SPONTANEOUS NEOPLASMS IN MICE 



205 



sity here to enter into the controversy over the exact tissue of origin and 

 whether they should be called melanocarcinomas or melanosarcomas. For 

 our purposes it is sufficient to designate them as either melanomas or 

 malignant melanomas. 



The histopathology usually shows a tumor whose architecture and cellu- 

 lar detail is heavily masked by the intense pigmentation (Fig. 113). Around 

 the edge of the tumor the cellular detail is visible and shows large and oval or 

 smaller and spindle-shaped cells whose cytoplasm is filled with a closely 



Epidermis 



Dermis 



Sebaceous gland 



Melanotic tumor 

 cells 



Fig. 113. — Melanoma of the skin (X200) 



packed fine brown pigment, melanin. The most rapidly growing foci show 

 the least pigmentation and the cellular detail is easily seen under the micro- 

 scope. ^Mitotic figures are often abundant and invasion of the adjacent 

 tissues is extensive. This is one of the most widely metastasizing types of 

 tumors found in the mouse. 



Rhabdomyosarcoma. — This is a malignant tumor originating from 

 striated muscle (Fig. 114). In the subcutaneous region it appears to occur 

 generally in mice of about the same age as animals bearing other types of 

 subcutaneous tumors. However, cases are sometimes observed in young 

 mice probably from embryonic rests in the striated muscle. The earliest 



