Observation of the abluminal surface of the atrioventricular endothelium which undergoes transition into cardiac cushion mesenchymal cells in the chick embryo.

In the developing chick heart, endothelial cells in the atrioventricular canal (AV) undergo a series of morphological changes and transform into cushion mesenchymal cells. In the present scanning electron microscopic study, we examined the abluminal surface features of the AV endothelium through an artificial window in the myocardial wall. The AV endothelial cell at stages 12 or earlier had a smooth, flattened basal surface with only a few blebs. In the successive stages, the abluminal surface exhibited remarkable changes; 1) the number of blebs increased, 2) elongated microvillous projections emerged, and 3) a thick filopodium, or a migratory appendage developed. It appeared, however, that these changes do not occur synchronously within the entire AV endothelium but were initially observed mostly in the proximity of the endothelial "crease" which was a limited invagination of the endothelial sheet towards the underlying acellular matrix. In addition, even in the proximity of the crease, endothelial cells with flattened basal surfaces were also observed next to endothelial cells that showed apparent morphological indications of transition into mesenchymal cells. These findings suggest that AV endothelial cells are possibly heterogeneous in the competency of transformation into mesenchymal cells and such heterogeneity would be important for maintaining the continuity of the AV endothelium.