i8o 



BIOLOGY OF THE LABORATORY MOUSE 



ing, epithelial lined cysts (Fig. 91). These lining cells form a single orderly 

 layer, are low cuboidal, closely packed and uniform in size, shape and stain- 

 ing properties. They have scant cytoplasm and somewhat rounded, deeply 

 staining nuclei. Mitoses are infrequent. The walls of the cysts are not 

 smooth as in simple adenoma, but have an irregular wavy appearance with- 

 out the formation of papillary ingrowths. 



The stroma is composed of coarse and fine, wav>% eosinophihc non- 

 nucleated fibrils, throughout which are scattered thread-like connective 



Large spindle cell 



Adenomatous foci 



Fig. 93. — Fibro-adenoma of the mammary gland (X200). 



tissue cells. It varies from the thin compact foci which barely separate the 

 adjacent walls of many of the larger cysts to the dense compact foci around 

 the smaller cysts. The stroma is quite cellular in the region of the capsule 

 and blends with it. Numerous thin-walled blood vessels are scattered 

 throughout the entire stroma. 



Papillary cyst adenoma of the mammary gland. — This type is so named 

 because of the characteristic architecture which shows large and small, 

 branching and anastomosing irregular growths extending into cystic epithe- 

 lial lined cavities (Fig. 92). These papillary structures may have one or 

 more broad or narrow points of attachment with the remainder of the tumor. 

 The stroma extends into these structures and thus makes up a considerable 

 part of the papillary formations. In the tumor in general some of the 



