SPONTANEOUS NEOPLASMS IN MICE 



20.; 



Fibrosarcoma. — Among the more common malignant tumors in the 

 subcutaneous region, the fibrosarcoma is the tumor most often observed. 

 However, no stock of mice shows a frequency of subcutaneous tumors which 

 would enable one to call it a high tumor stock in that respect. It is unusual 

 to find a stock showing over 15 per cent of the mice with tumors other than 

 mammary carcinoma in this region. Many fines of mice show considerably 

 less. 



Fibrosarcoma originates from the fibrous connective tissue cells. The 

 basic architecture is the same whether it develops in the mammary line and 



Fig. 112. 



-Fibrosarcoma of the subcutaneous connective tissue showing the typical 

 interlacing pattern of the spindle cells (X200). 



its branches or in any other subcutaneous focus. Grossly, the tumor is a 

 compact mass with a smooth, rounded, white surface. The cut surface is 

 uniform, bulging and varies from soft to firm. Microscopically it shows 

 closely packed spindle-shaped tumor cells (Fig. 112). Their architecture 

 presents a picture of solid masses of cells alternating with large and small 

 whorls and interlacing bands of fibrous connective tissue tumor cells. The 

 tumor cells exceed the stroma and the latter is difficult to identify, but 

 appears to be represented by inconspicuous stromal connective tissue cells 

 surrounding the individual tumor cells. Fibrosarcoma tumor cells often 

 appear slightly separated, as though shrunken away from the intercellular 

 stroma. Small, endothelial lined thin-walled blood vessels are abundant. 

 Invaded tissues such as striated muscle, nerves, large blood vessels and mam- 

 mary glands may be seen, for no capsule is present and infiltration occurs. 



