Ultrastructural comparison of spindle A, spindle B, and epithelioid-type cells in uveal malignant melanoma.

Eight cases of uveal malignant melanoma were studied with the electron microscope with the purpose of comparing the ultrastructural features of the three basic cell types (spindle A, spindle B, and epithelioid) originally described by Callender under the light microscope. Five of the eight tumors were of pure cell type, and three were mixed. This was undertaken because of earlier diverse opinions regarding the ultrastructure of those cell types based mainly on studies of spindle B, epithelioid and mixed-type melanomas. In the five melanomas of pure cell type (one spindle A, two spindle B, and two epithelioid), the following tendencies characterizing each cell type were found: The size and the degree of reticulation of nucleoli and the number of free ribosomes and mitochondria increased from spindle A to spindle B and epithelioid cells, while rough-surfaced endoplasmic reticulum was most prominent in the spindle B type. On the other hand, cytoplasmic filaments decreased in the same order. There were also other minor differences. The ultrastructural details of these are described and their significance is discussed. Judging from the behavior of nucleoli and free ribosomes, the results were consistent with the present consensus that the cellular malignancy increases from spindle A through spindle B to epithelioid-type cells. In mixed-type melanomas, the celhdar features were often more complicated because of structural variations and the presence of cells which could be regarded as intermediates.