HISTIOCYTES 21 



histiocytes can easily be observed. Here again there is formation 

 of giant cells which lie in close relationship to the reticulum and 

 which contain argentaffin fibrils. It is not clear why such focal 

 hypertrophy of histiocytic tissue should occur, but one may infer 

 that these large cells are not only morphological giants but 

 are also functionally powerful. This is suggested by the observa- 

 tion that such foreign-body giant cells arise particularly around 

 substances capable of being ingested and destroyed by phago- 

 cytosis. 



There seems also to be no doubt that the cells of the histiocytic 

 system can become free elements. The nucleus and protoplasm 

 of the sessile cell becomes hypertrophied ; soon the nucleus 

 becomes vesicular, and, although the cell is still attached to the 

 reticulum, it bulges into the reticular spaces. Gradually the 

 outlying processes of the cell are withdrawn and a rounded element 

 is set free. It appears that such liberated elements are particu- 

 larly active ; certainly particles of ingested material such as fat 

 and carbon are seen inside them in relatively large amounts. 



So much has been written about that special form of histiocyte, 

 the Kupffer cell in the liver, that we need say but little here. It is, 

 however, necessary to point out that recent observation has shown 

 the presence of at least three types of cells in the hepatic capillaries ; 

 there is the endothelium proper, then there are branched pericytes 

 lying between the endothelium and the liver cells themselves ; 

 and lastly there are endocytes that lie on the inner surface of the 

 endothelium, or even in the lumina of the capillaries ; these last 

 are presumably the true Kupffer cells. 



There is much argument as to the mechanism by which the 

 liver occasionally regains its embryonic function of blood-forma- 

 tion. Some writers contend that this occurs only by colonisation 

 of the capillaries by multi-potent stem-cells, whilst other investi- 

 gators are equally emphatic that such myeloid metaplasia may 

 arise in loco. Certainly some evidence is available that Kupffer 

 cells may become free and be converted directly in cells charac- 

 teristic of the blood. Further, some blood tumours of the liver 

 (angeiomata) contain immature blood-cells which presumably 

 must have been formed in situ, because such growths may occur 



