Intracytoplasmic eosinophilic inclusion bodies in breast cyst fluids are giant lysosomes.

Electron microscopic (EM) studies were performed on a benign breast cyst fluid to determine the nature of intracytoplasmic eosinophilic inclusion bodies (EIBs). EIBs were usually found in macrophages and had the ultrastructural appearance of giant lysosomes. EIBs contained cellular material and granular debris; when viewed by EM, the contents were somewhat variable, depending on the nature of phagocytosed material and the extent of enzymatic breakdown. Usually the phagocytosed material had a rather homogeneous, finely granular texture. Occasionally cell remnants, including cytoplasmic organelles, were preserved inside the EIBs. Since the "host" macrophages were in various states of degeneration, the formation of EIBs appears to be related to a severe disturbance in the macrophage cellular function. This study showed that EIBs in cells from breast cyst fluids are not viral particles.