Myoepithelial cells are seen in many secretory organs including salivary glands, where they are usually encountered in the acini and intercalated ducts (see the image below). These cells express a dual epithelial and smooth muscle phenotype and are likely of ectodermal origin. [1, 2] Their neoplastic biologic behavior in many salivary gland tumors, unpredictable at times, has and is still being characterized either as a major component of myoepithelial entities or as one of the components of many salivary gland tumors (eg, pleomorphic adenoma, adenoid cystic carcinomas).
In 1943, Sheldon was the first to identify myoepithelial tumors as a distinct salivary gland tumor entity.  Following Sheldon's reference, earlier series included those of Bauer and Fox and Bhaskar and Weinmann. [4, 5] The major myoepithelial neoplastic entities in which myoepithelial cells are the exclusive tumor component or a major neoplastic component include the following:
Epithelial-myoepithelial carcinoma (discussed in a separate article)
Myoepithelial-rich pleomorphic adenoma
Myoepithelioma was recognized as a histologically distinct entity by the World Health Organization (WHO) in 1991.  It is a benign salivary gland tumor that consists entirely of myoepithelial cells with variable cellular morphologies including spindle, epithelioid, plasmacytoid, or clear cells. A myoepithelioma can be composed of one or a mixture of those cell types, and a variable stromal component can be seen in these tumors. Among investigators, the level of "tolerance" for the presence of epithelial ductal elements in a myoepithelioma is variable and controversial (see Microscopic Findings). To date, more than 200 cases have been reported.
Some authors believe that myoepitheliomas are a monomorphic, single-cell type variant of pleomorphic adenomas (in this case, myoepithelial cells). [2, 7] Other authors, very convincingly, describe a spectrum of salivary gland tumors with monomorphic adenomas (composed of pure epithelial cells; eg, canalicular adenoma) at one end, and myoepitheliomas (composed of myoepithelial cells only) at the other end — with pleomorphic adenomas offering a diverse range of mixture of cells and stroma in between. [1, 8, 9, 10]
Myoepitheliomas are also known as myoepithelial adenoma or benign myoepithelial tumors.
Myoepitheliomas are unusual and uncommon tumors that constitute only 1-1.5% of all salivary gland neoplasms. [7, 8, 11, 12, 13] Equal sex distribution is seen in most case series [7, 11, 14] ; however, analysis of 15 cases arising in the minor salivary glands of the palate showed a 2:1 female preponderance.  The average age of tumor incidence in affected individuals is 44 years, with a wide range from 9 to 85 years. [11, 16] Although myoepitheliomas have been reported in children, this tends to be a rare occurrence. 
No clear etiologic factor has been identified for myoepitheliomas.
Although myoepitheliomas have been reported in all major and most locations containing the minor salivary glands, [18, 19] the parotid gland is the primary site of occurrence of most reported tumors (40-50%), [8, 11] followed by the minor salivary glands as the second most preferred site (of which the palate is the most common location [21%]). [7, 11, 12, 14] The submandibular gland accounts for 10% of the tumors.
Clinical Features and Imaging
Myoepitheliomas present as a slowly growing, painless tumor mass. No cases in the parotid have been reported to be accompanied by facial nerve weakness, and those occurring in the palate rarely ulcerate.  As the tumor has been reported in variable head and neck locations, the symptoms will vary accordingly.
A myoepithelioma is composed exclusively of modified myoepithelial cells in which the normal phenotypic expression of the non-neoplastic myoepithelial cells has been altered (see the image below). 
Tumors arising in the parotid gland are usually encapsulated, in contrast to those arising in other major glands (submandibular and sublingual) or the minor salivary glands, which tend to lack a capsule or only have a partial capsule but are invariably well circumscribed.
Myoepithelial cell morphologies
Myoepithelial cells exhibit 4 main cell morphologies: spindle (most common), epithelioid, plasmacytoid, and clear cells (least common). A mixture of these subtypes may be present in one tumor.
Spindle cell myoepitheliomas have central fusiform/cigar-shaped nuclei, eosinophilic cytoplasm, and tapered ends and are usually arranged in interlacing fascicles (see the following image).
Epithelioid cell myoepitheliomas have large polygonal cells with central nuclei, and eosinophilic or amphophilic, sometimes focally clearing, cytoplasm. Although epithelioid cells can form pseudoacini/pseudoglandular structures, true glands or lumina are not seen (see the image below). 
Plasmacytoid cell myoepitheliomas have round to ovoid cells with abundant eosinophilic cytoplasm and eccentrically located nuclei (see the following image). The cells tend to be discohesive and occur mainly in aggregates in abundant mucoid stroma.
The plasmacytoid cell type is particularly encountered in palatal myoepitheliomas and has been the subject of some controversy that is largely due to its low level or complete absence of of expression of myogenic markers. Some authors have doubted their identification as myoepitheliomas, [23, 24] even suggesting that these tumors should be classified as adenomas or plasmacytoid adenocarcinomas. Other authors believe that the low expression of muscle tissue markers should still qualify them as myoepitheliomas  and that plasmacytoid cells may represent, with the other morphologic myoepithelial cell types, different stages of evolution in myoepitheliomas.  Electron microscopic evidence suggests a myoepithelial origin for these cells (see the Electron microscopy examination section, below).
Clear cell myoepitheliomas, the rarest kind, have polygonal cells with clear cytoplasm (due to glycogen content) (see the following image). Sometimes the clear cells can exhibit a signet ringlike or lipoblastlike appearance.  Microcystic spaces containing amphorous material can be seen in clear cell myoepitheliomas, however, these features are also seen, albeit less frequently, with other cell types. When considering the diagnosis of clear cell myoepithelioma, it is important to remember that many salivary gland tumors and metastatic tumors to the salivary glands can exhibit clear cell morphology.
Foci of oncocytic differentiation/metaplasia may also be seen in clear cell myoepitheliomas, and on occasion, these features can predominate in the tumor.  Occasionally, the oncocytic nuclear pleomorphism, enlargement, and hyperchromasia can be concerning, but these have not been found to necessarily correlate with malignant change.
Different architectural patterns including trabecular, nested, or solid may be seen. An unusual reticular pattern variant have been reported by Dardick et al, [12, 28] in which narrow, interconnected cords of tumor cells in a netlike fashion are seen surrounded by abundant mucoid stroma or loose vascularized stroma.
A myxoid (see the following image), mucoid, or hyalinized stroma may be seen among tumor cells that tends to be scant in hypercellular tumors. Although chondromyxoid stromas, typically seen in pleomorphic adenomas, are not usually seen in myoepitheliomas, chondroid metaplasia has been reported in some cases of myoepithelioma and myoepithelial carcinoma, in addition to squamous, adipocytic, and osseous metaplasia. [1, 16, 22, 25]
Myoepitheliomas versus pleomorphic adenomas
At times, the morphologic similarity between pleomorphic adenoma and myoepithelioma can make the differentiation between them difficult (eg, myoepithelial-rich pleomorphic adenoma versus myoepithelioma). The differentiation, however, is based on the total absence of ductal elements. Although some authors allow 5-10% of ductal differentiation within myoepitheliomas, [8, 14, 29] other investigators favor the total absence of ductal elements as a prerequisite for the diagnosis of myoepithelioma. [22, 30]
Many authors believe the behavior of myoepitheliomas and pleomorphic adenomas are similar,  although some authors contend that myoepitheliomas tend to exhibit a more aggressive growth pattern and a less predictable biologic behavior on long-term follow-up. Different cell types have not been found to relate to differences in a patient's age, biologic behavior, frequency of recurrence, or prognosis [7, 21, 29] ; however, some investigators argue that the clear cell variant may be more prognostically unpredictable than the other variants. 
It is important to point out that some myoepithelial carcinomas can present with a very bland cell morphology that may defy the clinical impression of malignancy and make identifying a myoepithelial carcinoma a difficult task histologically, particularly on limited biopsies. Observation of the infiltrative pattern, focal necrosis, perineural invasion among other malignant indices may be the only clue to malignant change in a seemingly bland myoepithelioma,  a task that may only be possible on careful evaluation of the resected tumor.
Electron microscopy examination
Ultrastructurally, the myoepithelial cells contain abundant cytoplasmic filaments/fibrils, duplicity of basement membrane, perinuclear tonofilament bundles, attachment plaques on the cell membrane, and desmosome-type intercellular junctions. [7, 33, 34, 35, 36]
It is of interest that examination of the plasmacytoid cell types by electron microscopy in many studies showed the same fibrillar type as the spindle cell type; however, the fibrils in the plasmacytoid cel types were more disorganized  and did not exhibit focal densities as seen in spindle cells. In all other ways, however, there was homology of the ultrastructure of both cell types.
Cutaneous and soft-tissue myoepitheliomas rarely occur in the head and neck region [25, 30, 37] as well as in the lacrimal gland region  and should be differentiated from those of salivary gland origin.
Myoepithelioma cells have been shown to be usually positive for the following markers:
Cytokeratins (eg, AE1/AE3, CK 5/6, Cam 5.2, CK-7, and CK-14)
Vimentin (reported to be positive in neoplastic myoepithelial cells and negative in normal myoepithelial cells)
S-100 (usually positive in the neoplastic myoepithelial cells but not the normal salivary gland myoepithelial cells)
Smooth muscle actin (SMA)
Muscle-specific actin (MSA)
Smooth muscle myosin
Glial fibrillary acidic protein (GFAP)
Myoepithelial cells are typically negative for carcinoembryonic antigen (CEA), signifying no tubular differentiation. E-cadherin expression has also been detected in myoepitheliomas. 
Many investigators have found that cells with spindle cell morphology have the strongest immunoreactivity to smooth muscle markers, followed by the epithelioid cells, with the plasmacytoid and clear cells showing lesser if not absent activity on occasions. [30, 43] Neoplastic myoepithelium has been well established to not always retain actin expression. [29, 44]
Fortunately, many such confirmations can be resolved by using a combination of these markers. Usually, a combination of a keratin in conjunction with the detection of S-100, vimentin, and/or a myogenic marker is required for confirmation of the diagnosis of a myoepithelioma.
Note that some differences have been found in the expression of some markers in salivary gland myoepitheliomas in contrast to soft-tissue myoepitheliomas. 
El-Naggar et al reported their cytogenetic analysis of a parotid myoepithelioma in which there were alterations in chromosomes 1, 9, 12, and 13.  The investigators also showed that myoepitheliomas and pleomorphic adenomas share the chromosome 12q alteration reported in a subset of pleomorphic adenomas. 
Hungermann et al noted the paucity of genetic alterations in 12 myoepithelioma cases, in which only 3 cases showed chromosomal loss; these alterations preferentially involved chromosomes 2, 4, and 8. 
Weber et al detected mutations in p53 in 3 of 12 myoepitheliomas examined, with no mutations in p63 or p73 in the examined cases.  Vekony et al demonstrated that deregulation of the p16INK4a senescence pathway is involved in the development of myoepithelial tumors and that additional inactivation of p53 is seen in benign recurrences. 
Skálová et al examined the presence of the EWSR1 rearrangement in a variety of clear cell salivary gland carcinomas with myoepithelial differentiation. The study described for the first time EWSR1 gene rearrangement in a subset of myoepithelial carcinomas arising in minor and major salivary glands. The EWSR1-rearranged clear cell myoepithelial carcinomas de novo (CCMC) represents a distinctive aggressive variant composed predominantly of clear cells with frequent necrosis. Most EWSR1-rearranged CCMCs of salivary glands are characterized by poor clinical outcomes. 
Prognosis and Predictive Factors
According to Scibba and Brannon, when compared with pleomorphic adenomas, myoepitheliomas are less prone to recurrence after complete surgical resection  with only 1 recurrence of 16 cases observed over a period of 1 month to 7 years. However, other investigators have found higher recurrence rates.  Nayak et al found that of 42 previously reported myoepithelial tumors, 93% of them eventually followed a benign course, whereas only 7% eventually pursued a malignant course.