A myxoma is a benign tumor &127;thought to be of hamartomatous origin. It is composed of primitive connective tissue (mesenchymal cells) seen at arrow 1, endothelial cells at arrow 2, smooth muscle cells (not obvious here), and macrophages at arrow 3. These cells are surrounded by a large amount of mucopolysaccharide extracellular matrix seen as the large open spaces at arrow 4.