Chronic dermatitits characterized by alopecia may result from mite infestation. Two species commonly involved in the mouse are Myobia musculi and Myocoptes musculinis. Acanthosis with hyperkeratosis, infiltration of the dermis with mononuclear cells and cross sections of the mites on the surface of the skin are noted microscopically (Fig. 310).
Squamous cell papillomas occur as rare spontaneous lesions on the skin. They present a wart-like gross appearance and project above the skin surface. Microscopically, the multiple projections contain a connective tissue core covered by a highly keratinized squamous epithelium (Fig. 311). In skin painting studies, many typical papillomas and keratoacanthomas are produced. Some induced papillomas develop foci of squamous cell carcinoma which progress to invasive carcinoma.
Basal cell carcinomas arise from the basal cells of the epidermis. Although common in man and some animals, they are rare in mice. Characteristically, basal cell carcinomas are composed of ribbons of small basophilic cells with a scant amount of cytoplasm (Fig. 312).
Naturally occurring squamous cell carcinomas are rare in mice and must be distinguished from adenoacanthoma of the mammary gland in female mice. Neoplastic islands of squamous epithelium commonly invade the dermis. Keratin pearls my be prominent or infrequent depending upon the degree of differentiation (Fig. 313). Typical squamous cell carcinomas, often arising within papillomas (Fig. 313), are found in skin painting studies. Poorly differentiated squamous carcinomas may be identified by keratin immunostaining within some tumor cells (Fig. 314).
Sebaceous glands are found throughout the skin of mice, but sebaceous gland tumors are extremely rare. Adenomas present a typical appearance with cords of sebaceous cells (Fig. 315).
Spindle cell sarcomas are a relatively common tumor in the skin and subcutis of mice. They quite often are undifferentiated; and, thus, it is difficult to identify the specific cell of origin by light microscopy. They may be termed undifferentiated sarcoma or sarcoma, NOS (Sarcoma, not otherwise speecified). Sarcomas composed of plump fibroblasts producing collagen can be classified as fibrosarcomas. Microscopically the spindle cell sarcomas are composed of whirling fascicles of spindle-shaped cells with elongated nuclei (Fig. 316). The pattern may vary quite markedly within the same neoplasm. Electron micrographs of fibrosarcomas confirm the presence and production of collagen by tumor cells (Figs. 318 and 319).
The hemangiopericytoma is derived from the pericyte. In the dog it forms a very common concentric or onion skin pattern around small blood vessels. In the mouse the pattern is similar but not as striking (Fig. 317). The lesion is very rare in the mouse.