Electron microscopic demonstration of intercellular junctions between subendocardial smooth muscle and myocardium in...

The eustachian ridge of 11 sheep hearts was examined by light and electron microscopy to elucidate the morphologic relationship between cardiac smooth muscle cells and myocardial cells. Smooth muscle cells were commonly observed in the subendocardial connective tissue and they were usually arranged in strands. When mixed with myocardial cells at the margin of these strands, smooth muscle cells exhibited numerous direct connections with the myocardial cells. These intercellular junctions were composed primarily of simple appositions (undifferentiated regions), but occasional desmosomes and fasciae adherents (intermediate junctions) were seen. We found no nexus (gap junctions) at these sites. The existence of direct intercellular junctions between smooth muscle and myocardium suggests several possible electrical and mechanical interactions.