The cell-to-cell channel.

Cells of most organized tissues are interconnected at their junctions by aqueous membrane channels, shown in one instance to be at least 14--16 A in diameter. During junction formation, nascent channels are detected as quantal steps in cell-to-cell conductance, growing progressively in number. The channels detected are stable. Elevation of cytoplasmic free Ca2+ concentration can cause graded closure of channels.