General topography and dynamics of AJs in A431 cells. (a) Time-lapse images of EcGFP-expressing A431 cells acquired at 30-s intervals. Only the first (0 min) and final (60 min) frames of the 1-h sequence are shown. Bar, 20 μm. The zoomed cell–cell contact (indicated by dashed white box) from the same two frames is shown on the right (see Video 1). The apical (apl) and basal (bsl) edges of the contact are indicated. Bar, 7 μm. The bottom row shows a 7-min subset of Video 1 illustrating a fusion (white arrows) and a fission (red arrows) events occurring within the region marked by the blue dashed box (times in minutes are indicated). The rightmost image overlays the initial (green) and final (red) frames of this subsequence. Bar, 4 μm. (b) Tracked trajectories of major AJs within the region indicated by the white dashed box in a. The color code of trajectories corresponding to time is shown on the left. (c) Schematic representation of the net displacements of major AJs derived from the trajectories shown in b. Note that the lateral membrane is clearly partitioned into three zones containing apical AJs (gray), lateral AJs (red), and basal AJs (green). The lateral AJs exhibit nondirectional motion, while basal AJs move upward; two of them ultimately merge with apical AJs. The positions of apical AJs are shown approximately.