250 General Discussion 



this morning showed how with cells lying side by side in the same 

 organ, one can show morphological evidence of senescence in one 

 and yet the other can be a perfectly good non-senescent cell. 



Krohn: It indicates, though, that the ageing process is a local 

 process, not a general process. 



Dempsey: No, that is not true of all tissues. So far as I know, the 

 nerve cells have finished their divisions at approximately the time 

 of birth and then are maintained for the entire lifetime of the 

 individual; probably muscle does the same. In contrast to that, 

 there are many tissues in which there is a continuous growth and 

 continuous destruction of the cells going on simultaneously, so in 

 one organism you can have a cell, the age of which is equivalent to 

 the chronological age of the individual, or conversely you can have 

 a young cell in a young person or an old cell in a young person or a 

 young cell in an old person, in fact, all the combinations and 

 permutations. 



Villee : This implies that you can date a cell from the time of the 

 last cell-division, that it is then a new cell, and I wonder if you really 

 mean to imply that? 



Dempsey: Well, I think you have to have some data to operate 

 with and it seems to me that the two possible phenomena that you 

 can use for dating are: (1) the last previous mitosis and (2) the last 

 previous fertilization. 



Villee: Yes, those are two useful ones. 



Dempsey: But are there any others? 



Villee : Well, I am not at all sure that you can set up, other than 

 arbitrarily, some point at which to speak of a new cell. Certainly 

 the protein and other compounds in the cell are constantly being 

 renewed, but I do not think there is any time when they start com- 

 pletely afresh. 



Dempsey: But I am thinking of the fact that, in tissue cultures, 

 most cells are potentially immortal and that they continue to divide 

 and, therefore, so long as the process of division is maintained the 

 cell remains viable and does not become senescent, whereas post- 

 mitotically the cell does then begin to age and become senescent and 

 ultimately dies. 



Mitosis is rather orderly in the intestinal epithelium in that it 

 occurs at the base of the villi, and the older cells occur at the tips of 

 the villi, and there is an orderly progression. 



Fawcett : Is not one of the best examples to be found in the cartil- 

 age columns of the epiphyseal plate? There you have a row of 

 chondrocytes of all ages. At one end are cells that have just divided, 

 then a series of cells in various stages of maturation, and finally, 

 at the other end, are cells that are ageing and dying. It is essential 



