SPONTANEOUS NEOPLASMS IN MICE 195 



Tumors of the Skin, Subcutaneous and Body Wall Tissues 



Tumors of the dermis, subcutaneous tissues and body wall may be con- 

 sidered together. A tumor of the epidermis is fairly easy to determine 

 grossly, but tumors of the dermis might be confused with many of the new 

 growths occurring in the subcutaneous and body wall tissues. In the 

 mammary line and its branches the tumors not of mammary gland origin 

 would be the same as those of the subcutaneous tissues in general, except for 

 those of the axillary and inguinal lymph nodes. The majority of these 

 lymph node tumors belong in the groups to be discussed under lympho- 

 cytomas, myelocytomas and monocytomas. 



Tumors or the Epidermis 



Tumors of the epidermis are not common in any of our stocks. Papil- 

 lomas and epidermoid carcinomas have been found in small numbers in 

 many of the stocks, chiefly in the C57 black, X, W, ce, dba and their hybrids. 

 Papillomas occur most frequently on the external genitalia of the female, 

 around the anus, on the eyelids, ears, lower lip and occasionally on the skin 

 of other parts of the body. Epithelial horns are rare but have been found 

 about the head and shoulders in the C57 black and the dba mice. 

 Epidermoid carcinomas have been seen arising from the skin of the 

 dorsal and ventral surfaces (Fig. 106), the shoulders (Fig. 107), the lower 

 lip (Fig. 105), the eyelid and the skin around the anus and external genitalia 

 of the female. Frequently the epidermoid carcinoma occurs within a pre- 

 existing papilloma. 



The Papillomas. — These are benign epithelial tumors which are elevated 

 above the skin surface, often pedunculated, and contain varying amounts of 

 stroma. The epithelium is the active part of these tumors and shows thick- 

 ening and overgrowth. This results in the formation of the blunt, elevated 

 papillae and in the varying degrees of epithelial downgrowth into the dermis 

 (Fig. 104). Within the thickened epithelial layer the normal orientation of 

 epidermis is not lost and the basement membrane is intact, but marked 

 keratinization and cornification are usually present. The elevated mass 

 may consist almost entirely of epithelium with only thin finger-like processes 

 of stroma extending between the irregular epitheUal downgrowths and form- 

 ing a central core within the papillae. 



Epithelial Jwrns are really papillomas which show a marked degree of the 

 piling up of the cornified epithelium until grossly a horn-like growth about 

 two centimeters long may develop. This structure tapers from the base to 



