Microphotograph of MCF10A mammary epithelial cell’s acinar structure cultured under magnetic levitation for 48 hours. Acinar structures are 3D multicellular organizations in which epithelial cells are known to physiologically replicate under in vivo conditions. Usually in vivo, full functional differentiation occurs during pregnancy and leads to the formation of acinar structures at the end of the ducts, which produces the milk (Neville et al., (2002)). To achieve and maintain this remarkable level of tissue organization, mammary epithelial cells (MECs) and their surrounding extracellular matrix (ECM) must integrate their structure in a highly coordinated fashion. The above image shows mammary epithelial acinar structures where the green identifies the nuclei of the cells stained with 0.001% proflavine (green nuclear stain). Cells show to be polarized, which is indicated by the position of nuclei positioned towards the outer side of the acinar structure.