Signalment:  

Female, approximately 6 months of age, nu/+ mouse, congenic with BALB/c mouse (Mus musculus)

Gross Description:  

Firm anterior ventral cervical mass.

Morphologic Diagnosis:  

Myoepithelioma, submandibular salivary gland.

Condition:  

Myoepithelioma

Contributor Comment:  

Salivary gland tumors are generally rare in mice. An exception is myoepitheliomas in female BALB/c mice, where an incidence of 16.1/100,000 was reported in a large breeding colony at The Jackson Laboratories and 36/5090 in control groups in chronic studies (1,2). The lower incidence at Jax probably reflects the fact that breeding mice are rarely kept past 6-8 months of age, with most animals leaving the colony at 4-6 weeks of age. Myoepitheliomas are rarely observed in male BALB/c mice or in mice of other strains.

Myoepitheliomas in mice can vary from solid to having numerous large cavities resulting from necrosis of tumor cells. This tumor was more solid than most we see, with only small cystic areas embedded in a tumor consisting of sheets and swirls of epithelioid to spindle cells. This histologic pattern and location are considered sufficient for diagnosis, with the key differentials being a complex adenoma and carcinosarcoma. Tumors with extensive squamous differentiation can also be confused with squamous cell carcinomas.

Although not apparent in most of these sections, mouse myoepitheliomas are invasive. They can also metastasize to the lungs, although this is seen infrequently in the diagnostic health monitoring laboratory setting, where subgeriatric mice are killed for other reasons or when tumors first become apparent. 

JPC Diagnosis:  

Submandibular salivary gland: Myoepithelioma

Conference Comment:  

The myoepithelial cell is a modified epithelial cell that is located between the epithelial cell and the basement membrane. They contain long cytoplasmic processes that contract upon sympathetic or parasympathetic stimulation. Although myoepitheliomas can occur in any tissue, they most commonly arise from the submaxillary and parotid salivary glands, mammary tissue, and sweat glands.³ Neoplastic cells are positive for cytokeratin, actin, calponin and myosin.² Salivary myoepithelial neoplasms are rare in domestic animal species.²

References:

1. Botts, S., Jokinen, M., Gaillard, E.T., et al.: Salivary, Harderian, and Lacrimal Glands.  In: Pathology of the Mouse, ed. Maronpot, R.R., p. 59. Cache River Press, Vienna, IL 1999.
2. Seely, J.C.: Salivary Glands.  In: Pathobiology of the Aging Mouse, eds. Mohr, U., Dungworth, D.L., Capen, C.C., Carlton, W.C., Sundberg, J.P., and Ward, J.M., pp. 261-265. ILSI Press, Washington, DC 1996.

A virtual slide is not available for this case.

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