Cellular junctions in the tunicate heart.

The cell membranes of the tunicate heart exhibit an abundance of macular close junctions (also referred to as gap junctions or nexuses), distributed more or less randomly over the intercellular surfaces, and a system of zonal tight junctions limited to the cell borders adjacent to the epicardial surface. Three-dimensional images of these structures are displayed by the freeze-etch technique. In sectioned tissue, lanthanum appears to be excluded from the tight junctions, but is present in the close junctions. The presence of a primitive intercalated disk is suggested.