A primary cilium forms once the midbody remnant becomes proximal to the centrosome. (A–C) Videomicroscopic analysis of PC formation. (A) XY projection of GFP-tubulin and dsRed-centrin during PC formation (top). The enlargements show the fluorescent signal in the boxed region for the proteins analyzed (bottom). The blue and white arrowheads indicate a microtubular connection between the midbody remnant and the centrosome and a nascent cilium, respectively. The green and red arrowheads point to the midbody remnant and the centrosome, respectively. The dashed line indicates the cell contour. (B) 3D reconstruction of some of the images shown in A. (C) XY projection of GFP-tubulin and their corresponding differential interference contrast (DIC) images during PC formation (bottom). The enlargement of the boxed region shows the distribution of tubulin at apical planes (middle). Nascent cilia are visualized in the XY projections as dots because they are perpendicular to the substrate. Green arrowheads point to the midbody remnant. The blue and white arrowheads indicate a microtubular extension similar to that observed in A and a nascent cilium, respectively. An XZ image of the cells after 260 min is included to show that the new profile that appears close to the midbody remnant (white arrowhead) is a nascent cilium (top). The dotted line indicates the plane used for the confocal XZ image. Bars, 2 µm.