Spatial organization of MCF-7 and MCF-7/Doxo in co-cultures. To obtain phase contrast micrographs of growing MCF-7 variants in co-cultures, dishes were seeded with a 50:50 mixture of MCF-7:MCF-7/Doxo at day 0. Morphological differences permit an immediate identification of each cell subpopulation. MCF-7 appeared birefringent and round (boxes) whereas MCF-7/Doxo are more flat and spread (ellipses). Note that the cells remained organized in well-delimited islets.