**4. Pathological features**

UM originates from uveal melanocytes, which is limited in the early stage, and further develops into typical fungal changes. Under the electron microscope, UM tumors are composed of round and spindle cells with rich cytoplasm, large nuclei, and frequent mitotic phases. New blood vessels are scattered in the tumor. Most tumors contain melanin, and a few tumors do not. The histopathological classification of UM is currently widely adopted in the world according to the classification standard developed by the WHO in 1980, and it is divided into four categories: (a) spindle cell type: composed of spindle-shaped type A and type B tumor cells in different proportions. Tumor cells are relatively dense, arranged in bundles or swirls. (b) Epithelial cell type: The tumor body is mainly composed of epithelioid tumor cells. Epithelioid cells account for more than 75%, and the rest are spindle-shaped A cells or spindle-shaped B cells. (c) Mixed cell type: composed of spindle-shaped and epithelioid melanoma cells in different proportions. (d) Others: those that do not meet the above classification, such as necrotic type, balloon-like cell type.

Spindle cell melanoma accounts for 40% of all UM, of which more than 90% are spindle cells, and the 15-year mortality rate of patients is 20%; epithelial cell melanoma accounts for 3–5% of all UM, of which more than 90% are epithelial cells, and the 15-year mortality rate of patients is 75%; the remaining 50% are mixed cell melanoma. At present, it is believed that epithelioid cell type UM is the most malignant and has the greatest risk of metastasis, followed by mixed type UM, and spindle cell type UM is the least malignant. The main immunohistochemical markers of UM include S-100, Melan-A, and Vimentin, etc.
