Solitary adult xanthogranuloma: a case report with new observation on lipid droplet formation.

A case of solitary xanthogranuloma was studied by optical and electron microscopy. The tumor parenchyma cells were composed mainly of multilocular lipid droplets together with rough-surfaced endoplasmic reticulum, a few mitochondria, glycogen particles, and fine filaments sparsely scattered throughout the cytoplasm. The parenchyma cells bore a close structural resemblance to Ito's "fat-storing cells" which are situated in Disse's space, contain lipid droplets and are thought to be the only cells participating in intralobular fibrillogenesis in the liver. Another notable finding was that the lipid droplets of the tumor parenchyma cells originated directly from rough-surfaced endoplasmic reticulum. Thus, the tumor morphology was incompatible with the currently favored concept of a benign, reactive non-neoplastic process of prominently histiocyte cells, being characterized instead by prominently fibroblastic cells.