Histologically, in the absence of extraocular extension, the major determinants of prognosis are cell type, tumor size, and location. Cell type is generally categorized under the Callender classification system. Often a longitudinal fold of the nuclear membrane accounts for a prominent streak of chromatin. Mitotic figures are common. They exhibit variations in size and shape of their nuclei and may be multinucleated.
Chromatin shows coarse clumping.
Mitotic figures are abundant. Cells show loss of intercellular cohesiveness. Lesions composed of exclusively spindle-A cells are now regarded to behave almost like nevi. Spindle-B tumors may also form a fascicular pattern with palisading of the cells. A final classification is that of a necrotic tumor in which necrosis precludes identification of a cell type.