An unusual form of endoplasmic reticulum in mononuclear cells of a giant cell tumor of bone.

A peculiar configuration of endoplasmic reticulum (ER) was observed by electron microscopy in some mononuclear cells from a malignant giant cell tumor of bone that had been removed from a cervical vertebral body in a 54-year-old female. The unusual ER consisted of looped, smooth tubules, which originated from poorly developed rough ER present in the tumor cells. The tubular structures were approximately 23 nm in diameter and contained paracrystalline material. The curvilinear tubular ER closely resembles the vermiform tubular structures that have been observed mainly in histiocytes from different clinical forms of histiocytosis X.