Cell junctions and their role in transmural diffusion in the embryonic chick heart

Summary Studies of cardiogenesis in the chick embryo focus attention upon the intercellular junctions of epicardial, myocardial, and endocardial cells, and the role they play in diffusion across the cardiac wall. Cell membranes of apposed epicardial cells approach as close together as 40 Å; those of the endocardium additionally form focal tight junctions. In the myocardium focal tight junctions are restricted to the apposed membranes of the superficial layer of cells. The majority of close appositions in all parts of the myocardium are 40 Å gap junctions. Desmosomes and fascia adherens are distributed throughout the myocardium.Diffusion of horseradish peroxidase through the epicardium and endocardium occurs primarily through the intercellular junctions. The width of the cleft between cells, 200–300 Å, also permits the diffusion between cells of the larger ferritin particles. Pinocytotic activity, responsible for ferritin transfer across mesothelial and endothelial cells in the adult, is not significant.Tracers injected into the pericardial cavity or vasculature can be observed passing through the heart in the direction of their respective diffusion gradients. Unlike the apical junctions of epithelial cells, to which they have been compared, membrane specializations of the superficial myocytes do not form a seal separating the pericardial cavity, or subepicardial space, from the extracellular spaces of the myocardium.Supported by the Medical Research Council of Canada.The author wishes to express his gratitude to Mrs. J. Blackbourn for her excellent technical assistance.