SERIAL TRANSPLANTATION OF HAEMATOPOIETIC TISSUE 63 



number of leucocytes approached the normal control level. On the other 

 hand the mononuclear/polymorpli ratio remained consistently low; the poly- 

 morph count was two to three times greater than that of the mononuclear 

 count. Even at 75 days after irradiation and marrow therapy the ratio was 

 still less than 1 in all but the primary chimaeras. In the third and fourth 

 passage chimaeras the number of mononuclears was still less than 50% of the 

 control value. 



DISCUSSION 



We found that in isogenic chimaeras the composition of the peripheral 

 blood, in terms of the relative numbers of the various cell types present, is 

 abnormal and remains so for a considerable period. This reflects a delay in the 

 restoration of balanced haematopoiesis in the irradiated milieu. In lethally 

 irradiated animals the haematopoietic tissues are destroyed, consequently the 

 rapid restoration of granulo- and erythropoiesis becomes the most urgent 

 need. Such a demand is satisfied by the recolonization of the haematopoietic 

 sites, effected by the relatively small number of stem-cells present in the non- 

 irradiated donor tissue which has been injected. When the donor tissue is 

 marrow, the haematopoietic sites will be recolonized rapidly and only after 

 these sites are recolonized, does the restoration of the lymphopoietic organs 

 occur (Ford et al., 1957). When the donor tissue is spleen, recolonization of 

 the haematopoietic sites is delayed, due to the smaller number of myelo- 

 and erythropoietic stem-ceUs. Thus the number of the appropriate stem-cells 

 of the donor tissue is of primary importance to the recovery of the irradiated 

 host. 



The altered cell-composition in the blood of isogenic chimaeras observed 

 indicates that the balance between the haematopoietic components of the 

 donor marrow has been disturbed. When such an "unbalanced" marrow is 

 used again as donor tissue and transferred into another irradiated animal, it 

 might be expected that the balance m the cell composition would be further 

 distorted and this was found to be the case in our tertiary and quaternary 

 chimaeras. The gross disturbance in the relative proportions of the circulating 

 blood, in particular, the almost complete disappearance of the mononuclears 

 from the circulation might easily account for the death of the animals. Here 

 it may be relevant to mention the recent and very important observation of 

 Miller (1961). He found that the number of lymphocytes is extremely low in 

 the peripheral blood of mice thymectomized at birth, and the persistence of an 

 incompletely functionmg haematopoietic system in these animals results in 

 the "wastmg syndrome" and eventual death. 



Our experiment revealed that the cell population of the marrow is altered 

 by successive transfers into irradiated milieu. The alteration consists of a 



