1. The size of pleomorphic adenomas varies from several millimeters in diameter to more than ten centimeters. The tumor is round or ovoid, with smooth surface, or lobulated or nodular. The tumor may be surrounded by a connective tissue envelope, but the thickness varies. In different areas of the same tumor, the thickness can be different. In some cases, the envelope is incomplete and the tumor is connected to the surrounding normal tissues. According to the different structure of tumor cells, there are different manifestations: when there are more epithelial cells, the tumor is substantial, gray-white, and hard in texture. When mucus-like tissue is abundant, the texture is softer and there is much mucus. When cartilage-like tissue is more abundant, it is light blue, translucent and hard. Sometimes cystic cavities of various sizes are seen, containing colorless transparent or brownish fluid, and occasionally small areas of hemorrhage are seen. The adhesion between the tumor’s envelope and the tumor body is poor, and it is very easy to peel off. When enucleation is used, it is very easy to leave the stripped peritumor in the operation area, resulting in postoperative recurrence. Recurrent tumors are often multifocal, the tumor volume and number are very inconsistent, the small one is only the size of a grain of corn, but it can be as many as dozens or hundreds.2. The complex tissue structure of pleomorphic adenomas as seen by light microscope, different parts of the same tumor, or different tissue images are found among the pleomorphic adenomas. However, the main components of pleomorphic adenoma are glandular epithelial cells, myoepithelial cells, mucus, mucus-like tissue and cartilaginous tissue, which form the basic structure of duct-like structure, mucus-like tissue and cartilaginous tissue, myoepithelial sheet, etc. Squamous metaplasia was sometimes seen. According to the proportion of epithelial cells and stroma, they can be divided into cell-rich and mesenchymal-rich types. Generally, the cell-rich type is prone to malignant transformation, and almost half of the cases of cancer in pleomorphic adenomas come from cell-rich pleomorphic adenomas. Therefore, this type of tumor, especially in older patients, should be alerted to the possibility of malignant transformation. On the contrary, the mesenchymal-rich type is more likely to recur.3 Immunohistochemistry and electron microscopy are of great significance in elucidating the histogenesis of pleomorphic adenomas, especially in confirming the myoepithelial cells and their roles, but they are not necessary for general routine diagnosis. Pleomorphic adenomas may show positive immunohistochemical staining for cytokeratin, S-100 protein, glial fibrillary acidic protein (GFAP), actin, vimentin, epithelial membrane antigen (EMA), and carcinoembryonic antigen (CEA). Immunoelectron microscopic observation of myoepithelial cells in pleomorphic adenomas by Wang J et al. showed that the tumorigenic myoepithelial cells labeled within the epithelial area were dwarf cuboidal, spindle-shaped or polygonal. The nuclei were oval, pitted or incised, with a thick nuclear membrane and distinct nucleoli. The cytoplasm was rich in free ribosomes, more rough endoplasmic reticulum and mitochondria. These cells were located around the ducts or were present in patches and were connected to each other or to the glandular epithelium by bridging granules. Tumorigenic myoepithelial cells labeled in the mucus-like region were sparsely arranged, triangular, star-shaped or polygonal, with irregular nuclei, thicker nuclear membranes, obvious nucleoli, and protruding cytoplasm, with the protrusions connected to each other by bridge grains. Wang Jie et al. clearly believed that tumorigenic myoepithelial cells were the main proliferating tumor cells in pleomorphic adenomas, and the cytoplasm of some tumorigenic myoepithelial cells in the mucus-like region was enriched with glial fibrillar proteins and intermediate filaments of waveform proteins, which had the change of chondrocyte in the early stage. Thus, myoepithelial cells play an important role in the histogenesis of pleomorphic adenomas.