Synaptopodin retrograde flow and the evolution of apical stress fibers during junction development. (A) Frames from time-lapse of synaptopodin and ZO-1 in developing junctions. Retrograde synaptopodin flow originates from the apical junction. Rectangles track a row of synaptopodin densities as they flow inwardly away from the apical junction. Circles track single synaptopodin densities flowing from the junction inwardly from the apical junction into the medial-apical region. The graph shows the rate of synaptopodin flow by tracking individual synaptopodin densities. Scale bar is 2 μm. (B) Frames from time-lapse of synaptopodin and ZO-1 of maturing junctions. ZO-1 densities, marked by arrowheads, are temporarily clustered when synaptopodin stress fibers contract (circled). Scale bar is 1 μm. (C) Frames from time-lapse of synaptopodin and ZO-1 at a late stage of junction maturation. Synaptopodin puncta flow toward the junctional vertex. Circles track synaptopodin densities flowing into the junction vertex. Squares mark the vertex. Scale bar is 1 μm. (D) Frames from time-lapse of synaptopodin and ZO-1 at a late stage of junction maturation. Synaptopodin puncta flow correlates with the movement of junctional vertex against the direction of synaptopodin flow. Arrowheads track synaptopodin densities flowing into the junction vertex. Blue circles track the movement of the vertex. Red and white circles track ZO-1 densities moving towards the vertex. Scale bar is 500 nm.