2o6 



BIOLOGY OF THE LABORATORY MOUSE 



case in our records occurred in a two and one half months old dba female. 



This tumor is composed of cells which resemble embryonic muscle grow- 

 ing in wild confusion. These can be seen to originate from normal muscle 

 and may become spindle cells which resemble spindle cell sarcoma of fibrous 

 connective tissue origin. 



The tumor cells are eosinophilic but paler than normal muscle. The 

 largest cells are most differentiated and possess finely granular cytoplasm. 



Tumorous muscle 

 cell 





Fig. 114. — Rhabdomj'osarcoma that originated in the striated muscle (X400). 



There are usually some cells showing definite cross striations, as in normal 

 muscle, but most of these cells resemble embryonic muscle. Nuclei are 

 large, nearly round, fairly deeply staining and centrally located. The 

 stroma consists of an abundant blood supply and a small amount of 

 connective tissue. Where the cells are smaller and less differentiated, they 

 become more like fibrosarcoma in cellular characteristics and general 

 arrangement. 



Infiltration of normal tissue occurs. Mitotic figures are not very abun- 

 dant in the foci which are most like normal muscle. This tumor can be 

 distinguished from fibrosarcoma of the connective tissue invading normal 

 muscle, for in the latter, muscle is being destroyed, while in the former there 

 are foci resembling embryonic and regenerating muscle cells. 



