Multicentric Reticulohistiocytosis

Electron microscopic examination of multicentric reticulohistiocytosis (MR) cells is reported. Most of the cells contained numerous dense bodies which seemed to be lysosomes. Complex interdigitations of the membranes of the adjacent MR cells were distinctive findings. Some of the cells contained myelinated dense bodies in the cytoplasm. Histochemically, the cells were strongly positive for acid phosphatase, alpha‐naphthyl butyrate esterase and lysozyme. From these observations, the majority of MR cells appeared to be proliferations of monocyte/macrophage derived cells. In the synovium, these cells resembled type A synovial cells. Other smaller and fewer cells had many rough endoplasmic reticulum (RER), and these resembled type B synovial cells. In the synovium, MR cells might be formed by proliferation and fusion of synovial cells.