Fig. 4. Retinoic-acid-induced lumen morphogenesis in a collagen gel sandwich assay. (A,B) Phase contrast micrographs (asterisks indicate cell-free gel areas). (C,D) Semi-thin (1 µm thick) sections perpendicular to the culture plane. When sandwiched between two collagen layers (see Materials and Methods) for 48 hours, untreated J3B1A cells form flattened cell cords containing only a few small lumina (A,C). By contrast, in the presence of 1 µM RA (B,D), J3B1A cells form large lumina (arrows in B). (A,B) Bar, 200 µm. (C,D) Bar, 100 µm.