400 DISCUSSION 



there, then passed on and repopulated the lymphoid tissue. Is there 

 anything in your evidence that would shoot down that hypothesis ? 



Miller: I don't think so. The barrier that I mentioned has been shown 

 to exist only in normal adult animals. It is possible that after irradiation 

 the barrier temporarily breaks down, and during this time the bone 

 marrow cells would get to the thymus area, repopulate it and later go 

 out again. 



Woodruff: Would it be worth trying to re-equip these animals with 

 one or more thymuses isolated in diffusion chambers ? It might cast 

 some light on whether or not endocrine factors are concerned in your 

 phenomenon. 



Miller: Unfortunately the thymus doesn't seem to grow very well in 

 diffusion chambers, and if you have negative results you don't know 

 whether it is because the thymus could not estabhsh itself in the chamber 

 or function properly, or whether it is because there is no humoral 

 element from the thymus. 



Woodruff: This prompts me to ask about the structure of the thymus 

 in these young mice. Is there an appreciable epithelial element ? 



Miller: Yes, there is an epithehal component. There are some very 

 large cells, which look like reticulum cells, but which have a pinker 

 cytoplasm, and these are probably the remains of the original thymus 

 epithelium from which the thymus develops. 



Brent: It might be interesting to try and re-equip your mice with the 

 thymuses taken from tolerant animals — animals made tolerant by intra- 

 venous injection of allogeneic cells at birth. You could then discover 

 whether the animals have restored to them a general reactivity which 

 also applies to skin grafts from the strain with whose cells the thymus 

 donors had originally been made tolerant, or whether you get restora- 

 tion of general reactivity minus reactivity towards those antigens with 

 which the thymus donor had been injected. 



Miller: I think the thymectomized newborn animal offers unlimited 

 potentialities for experimental work. 



G. Klein: Can you restore the reactivity of thymectomized mice 

 with spleen cells from adult syngeneic mice or must you have a thymus ? 



Miller: The reactivity of thymectomized mice can be restored by 

 spleen or lymph node cells of adult syngeneic mice in the absence of 

 a thymus graft. 



