Light and electron microscopic study of cellular proliferation in carcinoid heart disease.

An ultrastructural and histochemical study of the subendocardial lesion in carcinoid heart disease showed six different cell types within a myxoid matrix. The matrix, composed of a mucopolysaccharidic ground substance, collagen, and reticluin fibers, contained stem cells, four types of fusiform cells (fibrocytes, fibroblasts, myofibroblasts, and smooth muscle cells), and intermediary cell type. Our observations suggest that the humoral mediators of the carcinoid syndrome may induce the differentiation of a subendocardial stem cell into contractile elements.