834 IMMUNOLOGY 



In addition to the cells which are generally admitted to retain mesen- 

 chymal potencies, free cells occur in the loose connective tissue, which 

 we have called macrophages and which are variously known as histio- 

 cytes, clasmatocytes, rhagiocrine cells, or resting wandering cells. Just 

 as in the case of the phagocytic mesenchymal cells, there is no unani- 

 mity of opinion as to whether these free cells retain all hematopoietic 

 functions, but in any case they can become phagocytic without morpho- 

 logical change, can reproduce by mitotic division, and can transform 

 into fibroblasts. 



Many other macrophages occur throughout the body, the develop- 

 mental capacities of which have not been adequately studied. Thus the 

 stroma cells of the lamina propria of the intestine probably have develop- 

 mental potencies identical with those of the reticular cells. 



As would be expected, macrophages in different locations and before 

 and after becoming phagocytic vary somewhat in structure with regard 

 to the amount of their cytoplasm, the size and shape of their nucleus, 

 and the amount and size of the chromatin granules and nucleoli in their 

 nucleus. They generally possess, however, well defined cytoplasm and a 

 large, vesicular, often indented nucleus, in which are found fine chro- 

 matin granules and a few small nucleoli (see reticular cell and macro- 

 phage in PI. 3, Fig. 1 ) . 



Fibroblasts of loose connective tissue have outstretched, ill-defined 

 cytoplasm and a large, regularly oval, vesicular nucleus containing dust- 

 like chromatin granules and small nucleoli. They can divide by mitosis, 

 are instrumental in repair and in walling off foreign material, but are 

 rarely phagocytic and do not generally develop into other cells (except 

 in bone and cartilage) . 



Endothelial cells line the larger blood vessels and capillaries. (The 

 term as herein used, does not include the littoral cells lining the sinuses 

 and sinusoids of the reticular tissues and elsewhere, which have wide 

 developmental potencies). The endothelial cells can divide by mitosis, 

 can form endothelium of new blood vessels, and can develop into fibro- 

 blasts, but are rarely phagocytic and do not generally develop into other 

 cells. 



B. Free Blood and Connective Tissue Cells. — In accordance with 

 common usage, cells of the blood and lymph are classified according 



