168 The Lymphocyte and Lymphocytic Tissue 



the dermis (Fig. 13-3), isolate the collagen bundles, and extend into the 

 subcutis to grow between the fat cells (Fig. 13-4). Mitoses are common. In 

 many respects, therefore, this lesion simulates a locally invasive, malignant 

 neoplasm, but it actually is benign, readily cured by simple, even incom- 

 plete excision, and is often self-limiting. Of some help to the pathologist 

 encountering this lesion is the knowledge that malignant lymphoma in the 





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Fig. 13-5. Canine mastocytoma. Neoplastic mast cells invade the dermis 

 and are often arranged concentrically around hyaline, thick-walled blood 

 vessels. The metachromatic granules in this cytoplasm of the mast cells 

 differentiate the cells from the lymphocytic series. Giemsa's stain, 

 (x 375) (AFIP 186765.) 



dog rarely involves the skin, and when it does, other organs are usually 

 recognizably affected. 



The canine mastocytoma may present a problem in differential diagnosis 

 unless the pathologist is familiar with its characteristics. This tumor involves 

 the dermis and sometimes the subcutis. It is made up of mast cells of varying 

 degrees of differentiation and is mixed with eosinophils and occasionally 

 more primitive myeloid cells. The metachromatic granules in the cytoplasm 

 of the mast cells are of definitive value in distguishing these cells (Fig. 13-5). 



The term "malignant lymphoma" as used by Gall and Mallory 12 most 

 clearly encompasses the histologic features of the various forms of lympho- 



