204 BIOLOGY OF THE LABORATORY MOUSE 



The sarcoma cells vary in size from medium to large, while very large 

 tumor giant cells are sometimes present. In shape the tumor cells range 

 from blunt, to long, narrow spindle cells. The cytoplasm is pale, eosino- 

 philic and appears to have faint, longitudinal striations. Nuclei are elon- 

 gated, moderately hypochromatic, more or less irregular in outline, and have 

 one or more large nucleoli. The nuclei are centrally located and between the 

 nucleated cells are many smaller non-nucleated fragments. These frag- 

 ments represent the tapering ends of long cells cut at such an angle that the 

 nuclei are not included. Mitotic figures are abundant. 



In some undifferentiated fibrosarcomas the spindle-shaped cells are often 

 in the minority. These tumors show many polyhedral cells that are large, 

 pale and closely packed. They grade into very large mononucleated and 

 multinucleated tumor giant cells. These have an irregular outline and 

 abundant, rather deeply eosinophilic, cytoplasm. Some of the largest cells 

 may have a stippled appearance due to the presence of tiny vacuoles. This 

 is a degenerative change which can advance into a signet ring type of cell 

 where the nucleus and cytoplasm are compressed into a small peripheral mass. 

 As a rule the more undifferentiated the cells, the less the amount of stroma 

 and the more rapid the growth of the tumor. 



Liposarcomas. — These are malignant tumors originating from fat tissue 

 as in a lipoma. They are among the rare tumors in mice, but have been 

 observed in yellow stock animals. 



Neurogenic fibrosarcoma. — This type is difhcult to separate from fibro- 

 sarcoma of connective tissue origin in the mouse. However, it can be 

 identified when the origin is definitely traced to nervous tissue. There is 

 also a more marked tendency to show a herring bone pattern type of arrange- 

 ment of the spindle-shaped tumor cells in neurogenic fibrosarcoma. It is 

 probable that the rapidly growing undifferentiated tumors of this group are 

 often classed with the fibrous connective tissue tumors, fibrosarcomas. 



Melanomas. — These tumors have occurred for the most part in our dba 

 stock and the common site has been on or near the tail. There have been 

 cases of melanoma of the eye, ear and the skin in general. The tumor is 

 grossly brown to black and the color is often visible through the skin of the 

 living mouse. The tumor is smooth and rounded and may show tiny black 

 foci extending into the adjacent tissues. Lymph nodes when involved often 

 appear black to the naked eye, and lung metastases may be so extensive that 

 the lungs are sometimes solid and nearly black in color. The cut surface is 

 bulging, smooth and black or nearly black. 



These tumors are usually malignant and the majority of them may 

 properly be called malignant melanomas. There is neither space nor neces- 



