INTERRELATIONS 



comment. Specialized mesenchymal cells concerned with 

 bone, cartilage or dense connective tissue can also be neg- 

 lected. This leaves for consideration cells which can be 

 placed in one or other of five categories or in some inter- 

 mediate position. 



(i) Primitive reticulum cell, haematoblast. 

 (ii) Lymphocyte. 

 (iii) Plasma cell. 



(iv) Macrophage (tissue) or monocyte (free), 

 (v) Fibroblast. 



(i) The primitive reticular cell, representing an almost 

 undifferentiated type of mesenchyme, is usually regarded as 

 a stem cell from which under intense enough call other types 

 may develop. Much evidence suggests, however, that this 

 type of cell is not primitive in the sense that it is descended 

 wholly from similar undifferentiated ancestral forms running 

 back to the earliest mesenchymal cells. There is a growing 

 body of opinion that under appropriate physiological con- 

 ditions any type of mesenchymal cell may take on the func- 

 tions of a stem cell. Yoffey and Courtice (1957), for instance, 

 summarize the evidence that lymphocytes lodging in the 

 bone marrow may be transformed to haematoblasts with 

 potentiality to produce either lymphoid or myeloid cells. 



(ii) Lymphocytes are produced in the germinal centres 

 of lymphoid tissue where the cells undergoing mitosis have 

 the histological character of medium or large lymphocytes 

 and sometimes primitive reticular cells. There is increasing 

 evidence that lymphocytes are constantly being returned 

 from the blood and tissue fluids to the lymph nodes and other 

 aggregations of lymphoid tissue. Farr (1951), using lympho- 

 cytes labelled with a non-toxic dye, considered that a propor- 

 tion of those returned to the body intravenously settled in 

 lymph nodes and gave rise to new lymphocytes. 



Gowans (1957) finds that when lymphocytes labelled 

 with 32p are injected intravenously into rats, some rapidly 

 appear in the thoracic-duct lymph. 



Ill 



