Changes in endocardial cell morphology during development of the endocardial cushions

This paper presents a scanning electron microscope study of the morphologic changes undergone by the endocardium during development of the atrioventricular (A-V) endocardial cushions. Prior to cell seeding into the cushions, endocardial cells show a regular, uniform morphology, many of them appearing to be oriented in the direction of the blood flow. Concomitant with the appearance of cells in the A-V cushions, endocardial cells lose their elongated appearance, become more flattened and adopt a variable morphology. Endocardial cells at this stage develop filopodia and lamellipodia, overlap each other and show a variable number of microvilli and different degrees of flattening. After completion of endocardial migration, the endocardial cells that remain in the plane of the endocardium become extremely flattened, the degree of overlapping decreases and the cells adopt a polygonal morphology. These changes in endocardial cell morphology appear to be related to endocardial cell activity. It is suggested that the whole endocardial cushion, not just the cells that migrate into the cushions, is involved in cushion development. The activity of the endocardial cells may be related to the maintenance of the integrity of the endocardium.