Introduction of fibronectin into differentiated cultures reverses growth arrest. A, after 10 d in three-dimensional culture, medium was replaced with medium containing or lacking 50 µg/mL of rat fibronectin. Cultures were fixed and stained with DAPI and anti–Ki-67 antibody 2 or 4 d later. Percentage of Ki-67–positive cells was calculated. Between 150 and 250 cells were examined per condition per experiment. Results are representative of three independent experiments. *, P < 0.05. B, using the data presented in Supplementary Fig. S2, the percentage of acini with more cells than the Matrigel control’s average is graphed for cells with Matrigel alone or Matrigel with fibronectin. *, P < 0.05 by a t test of proportions.