Mammary extracellular matrix (ECM) isolated from tamoxifen-treated rats inhibits tumour cell invasion in 3-dimensional (3D) culture. (a) Whole-mount 3D organoid images of MCF12A-Ras cells plated onto control and tamoxifen derived mammary ECM. Left panel arrow depicts commonly occurring tumour cell filapodia invading control ECM. Right panel shows small compacted 3D organoids that formed on mammary ECM isolated from tamoxifen treated rats. (b) 5 μm H&E-stained sections of organoids on control and tamoxifen ECM 200× magnification. Arrow depicts locally invasive tumour cells present in control ECM cultures. (c) Quantitation of organoid size that developed on control mammary ECM, tamoxifen ECM and Matrigel. Tamoxifen and Matrigel organoid size do not differ, but are different from control ECM organoid size. *p < 0.0001. (d) upper panel: The addition of 20 μg/ml fibronectin (FN) to tamoxifen ECM increased organoid size by about 20%. (d) lower panel: Quantitation of organoid size. *p < 0.002.