THE MAST CELLS 



cells stain uniformly in a blue, orthochromatic tint and so densely as to obscure 

 the nucleus. For convenience these may be called type-I mast cells. 



If we now follow the vessels distally it is seen that there is a gradual change 

 in the type of perivascular mast cell. Where the muscular vessels are enveloped 

 in fat the type-I cells remain close to the vessel between the muscle and the 

 surrounding fat. However, where the muscular vessels traverse loose connective 



Fie;. 11 



Mast cells in connective tissue near mammary gland. Chains of type-I cells accompany the main 



vessels with muscle coats. Scattered type-I I cells are associated mainly with capillaries, best seen 



to the right of the field (. 40). (Riley (1953), J. Path. Bad. 65, 461.) 



tissue the type-I cells appear free to move away from the vessel wall and to 

 become larger and irregular in shape. In these cells the homogeneous staining 

 is gradually resolved into orthochromatic granules set in a clear cytoplasm and, as 

 the granules become further dispersed, they stain purple to red (metachromasia). 

 These cells have been designated type-I I mast cells. In some of them the round 

 nucleus is represented by a clear space from which the granules have been 

 pushed aside. Around the capillaries type-I I cells predominate, though an 

 occasional small type-I cell remains close to the capillary itself. In addition, 

 occasional dense clumps of 3-20 type-II cells are found isolated in the inter- 

 spaces of the dorsal subcutaneous connective tissue remote from any demon- 

 strable vessel. These cells frequently show a cross-scored appearance from 



50 



