Intranuclear rod-shaped actin filament bundles in poorly differentiated axillary adenosquamous cell carcinoma

An axillary cystic poorly differentiated adenosquamous cell carcinoma was studied by electron microscopy and immunohistochemistry for keratin, vimentin, desmin, and actin. Many tumor cells had rod-shaped intranuclear fibrillary inclusions which consisted of 60 A microfilaments. These intranuclear inclusions reacted with antiactin antibody and were considered to be F-actin bundles. The cells having intranuclear inclusions were frequently found in areas close to the cystic space, and inclusions of tumor cells close to the cyst were thicker. However, mitotic figures were predominantly found in the periphery of the tumor. Intranuclear actin is known to be mainly soluble G-actin, and one of its important functions is reported to be the chromosome condensation in mitosis. It is speculated that the cells having intranuclear fibrillary inclusions of actin have impairment of mitosis.