MAST CELLS IN CATTLE AND RAT 



bundles of voluntary muscles, especially in the tongue. However, they are 

 rare immediately under the surface epithelium of the tongue, which lacks the 

 usual layer of subepithelial connective tissue and cannot be lifted off the under- 

 lying muscle (Fig. 17). Where such connective tissue is present, mast cells are 

 always to be found and their number is roughly proportional to the vascularity 

 of the part. They are thus very common in the subcutis of the nose and paws 

 and in the immediate vicinity of the nipples and ano-genital orifices. They are 

 exceptionally common in the thin subcutis of the ears in which they often sur- 

 round the hair follicles in characteristic circular patterns (Fig. 18, and compare 

 also Fig. 56). 



Mast cells are seen in the wall of the oesophagus mainly around the veins. 

 A few occur in the deeper mucosa and the submucosa of the stomach, whence 

 they may be traced in diminishing numbers down the rest of the alimentary 

 tract. 



In contrast to the general paucity of mast cells in paraffin sections of the 

 various parenchymatous organs (Fig. 16), a striking picture is seen when tissue 

 spreads stained with toluidine blue are examined. Here the predominant cell 

 is the mast cell and the relationship of the mast cells to the blood vessels in such 

 material can be accurately determined. The reader is referred to the original 

 publication for technical details of the methods which were devised for injecting 

 the blood vessels of the rat and for preparing and staining the tissue spreads 

 (Riley, 1953a). 



Dorsal subcutaneous connective tissue 



Vessels. The dorsal subcutaneous tissue of the rat is characterized by the 

 presence of parallel neurovascular bundles which run in segmental fashion 

 round the flanks and ultimately contribute to the rich vascular network of 

 the mammary glands (Figs. 11-13). These vessels themselves often throw off 

 branches laterally into the transparent connective tissue separating them from 

 adjacent neurovascular bundles, either as a tree-like arborescence (Fig. 13) 

 or, more rarely, a closed vascular loop (Fig. 12). Not infrequently their 

 branches anastomose with coi responding branches from the next main vessel. 

 At their origin the smaller twigs have usually a double muscle coat and this, 

 in turn, is surrounded by an adventitial sheath of connective tissue from which 

 are derived such perivascular fat cells as may be present. As the calibre of 

 the vessel diminishes, first the outer layer of muscle is lost, then the inner, and 

 finally the adventitia until around the capillaries only an occasional adventitial 

 cell remains. 



Mast cells. The neurovascular bundles themselves are enveloped in a 

 swarm of elongated ovoid mast cells lying in the adventitia close to and parallel 

 with the walls of small vessels (arterioles and venules) with muscle coats. These 



d 49 



