206 



CELLS, TISSUES, AND ORGANISMS 



Temperature and aggregation 



Some support for such assumptions can be readily drawn from the 

 finding that cell-cohesion and aggregation processes are temperature- 

 sensitive. Taking aggregation patterns at 38° C. as basal, it is found 

 that the tendency of normally cohesive cells to aggregate decreases 

 sharply at lower temperatures. There is, thus, a distinct reduction in the 

 size of aggregates, and eventually cell-bonding and aggregation are 

 completely inhibited, though the cells are brought in contact by rota- 

 tion. For instance, neural-retina cells from 7-day chick embryos which 

 cohere readily at 38° C. remain completely dissociated at 15° C, in 

 spite of medium and rotation conditions conducive to aggregation. If 

 continuously rotated at this temperature, the cells remain alive for 

 several days, and when transferred to 38° C. they aggregate. As might 

 be expected, aggregation-limiting temperatures vary somewhat with 

 diflFerent types of cells and with the stage of development. 



This correlation between temperature and aggregability invites 

 certain obvious questions. Consider, for instance, in the light of this 

 information, the well-established role of calcium ions in cell-bonding 

 (Steinberg, 1958). At temperatures around 15° C, which effectively 



mm 



0.5 









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7 9 11 



14 



17 19 days 



Figure 7. Age of cells at dis- 

 sociation and size of aggre- 

 gates: neural-retina cells, em- 

 bryos at 7, 9, 11, 14, 17, and 

 19 days. 



