Sl/Sl '^, put them into an anemic animal of 

 genotype W/W and cure the anemia. This 

 experiment has been done by Russell and 

 Bernstein, with whom we collaborated in these 

 experiments, and it worked perfectly well. The 

 animals are cured of their anemia at least as 

 well with cells from Sl/Sl '^ animals as with 

 genetically normal cells (9). This is comple- 

 mentation of the cellular level. 



KAHN: Where did the stem cells come from 

 in the SI type? If you have no stem cell renewal, 

 how do you get the spleen to pick up cells? 



TILL: We don't know the answer to that. 

 Maybe this is not as serious a defect in the 

 embryo and enough multiplication of the stem 

 cells can occur. Also, we can't rule out that 

 there's slow multiplication. Perhaps over a 

 long period of time these cells can build up in 

 numbers. We don't know yet which is right. 

 Our experiments on growth in these hosts have 

 been carried out over a fairly limited period 

 of time - about two weeks. I think the fact that 

 these animals are alive and that they do have 

 normal numbers of stem cells in them means 

 that there must be slow growth of stem cells 

 until the equilibrium level is reached. However, 

 the evidence does suggest that the W gene acts 

 intrinsically and the SI gene extrinsically to 

 the stem cells. 



In the Sl/Sl '^ animals one would like to 

 know, what is the external regulation? Is it by 

 means of a molecule? Can the molecule be iso- 

 lated? We did standard experiments to test for 

 such a molecule. An Sl/Sl '^ animal was joined 

 in parabiosis with a normal animal of the same 

 inbred strain; that is, they had a shared cir- 

 culation. We demonstrated the existence of the 

 shared circulation by putting chromium-labeled 

 erythrocytes into one animal and showing that 

 they appeared in the other. Then both were 

 irradiated and inoculated with the same number 

 of normal bone marrow cells. If the failure of 

 cells to grow in the Sl/Sl'^ irradiated host is 

 due to an inhibitor which is circulating in the 

 peripheral blood, then we should get growth in 

 neither member of the parabiotic pair. If, on the 

 other hand, it's the lack of a stimulatory factor 

 that accounts for the failure of cells to grow in 

 Sl/Sl'^ hosts, in the parabiotic situation this 

 factor should be supplied by the normal member 

 of the pair and there should be growth in both. 

 In fact, what we got was exactly the same 

 situation as if they hadn't been joined to- 

 gether (9). In other words, the cells grew in 

 the normal animal and they didn't grow in the 

 mouse of genotype Sl/Sl ^ . Either there is not 

 a factor that circulates in the peripheral blood 



or it's so unstable that before it can get from 

 the normal to the anemic mouse it's gone. We 

 don't know which it is, but this just makes the 

 whole system difficult to study, because it 

 means one is looking at relatively short range 

 factors. We haven't yet had any good ideas on 

 how to investigate a short range factor of this 

 type. 



The third type of mutant that we have studied, 

 f/f has quite a different kind of anemia. It's 

 a transitory anemia which just shows up in the 

 embryo and the new born and apparently cures 

 by the time the animal is two weeks of age (11). 

 The first experiment we did on mice of geno- 

 type f/f was as follows : we thought a transitory 

 anemia might be more interesting because we 

 might see shifts in the properties of the cell 

 depending on the age of the animal we took them 

 from, so we took animals of this genotype of 

 various ages and tested their marrow cells for 

 ability to form colonies, as compared with cells 

 from normal animals of the same inbred strain. 

 We found that marrow cells from the controls and 

 from the mutant animals produced spleen col- 

 onies with the same efficiency. In other words, 

 we got the same number of colonies from the 

 same number of marrow cells transplanted. 

 However, in the case of cells from /// donors, 

 independent of their age, the colonies were dif- 

 ferent in composition. They were smaller, and 

 when we tested them in various ways, they 

 showed fewer erythrocyte precursors. Experi- 

 ments done in collaboration with Dr. Margaret 

 W. Thompson and Dr. John Fowler (12, 13), have 

 indicated that this deficiency is specific, in that 

 both the number of granulocytes and the number 

 of new stem cells, produced by rapidly pro- 

 liferating transplants derived from /// donors, 

 appears to be normal. Apparently, the defect in 

 cells from mice of genotype f/f causes them to 

 be late in arriving at the stage where they begin 

 hemoglobin synthesis. If the cells are stimulated 

 to proliferate at their maximum rate, then this 

 defect appears in cells from f/f donors, inde- 

 pendent of their age. Thus, the "curing" of the 

 anemia in adult mice is not due to repair of 

 the defect, but is apparently, the result of its 

 being masked by the decreased demand for pro- 

 duction of new erythrocytes in the adult animal. 



In any event, the defecting// mice appears 

 to be intrinsic to the stem cells, and seems to 

 affect their ability to differentiate toward the 

 production of red cells. What the exact nature 

 of the defect is, we don't know. Of course, there 

 is a hormone which specifically stimulates cells 

 of the blood-forming system to proliferate and 



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