396 J. F. A. p. MILLER 



(e.g. Miller, 1962). Metcalf (1958) has postulated the existence of a 

 lymphocytosis-stimulating factor in the thymus which appears to 

 exert some influence on lymphopoiesis. On the other hand, there 

 appears to be some evidence that thyroid hormone may act as a 

 specific lymphoid tissue stimulant (Gyllensten, 1959). In our 

 laboratory we have found that radioactive iodine uptake by the 

 thyroid of neonatally thymectomized mice is extremely low, 

 suggesting that thyroid function may be depressed, and that some 

 feedback mechanism may operate between the thymus and 

 thyroid or pituitary. 



Summary 



Thymectomy of the newborn mouse is associated with severe 

 depletion of the lymphocyte population and serious defects in the 

 immunological capabilities of the mature animal. Such mice 

 show striking atrophy of the spleen and lymph nodes and marked 

 deficiency of germinal centres and plasma cells and many die 

 from a syndrome very similar to the runting syndrome. The 

 survival of allogeneic skin grafts is prolonged by two or more 

 months even in strain combinations which differ at the H-2 locus 

 and some of the mice even fail to reject rat skin grafts. 



It is suggested that the thymus during foetal life and even in 

 postnatal hfe produces the originators of immunologically com- 

 petent cells which mature and migrate to other sites. 



Acknowledgements 



I wish to thank Professor Alexander Haddow, F.R.S. for his continued interest 

 and support throughout this work, Dr. P. M. Sutton of University College 

 Hospital Medical School for helping with the histopathology, Mr. K. Moreman 

 and the Photographic Department of the Chester Beatty Research Institute for 

 their help and Miss Violet Grassom, Miss Winifred House and Mrs. Dianne 

 Millican for excellent technical assistance. Tliis investigation has been 

 supported by grants to the Chester Beatty Research Institute (Institute of 

 Cancer Research : Royal Cancer Hospital) from the Medical Research Council, 

 the British Empire Cancer Campaign, the Anna Fuller Fund and the National 

 Cancer Institute of the National Institutes of Health, U.S. Public Health 

 Service. 



