Ultrastructural study of nuclear inclusions in spindle and epithelioid cell nevus cells

The nuclei of spindle and epithelioid cell nevus (SECN) cells often contain inclusions. To determine the origin and pathogenesis of the nuclear inclusions, 12 cases of SECN were electron microscopically examined. True inclusions, pseudoinclusions, and electron-lucent areas were frequently found in the nuclei of SECN cells in all cases. The true inclusions were composed of various structures including vesicles and electron-dense homogenous substance. The pseudoinclusions were a cytoplasmic structure engulfed by nucleus; they were surrounded by a nuclear membrane and rich in cytoplasmic vesicles and electron-dense materials. Occasionally, a partial dissolution of the covering membrane of the pseudoinclusions was seen, leaving their contents in the nuclear matrix. It was concluded that a sequential process from the pseudoinclusions to the true inclusions in nuclei was morphologically present in SECN cells.