104 Discussion 



mitochondria in electron micrographs of liver that appeared to be under- 

 going division but we must admit that the pictures could equally well 

 be interpreted as two mitochondria coalescing. I believe the electron 

 microscope will ultimately settle the question as to whether mito- 

 chondria divide or are formed anew from other components of the 

 cytoplasm ; but I have been disappointed in our own work to date, in 

 that we seldom obtain pictures that provide clear-cut evidence for one 

 or the other of these alternatives. 



Dawes: I was puzzled by the use of the word "membrane" ; could you 

 explain that, Prof. Dempsey ? It seemed rather like the story of Frankie 

 and Johnny — that some of the membranes had no beginning and no 

 end. Are they partitions? 



Dempsey: Well, they are thought to be membranes in that mito- 

 chondria may be shown to be active osmotically. I quoted part of the 

 evidence. It can also be shown that they can be swollen and shrunk 

 experimentally by changing the tonicity of the suspending medium. 

 The structure that we see is a line in a micrograph which has a density 

 greater than the surrounding. The density, we strongly suspect, is due 

 to the fact that it has reduced osmium tetroxide. We have to have 

 some kind of terminology, and "membrane", in view of these two facts, 

 seems to be tentatively reasonable. 



Wislocki: We shall have to experiment with the effect of fixatives 

 upon the preservation of the mitochondria in their most natural state. 

 Besides artifacts introduced by fixation, mitochondria are subject to 

 functional as well as age changes which must be recognized. It is 

 difficult, with just the one or two fixatives now at our disposal, to say 

 whether an organelle is normal or pathological, or merely showing arti- 

 factual changes as a result of fixation. 



Dempsey: There is only one technical way that I know in which this 

 difficult problem can even be approached. If in a tissue with a mixed 

 cell population, it is found that one cell type exhibits unaltered mito- 

 chondria whereas the other cell type exhibits altered mitochondria, it 

 seems difficult to implicate the quality of fixation as an explanation for 

 the alterations. This is the criterion we have used. 



Wislocki: I have observed that identical cells will vary considerably 

 in their appearance in the electron microscope, depending upon whether 

 they were fixed in either osmic acid or a chromic acid fixative. 



Dempsey: Differing in some details, but I think not in the configura- 

 tion of these so-called membranes in the mitochondria — I believe it is 

 right, Prof. Fawcett, that the density of the matrix of mitochondria may 

 be reduced by over-fixation with Palade's fluid? If one fixes for a long 

 time, this material leaches out of the specimen, so that by removal of 

 the matrix, one obtains an enhanced contrast of the essential skeleton 

 of the mitochondria. 



Fawcett: Yes, prolonged fixation in Palade's buffered osmium solution 

 will extract some of the mitochondrial matrix, but the membranous 

 structures are not significantly altered. For some purposes it is ad- 

 vantageous to decrease the density of the background somewhat by 

 longer fixation so that the membranes stand out in sharper contrast. 



