The midbody positions the apical surface during cyst development. (A) Caco-2 transfected with control or Cdc42 siRNA was fixed and stained for DNA (blue), tubulin (green), and aPKC (red). (top) A control cyst at the two-cell stage (note that abscission appears to have occurred symmetrically). (middle) A larger control cyst, with the midbody in the center of the developing structure (apical region of dividing cell) reflecting asymmetric abscission. (bottom) Cdc42 siRNA structure with one midbody positioned normally at the center and another midbody (located in a different z section) abnormally positioned. (B) Quantitation of midbodies at the center of the cyst from three independent experiments. The total number of midbodies is indicated (N). A midbody is regarded as being in the center if it is located at a distance from the centroid that is less than one third the radius of the structure.