Fig. 3. Structural organization of RA-induced cysts. (A) Semi-thin sections of a 12-day-old collagen gel culture incubated with solvent (DMSO) alone show that the colonies of J3B1A cells consist of compact cell aggregates. (B) Semi-thin sections of cultures treated with 10 nM RA show cystic structures enclosing a wide lumen. (C) Thin sections of the wall of a cyst. The cells have a clearly defined polarity, with a microvilli-bearing surface facing the lumen (lu) and a basal surface in contact with the collagen matrix (col). (D) Detail of an apically located junctional complex (arrows). (A,B) Bar, 50 µm. (C) Bar, 2 µm. (D) Bar, 0.5 µm.