Fig 1: Ductal-type mouse breast cancer organoid models acquire basal features and align Collagen-I rich environments during collective invasion.A Non-invasive tumor PyMT organoid clones B6 (left panels) or invasive clones H7 (right panels) were placed in BME (top panels) or Collagen-I (bottom panels). Note the overt invasion of H7 in collagen (black arrow heads) in contrast to the non-invasive clone B6. Size bars indicate 50 µm. B Invasion is accompanied by expression of basal cytokeratin (CK) expression. Invasive PyMT H7 organoids show expression of CK14. In contrast, the non-invasive clone B6 does not acquire CK14 expression but instead shows homogenous CK8 expression. White arrow heads depict the basal-type leader cells. Size bars indicate 75 µm. C, D Invasive organoids align the collagen network. Collagen alignment (collagen bundle angle variability perpendicular to the organoid) by collectively invading H7 organoids was visualized using confocal reflection microscopy (insets) (C) and quantified using OrientationJ in (D). Pseudo coloring in the insets indicate orientation angles from -90° to 90°, perfect alignment with the organoids is defined as 1, and the cut-off for non (random)-alignment is 0.2 in (D). Alignment persisted over long distances (>150 µm). In contrast, non-invasive PyMT B6 organoids did not induce collagen matrix alignment. Size bar indicates 10 µm. The Mann–Whitney test was done to assess significance. **p < 0.01, ***p < 0.005, ****p < 0.001. All experiments are biological replicates and were repeated at least three times. Error bars show standard error of the mean.