[Ultrastructural changes in atrial myxoma].

Ultrastructural changes of 20 cases of atrial myxoma were studied. Under the electron microscope, in a large amount of amorphous matrix, the myxomal cells were either scattered or aggregated. There were numerous microvilli-like cytoplasmic processes on the surface of the tumor cells. In many aspects, the ultrastructural features of the tumor cells were similar to those of the smooth muscle cells. The abundant cytoplasmic filaments were one of the most prominent ultrastructural features. Sometimes, some of them could form dense bodies. There were cytoplasmic filaments in the cytoplasmic processes of a certain tumor cells and lots of micropinocytic vesicles beneath the inner surface of the plasmic membrane. Nuclear membrane of the tumor cells often had marked indentation which was probably related to contraction of the cytoplasmic filaments. 2 of 20 lesions showed malignant characteristics. One lesion infiltrated into the atrial myocardium, and the other involved the left and right atria. It is suggested that the atrial myxoma be a true neoplasm of the atrium with potential malignant tendency. It originates from the multipotential mesenchymal tissue and differentiates mainly towards smooth muscle cells.