178 



BIOLOGY OF THE LABORATORY MOUSE 



staining properties. The basement membrane is well preserved. These 

 cells do not differ strikingly from inactive, normal mammary gland epithelial 

 cells. In some cases the lining epithelial cells of the cysts range from the 

 small cuboidal type with moderately deeply staining nuclei and scant cyto- 

 plasm to fairly large, oval cells which bulge into the cyst cavity. These 

 latter cells have large, pale, centrally placed oval nuclei which contain 



Fig. 91. — Polylocular cystadenoma of the mammary gland showing intercommuni- 

 cating cysts (X200). 



scattered chromatin granules and multiple nucleoli. The cytoplasm is 

 eosinophilic, uniformly staining and fairly abundant. In simple adenomas 

 the stroma does not bulge into the epithelial-lined cavities. These gland- 

 like arrangements may have foci that are uniformly large or small but are 

 usually distributed so that the whole range can easily be found in a single 

 low power field. Mitoses are seen but are infrequent. 



The stroma is rather loose in the foci where the gland-like structures are 

 most widely separated and contains scattered strands of connective tissue. 

 Beneath the basement membrane of each of the gland structures the connec- 



