Par1b promotes mitotic spindles that are oriented toward the lateral lumen and not aligned with the basal surface in MDCK cells, a characteristic of hepatocytic cells. (A) MDCK cells overexpressing Par1b under a doxycycline (dox)-dependent promoter (Par1b-MDCK-Tet-Off) were fixed and stained for the apical protein gp135, microtubules (α-tubulin), and chromatin with DAPI. The β angle was quantified (right panels). MDCK cells had significantly more cells with β angles in the 0–10° than in the 10–20° category, whereas the opposite was the case for MDCK-Par1b cells. (B) MDCK-Par1b, WIF-B9, and HepG2 cells were fixed and stained for α-tubulin, chromatin, and the apical domain (ezrin). The γ angle was quantified in cells with one spindle pole oriented toward the lateral lumen (right panels). In all three cell lines the percentage of cells with γ angles in the 0–30° category was significantly different from that of cells in the 30–60° category, indicating that the orientation was not random. Red arrowheads, BC-like lumina; black arrowheads, predicted cleave furrow plane. Error bars indicate ± SEM (dot graphics) and ± SD (bar graphics). *, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001.