Discussion 103 



DISCUSSION 



Montagna: I would like to ask Prof. Dempsey whether or not he con- 

 siders the mitochondria as fixed rather than labile structures. I find it 

 difficult to reconcile the idea of mitochondria as rigid structures with 

 the observations of Frederic and Chevremont and with those of the 

 Lewises. These investigators all observed living mitochondria undergo 

 numerous vicissitudes around the nucleus. 



Dempsey: I think there is no question but that the mitochondria are 

 extremely labile structures. Bensley showed many years ago that if 

 one places a rat on a diet of nothing but sugar and water, within a week 

 the mitochondria practically disappear from the cells of the pancreatic 

 acini. Dr. Weiss in my laboratory repeated that experiment a year or 

 two ago and confirmed Bensley's results. He then continued by re- 

 feeding the animal a normal diet, after the exhaustion of the mito- 

 chondria, and was able to show that the organelles regenerate with 

 extraordinary rapidity. Within a matter of hours after administration 

 of protein materials in the diet, the pancreatic cells again have their 

 normal complement of mitochondria. In this kind of experiment it 

 seems to me perfectly clear that they are labile structures. Also, in the 

 adrenal cortex, there are many more mitochondria in the secretory zones 

 than there are in those cells in the degenerating zones. If one permits 

 the assumption that these are two different phases in the life-cycle 

 of the same cell, then it would appear that mitochondria had been 

 exhausted from the cell during its life-span. 



Wislocki: This paper raises many questions about mitochondria; for 

 example, what are their life-spans, to what degree do they degenerate, 

 to what extent and how do they divide and renew themselves and what 

 changes do they undergo under different physiological and pathological 

 conditions? The electron microscope should, in the next several years, 

 provide answers to many of these questions. 



Dempsey: Prof. Wislocki's remark reminds me of something I had 

 intended to say and forgot. You, Prof. Wislocki, I believe inadvertently, 

 remarked that the mitochondria perhaps divide and renew themselves. 

 I am not sure that we should assume that the organelles within a cell 

 derive themselves from pre-existing organelles. We are so familiar, 

 I believe, with the cell doctrine and with the dictum that all cells come 

 from previously existing cells, that imperceptibly our minds tend to 

 slide into the similar syllogism that all structures come from pre- 

 existing structures, but they do not necessarily; and it could well be 

 that mitochondria are not produced from previously existing mito- 

 chondria but arise from some other structure in the cell. 



Wislocki: That is a possibility. 



Dempsey: I think the question ought to be kept open. 



Fawcett: The origin of mitochondria from pre-existing mitochondria 

 has been well documented by phase contrast cinematography of living 

 cells in tissue culture. In these films one sees long filamentous mito- 

 chondria dividing into several segments, and these detached segments 

 continue their activity in the cytoplasm and apparently constitute new 

 mitochondria. Under certain experimental conditions, we have seen 



