Left atrial myxoma. An ultrastructural study.

Electron microscopic observations of 4 excised cardiac myxomas were made. All were present in the left atrium and all but one had a stalk. The hearts were otherwise normal. Several types of mesenchymal cells in varying stages of differentiation were identified. Cells having some features of smooth muscle, fibroblasts, and endothelium were observed. Some cells having features of both smooth muscle and fibroblasts namely, myofibroblasts were also encountered. Macrophages, lymphocytes and plasma cells were also seen in small numbers. Hematopoietic foci and calcification were not seen in any of our cases. Ultrastructurally, fine electron dense granules and only stray collagen fibres were noted in the matrix. Myxomas possibly represent either hyperplasia or a true neoplasm of the subendocardial multipotential cells that are capable of differentiating into various types of mesenchymal elements.