MESENCHYMAL CELLS 



findings, therefore, would support an hypothesis that activity 

 in spleen and lymph nodes could be initiated by the entry 

 of activated cells from the blood when they settle down and 

 initiate replication. 



In the blood the small lymphocyte and the monocyte are 

 very different cells but there is a continuous range of morpho- 

 logical intermediates and most authors seem willing to allow 

 lymphocytes to give rise to monocytes under some conditions. 

 Lymphocytes are not phagocytic but they are actively mobile 

 and are constantly seen to migrate out of the vessels into the 

 tissues. There is doubt as to whether lymphocytes can give 

 rise to macrophages. Marshall (1956), who uses silver- 

 staining methods to define macrophages (his metalophil cells), 

 is sceptical of the possibility and believes that many of the 

 small cells called lymphocytes by Maximow, which are seen 

 escaping into the tissues from the blood, are small metalophil 

 monocytes. When large numbers of cells from lymph nodes 

 of the rabbit are inoculated intravenously into another 

 rabbit, they tend to accumulate in the small blood vessels of 

 the lymph nodes, spleen and periportal tissues of the liver 

 (Farr, 1951). Gowans (1957) has shown that the supply of 

 lymphocytes in the thoracic duct can only be maintained if 

 the cells are returned to the circulation, and current opinion 

 is now almost solid that the lymphocyte has a longer life than 

 is deduced by the numbers passing through the thoracic 

 duct; in other words, lymphocytes can return to the lym- 

 phoid tissues several times in their life span. 



It should be stressed that there can be no more than cir- 

 cumstantial evidence that a (small) lymphocyte suitably 

 activated can settle in reticular tissue and there give rise by 

 division to a focus of lymphocytes or plasma cells, depending 

 on the site of lodgment or other factors. Equally at the pre- 

 sent time there is no direct evidence to prove or disprove 

 the claim that a similar activated cell reaching the bone 

 marrow may develop into a haematoblast from which 

 eosinophil cells develop as progeny. 



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