These results show that monolayer cultured MECs do not cease proliferating because they become confluent, but rather they enter an apparently irreversible quiescence. In contrast to cell lines, this quiescence cannot be rescued by trypsinising and replating the cells and appears to be irreversible under 2D growth conditions. Moreover, the cells do not undergo senescence, as BEZ235 supply judged by b-gal staining. 3D ECMs such as a BM-matrix have been used extensively to the study cell behaviour because they bestow an environment more similar to that found in vivo than planar dishes. Consequently, we explored whether 3D culture might provide an opportunity to maintain or extend the proliferation potential of primary cultures. P16�C18 MECs form spherical acini when they are cultured in 3D BM-matrix. The proliferation rate of primary cells from late pregnancy in these 3D structures over the course of 7 days had a similar profile to that cultured in 2D, with an initial burst of cells in S-phase at day 2, which steadily decreased. Notably, the behaviour of primary MECs in 3D culture is different to non-malignant human MEC lines such as MCF-10A, which proliferate steadily over a period of 7�C10 days before exiting cell cycle. The culture of primary MECs in 3D BM-matrix mimics some of the conditions the cells are exposed to in vivo, with the presence of basement membrane proteins and a 3D structure. We hypothesised that, despite this loss in proliferation whilst culturing in 3D, the intrinsic potential to undergo cell cycle may not be lost in those conditions. We therefore tested whether the proliferation potential of primary MECs could be maintained in 3D culture over a period of several days, such that when acini were Tubulin Acetylation Inducer HDAC inhibitor isolated and replated onto 2D ECM, the cells efficiently enter S-phase again. Mammary acini were isolated from the 3D BM-matrix in sterile PBS containing 5 mM EDTA, which retained the acinar structure of the MECs but removed the BM-matrix, and then transferred to pre-coated collagen I culture dishes. The cells migrated out of the 3D structure, proliferated and formed a monolayer on the 2D collagen. When MECs were cultured as 3D acini for 2 days and then replated into 2D, the number of cells in S-phase peaked 2 days later, similar to primary MECs. Interestingly, these cells maintained a high level of cell cycle for a longer time period than cells plated onto a 2D substratum directly after tissue isolation. The duration of 3D culture before replating the cells did not affect the ability to MECs to proliferate when entering 2D culture. For example, even after culture in 3D for 7-days when proliferation was reduced to 2%, the cells showed a significant and dramatic cell cycle burst.