Myoepitheliomas in Inbred Laboratory Mice

JAX® NOTES Issue 450, Summer 1992

John P. Sundberg, DVM, PhD; Kathy S. Brown, BS, MA; and Hendrick G. Bedigian, PhD

Description

Myoepitheliomas are tumors arising from modified epithelial cells that have contractile properties. These cells are found in simple and compound glands which contract to force secretions out of acini and down ducts. Primary tumors of these types of cells are rare in domestic animals, which creates a diagnostic dilemma for veterinary pathologists not familiar with this lesion.

Structure

Myoepitheliomas present as large, thin walled, soft fluctuant masses, primarily located in the region of the ventral neck . When opened, they contain thick, opaque, red-brown fluid (the result of liquefactive necrosis, (Fig.2). Mestastases are rare. Microscopically, the wall of the tumor is well circumscribed and consists of spindle-shaped cells with abundant eosinophilic cytoplasm in a palisade-like pattern. (Fig. 3). Fibrovascular stalks penetrate into the tumor from the surrounding connective tissue. These may contain efferent ducts from mammary glands (Fig. 4). Ultrastructurally the neoplastic cells have prominent microfilament bundles surrounding the nucleus. Because these lesions are found in the neck region, often adjacent to the salivary glands, it has been commonly believed that they originate from myoepithelial cells surrounding salivary gland acini. This is difficult to prove or disprove based on routine histologic sections, however, the fact that these tumors can be found in the lower genital tract, ventral abdomen, around the eyes, and other locations where various glands, other than salivary glands are present, suggests that myoepitheliomas can arise from any exocrine gland. Extensive immunohistochemical studies further suggest that myoepitheliomas arise from cells around ducts rather than the gland themselves.

Incidence

In evaluating the epidemiology of this tumor type in our production and research colonies, we diagnosed myoepitheliomas in A/HeJ, A/J, BALB/cByJ, BALB/cJ, LLC.A/Ckc, and NOD/Lt strains. With the exception of the NOD/Lt, the affected strains has a similar derivation, suggesting a genetic predisposition. This is even more striking since 84% of the tumors were in the BALB/cJ strain. When corrected for colony size, the discrepancy remained (BALB/cJ, 14/105; A/J, 5.7/105; and BALB/cByJ, 1.9/105).

Figure 4

Interpersed between the spindle-shaped cells of the myoepithelioma are islands of necrosis (arrow) as well as supporting fibrovascular networks that often contain ducts (arrowhead).

Kodachromes

Southern blot analyses of DNAs extracted from a limited number of myoepitheliomas did not detect additional mouse mammary tumor virus (MMTV) proviruses or rearrangements with the Int-1 locus, which suggested that a retrovirus was not the etiological agent.

A set of kodachrome slides is available for teaching purposes on the myoepithelioma. For information on obtaining the set, contact a customer service representative at 1-800-422-MICE.

Reference

Sundberg JP, Hanson CA, Roop DR, Brown KS, Bedigian HG: Myoepitheliomas in inbred laboratory mice. Vet Pathol 28:313-323, 1991.