITIAL CELL DIFFERENTIATION 



The role of interstitial cells in hydra regeneration has been 

 emphasized by many in\ estigators (1, 7, 9, 10). While the ability 

 of interstitial cells to differentiate into any other cell type in hydra 

 is disputed, most authors agree that cnidoblasts arise from inter- 

 stitial cell differentiation (2, 10, 14). The differentiation of inter- 

 stitial cells into cnidoblasts (specifically those containing desmo- 

 nemes) was selected as a process deserving study, since it can be 

 directly related to the rate of normal and abnormal regeneration 

 in hydra. Cell counts of differentiated stages of interstitial cells 

 in the pre-tentacle area of non-regenerating hydra (severed and 

 then immediately killed ) revealed that those hydra whose tentacles 

 and hypostomes had been removed close to the hypostomal area 

 (while the hydra were in a stretched condition) contained more 

 desmonemes and late-state interstitial cells than did hydra which 

 had been cut in the mid-stomach region. This and other observa- 

 tions indicated that along the body of a hydra there is a qualita- 

 tive gradient in the distribution of the various stages of interstitial 

 cells, with the greatest concentration of desmonemes and late-stage 

 interstitial cells at the base of the tentacles (11). Tardent (14), 

 using Chlorohijdra, reports a quantitative gradient in the distribu- 

 tion of all types of interstitial cells. The existence of a qualitative 

 gradient explains the decreased regenerative ability observed in 

 normal hydra severed at the mid-stomach region when compared 

 to those severed at the hypostomal region, since in the former 

 case most of the reserve of partially differentiated interstitial cells 

 is removed at the time of the cutting. It is postulated, therefore, 

 that any factor w4iich is apt to interfere with normal differentiation 

 of interstitial cells is a more effective inhibitor in hydra cut at the 

 mid-stomach region than in those cut at the hypostomal region. 



Some of these factors which have been observed to affect in- 

 terstitial cell differentiation are a depletion of nutrients (fasting of 

 the animal prior to testing ) , and the presence of intracellular para- 

 sites in hydra (11). Also, the inhibition produced by exposure 

 to lipoic acid and to certain other chemical agents is much more 

 effective in hydra cut in the mid-stomach region than in those cut 

 at the hypostomal region. 



