Fig. 1. Retinoids substitute for serum in inducing lumen formation by mammary epithelial cells. (A) J3B1A cells suspended in a collagen gel at a concentration of 3x10⁴ cells/ml and grown in serum-free, chemically defined medium for 8 days form small solid colonies devoid of a discernible lumen. (B) J3B1A cells grown for 8 days in defined medium supplemented with 0.1% DCS form cystic structures containing a central cavity. (C) Addition of 1 nM RA to the defined medium mimics the lumen-inducing activity of DCS, resulting in the development of small, irregularly shaped cystic structures. (D) At higher (1 µM) concentration, RA elicits the formation of large spheroidal cysts. (E,F) J3B1A cells were grown for 7 days in defined medium to allow the formation of compact multicellular colonies and subsequently incubated in the presence or absence of 1 µM RA for a further 3 days. Whereas control colonies remain solid (E), RA treatment induces lumen formation (F). (G,H) Effect of RA on preclustered cells. J3B1A cells were grown in suspension on agarose for 3 days to obtain discrete cell aggregates, which were subsequently embedded in collagen gels. After 48 hours of incubation in the absence of RA, the aggregates extend branching cords in the collagen matrix but do not form cystic structures (G). Addition of RA (100 nM) at the time of embedding reduces the extent of branching and induces cavitation of the aggregates (H). Bar, 100 µm.