THE MAST CELLS 



one or more venules which return parallel to the afferent arteriole or first link 

 up with other milk spots before joining the main vein. 



Mast cells. Type-I mast cells lie close to the muscular walls of the main 

 arterioles and venules and their branches. In the milk spots a new arrangement 

 of mast cells is seen. The milk spot itself consists of a knot of small mesenchymal 

 cells and a few fat globules. Many of the small cells have a narrow rim of 



V 1 



Fig. 15 



A milk spot from the omentum. There is a leash of tiny vessels running through the mass of 

 relatively undifferentiated mesenchymal cells which constitutes the " tache laiteuse ". Emerging 

 from this zone are numerous rounded mast cells ( 50). (Riley (1953), J. Path. Bact. 65, 146.) 



cytoplasm which stains orthochromatically, and as the cells enlarge this staining 

 is most pronounced at the poles of the cells. In other cells small granules are 

 present around the nuclear space, and with further development and packing 

 of the granular material the cells gradually assume the appearance of type-I 

 mast cells, except that they are more rounded than the type-I cells seen elsewhere. 

 These type-I cells appear to change progressively into large type-II cells, some 

 of which lie free in the adjacent peritoneum. 



Uterine fringes 



In female rats the leaf-like fringes attached to the horns of the uterus 

 furnish excellent preparations for mast cells, as here much of the fat is deposited 



54 



