Thick (myosin) filaments in a glomus tumor

An otherwise classic digital glomus tumor is presented with the unusual ultrastructural finding of cytoplasmic thick (myosin) filaments together with thin (actin) filaments in many of the cells. In places, sarcomere-like orientation was seen. It is little appreciated among diagnostic pathologists, but is well-established, that thick (myosin) filaments occur in smooth muscle type cells. They are present in vivo and can be demonstrated ultrastructurally if rather stringent preparative conditions are met. Whether or not the contractile process in smooth muscle is analogous to skeletal muscle is a debated issue. In the context of diagnostic electron microscopy, it is stressed that thick filaments are not, as often stated, pathognomonic of skeletal muscle neoplasms, and may potentially be found in smooth muscle neoplasms and neoplasms of related cell type (glomus tumors, hemangiopericytomas, tumors of myofibroblasts, etc.).