192 BIOLOGY OF THE LABORATORY MOUSE 



rounded epithelial tumor cells and represent a more undififerentiated form of 

 carcinoma simplex. These spindle cells often grow in nests and strands 

 with dense irregular strips of connective tissue cells scattered between them. 

 In some respects they may be confused with fibrosarcoma. However, 

 fibrosarcomas have more distinct cell boundaries and the cells are more 

 definitely tapering and spindle shaped. Also, these cells usually have less 

 cytoplasm and possess smaller, more hyperchromatic, nuclei which are more 

 pointed at the poles. The spindle-shaped carcinoma cells are greatly 

 elongated epithelial cells and grade into polyhedral and rounded epithelial 

 tumor cells at the periphery of the tumor cell nests. 



The polyhedral cells are the most uncommon carcinoma simplex cell in 

 our stocks. The cases we have show pale, rather large, many sided cells 

 which in some respects resemble squamous cells of the epidermis, but are 

 without keratinization. They grow in closely packed, irregular masses with 

 dense stroma between them. Their origin can be traced to mammary gland 

 epithelium. Mitoses are frequent and the tumor freely invades surrounding 

 tissues. 



Carcinoma simplex as a whole grows rapidly, shows extensive infiltration 

 into adjacent tissues and metastasizes to the lungs. Epithelial pearls may 

 be found, especially in the branching and anastomosing forms. 



Carcinosarcoma of the Mammary Glands 



This type originates from a pre-existing fibroadenoma. In this tumor 

 the adenomatous elements become malignant as shown by invasion of the 

 basement membrane, followed by the spreading of the epithelial tumor cells 

 into the stroma in dense, irregularly arranged nests. There is also a malig- 

 nant change involving the large connective tissue cells found in the stroma. 

 These multiply and spread as interlacing strands of connective tissue tumor 

 cells. Thus the resultant histological picture is that of a fibrosarcoma grow- 

 ing around nests of adenocarcinoma (Fig. 103). Both types of tumor show 

 frequent mitoses. Should the sarcoma outgrow the carcinoma, the picture 

 is predominantly that of fibrosarcoma. 



Fibrosarcoma of the Mammary Gland Stroma 



This may originate from a carcinosarcoma, as above, or from an adeno- 

 fibroma in which the fibromatous elements alone have become active. This 

 tumor can also originate from the stroma about the mammar}^ glands in the 

 absence of an adenoma. The resultant fibrosarcoma forms a dense tumor 

 mass composed of closely packed spindle-shaped connective tissue tumor 



