MESENCHYMAL CELLS 



plates which are subsequently transferred (by phagocytosis) 

 to macrophages. These are then converted to plasma 

 cells. Such a view is neither in keeping with the clonal 

 selection approach nor adequately supported experimentally. 

 It is much more likely that the eosinophil has some imme- 

 diate function in the immune response. 



If it can be shown conclusively that eosinophils are im- 

 munologically ' labelled ' cells this will provide strong support 

 for the unitarian view that all mesenchymal cells can give 

 rise to the whole range of forms under appropriate conditions. 

 Evidence has already been cited that lymphocytes may give 

 rise to haematoblasts in the bone marrow. Some rather 

 direct evidence in the same direction has recently been 

 obtained by Ford, Ilberry and Loutit (1957). They studied 

 the recolonization of bone marrow spleen and lymph nodes 

 of mice which had been saved from lethal doses of X-radia- 

 tion by injection of heterologous (rat) cells. Eventually 

 mouse cells completed recolonization and in some individuals 

 it could be shown that up to 44 % of the cells showing mitosis 

 carried common chromosome markers, indicating their deriv- 

 ation from a single damaged stem cell. It is almost incon- 

 ceivable that such a high proportion of the mitoses in bone 

 marrow should not include both granulocytic and erythro- 

 cytic precursors or that a similar proportion in lymph nodes 

 and spleen should not include lymphoblastic cells. 



If this is a reasonable summary of current histological 

 opinion, it can be used to provide a cellular basis for the 

 clonal selection theory. This is that cells of any mesenchymal 

 clone can, under appropriate sequences of intrinsic or en- 

 vironmental stimuli, take on any of the forms mentioned 

 which represent merely adaptations to the functional activity 

 called for by circumstances. The small lymphocyte represents 

 the central figure in the situation : it is the dominant product 

 of mesenchymal proliferation and it is highly mobile, passing 

 incessantly in and out of the circulating blood and lymph. 

 Vast numbers are expendable but there is everything to 



118 



