Transmission electron microscopic images of MDA -MB- 231 cells (A- D) and a 4 :1 mix of MDA -MB- 231 and MDCK (E- H) after culturing in 3D for 8 days. (A-D) MDA-MB-231 cells cultured alone show microvilli-like structures on the cell surface. The cells are attached to others only loosely (B, C). The cytoplasm is full of ribosomes, however, organelles such as Golgi apparatus and mitochondria are rarely seen. Scale bars: A, 10 μm; B, 2 μm; C, 2 μm; D, 0.5 μm. (E-H) MDA-MB-231 co-cultured with MDCK at a ratio of 2:1. The MDCK cells exhibit acinus structures, with numerous microvilli on their apical surface (triangles in E and H) and the nuclei located on their basal side. The MDCK cells show higher electron density compared to the MDA-MB-231 cells (asterisks in E, H). F is a magnified view of the area shown by one of the rectangles in E, where MDCK (upper left) and MDA-MB-231 (lower right) are closely attached. Many mitochondria, ER and Golgi apparatus are seen in the MDA-MB-231 cell. G shows another area in the same MDA-MB-231 cell, showing multi-vesicular bodies. H shows another example of the attachment of MDA-MB-231 cells (low electron density, the middle one marked with asterisk) to MDCK cells (high electron density, the right-hand-side). While the MDCK cells form an acinus-like structure with microvilli on the apical side (triangle), the basal side is deformed, with sharp processes attached to the MDA-MB-231 cells (arrows). Scale bars: E, 10 μm; F, 0.5 μm; G, 1 μm; H, 5 μm.