Left ventricular endocardium obtained by transvascular endomyocardial biopsy from nine infants and children with various congestive cardiomyopathies showed thickening in five, with hyperplasia of endocardial cells and increase in elastic and collagen fibers (endocardial fibroelastosis). Based on the morphologic findings, we suggest a chronologic sequence of hyperplasia of smooth muscle (SM) cells followed by transformation and translocation. The sequence appears to be (1) proliferation of dark SM cells with many surface vesicles, many myofilaments, and fusiform densities; (2) possible proliferation of light SM cells containing fewer surface vesicles and fewer myofilaments than the dark SM cells; (3) transformation of SM cells to leiomyoid cells that resemble both SM cells and fibroblasts; and (4) transformation to typical fibroblasts. The cells producing the increased elastin and collagen are believed to be the SM cells and the leiomyoid cells.

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