JPC SYSTEMIC PATHOLOGY
Signalment (JPC Accession # 4035521): 8-month-old female Balb/c mouse
HISTORY: The mouse was culled during routine surveillance. It had a 3cm diameter, cystic, subcutaneous mass of the ventral neck and cranial chest composed of pale, friable tissue with a rich blood supply. The mass had a central cavity containing blood and viscous material. There were no other gross abnormalities.
HISTOPATHOLOGIC DESCRIPTION: Haired skin: Expanding the subcutis, elevating the overlying dermis and epidermis, and compressing atrophic pre-existing mucinous and mixed salivary glands, is a 1.25x1cm expansile, well-demarcated, partially encapsulated, moderately cellular, multilobular, multicystic neoplasm. The neoplasm is composed of spindle cells arranged in short streams and nests divided into lobules, and often palisading along a moderate fibrovascular stroma. Neoplastic cells have indistinct cell borders, a moderate amount of finely vacuolated basophilic cytoplasm, and oval to elongate nuclei with finely clumped chromatin and 1-2 small basophilic nuclei. Mitoses average 1-2 per 400X high power field. Many of the neoplastic lobules contain large central areas of lytic necrosis. At one edge of the section, there is a large area of mature granulation tissue with a focus of the previously described neoplasm. The neoplasm is bordered on one side by an extensive area of granulation tissue admixed with granulomatous inflammation admixed with hemorrhage and hemosiderin-laden macrophages
MORPHOLOGIC DIAGNOSES: Haired skin, mixed and mucinous salivary glands: Myoepithelioma, Balb/c mouse, rodent.
- Myoepitheliomas arise infrequently in most strains of mice, but are more common in some strains (BALB/c, BALB/cBy), especially females
- In mice, myoepitheliomas arise most frequently from submaxillary and parotid salivary glands, but may also be associated with mammary, preputial, and Harderian glands
- Metastasis to the lungs may occur with large tumors
- There is frequent concomitant bone marrow and splenic myeloid hyperplasia apparently related to a secretory product of the tumor
TYPICAL CLINICAL FINDINGS:
- Most animals with salivary gland neoplasia present due to the development of a mass
- Other symptoms include halitosis, weight loss, anorexia, dysphagia, exophthalmos, Horner’s syndrome, sneezing, and dysphonia
TYPICAL GROSS FINDINGS:
- Myoepitheliomas can become very large with cystic chambers containing mucinoid fluid
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Tumors composed of large, pleomorphic, spindle cells with epithelial and mesenchymal features lining a cystic center
- Cystic areas due to necrosis
- Immunohistochemistry: Myoepithelial cells are immunoreactive for smooth muscle actin, calponin, vimentin, and p63
- Salivary gland neoplasms:
- Epithelial: pleomorphic adenoma (benign mixed tumor), oncocytoma, adenocarcinoma (see D-N11), malignant mixed tumor, cystadenocarcinoma, myoepithelial carcinoma, squamous cell carcinoma, salivary duct carcinoma, basal cell adenocarcinoma
- Non-epithelial: rare reports of osteosarcoma, infiltrative angiolipoma, melanoma, and metastatic neoplasms of other origins
- Non-neoplastic salivary gland masses: salivary gland cyst (pseudocyst AKA sialocele, true salivary cyst), salivary gland infarction (AKA necrotizing sialometaplasia, see D-M12), lipomatosis (rare, unilateral, slowly progressive enlargement of salivary gland by lobules of well-differentiated adipocytes)
- Salivary gland neoplasms are rare in all species; they have been reported in cattle, sheep, goats, horses, dogs, and cats but not swine; and are almost exclusively carcinomas (see D-N11); rarely other neoplasms including epithelial (myoepithelioma, squamous cell carcinoma, cystadenocarcinoma) and mesenchymal (angiolipoma, fibrous histiocytoma, osteosarcoma, fibrosarcoma) malignancies arise in salivary glands
- Myoepitheliomas are generally considered rare in domestic animals and man; myoepitheliomas of non-salivary glands include:
- Mammary myoepitheliomas (see R-N14) are described in dogs, are rare, benign neoplasms composed of spindloid cells in short bundles on a myxoid matrix, there are also malignant variants (i.e. malignant myoepithelioma); mammary neoplasms are common in dogs, relatively uncommon in cats, and rare in other domestic species
- Myoepithelioma of the gland of the third eyelid has been reported in dogs; neoplastic spindle cells were strongly immunoreactive for CK14, p63, SMA, and calponin in one study (Miyazaki 2015) and p63, α-SMA, and calponin but were negative for AE1/AE3 in another study (Bondoc 2014)
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