igS 



BIOLOGY OF THE LABORATORY MOUSE 



the tip. At the base the epithehal downgrowths may penetrate below the 

 level of the epidermis and contain only thin, loose strands of stroma (Fig. 

 104). The basement membrane remains intact and the arrangement of the 

 epithelium at the base is that of a papilloma. 



Epidermoid carcinomas of the skin.— These are malignant tumors of the 

 epidermal epithelium. The\- vary from early forms beginning in papillomas, 

 such as on the lip (Fig. 105A) or external genitalia, to wildly growing types 



Sebaceous glands 



Stroma of dermis 



Stroma of tumor 



Base of papilloma 



Fig. 104. — Subcutaneous focus from the base of an epithelial horn (papilloma) of the 



skin (X200). 



with marked anaplasia. This is accompanied by extensive invasion and 

 occasional metastases to lymph nodes (Fig. 108A) and lungs. The low 

 grade forms show loss of orientation, extension through the basement mem- 

 brane and invasion of the adjacent normal structures. Except in the most 

 rapidly growing forms, marked keratinization and extensive formation of 

 large and small epithelial pearls are common (Fig. 106). The epithehal 

 tumor cells grow in nearly solid masses without much stroma. Epithelial 

 pearls are usually scattered throughout and show concentrically arranged, 

 flattened, cornified epithelial cell debris that takes an eosinophilic stain. 

 Around these pearls are irregular clumps of large polyhedral cells with large. 



