Fig. 8. Medium conditioned by embryonic fibroblasts (CEF) induced the formation of heterogeneous tight junctions. RPE was isolated from E14 embryos and cultured with CEF medium in the basal chamber and either SF3 (A) or rcSF3 (B) in the apical chamber. (A) CEF medium alone induced the formation of loosely organized junctions that could be very deep with many strands parallel to the plane of the monolayer. Discontinuities (arrowheads) were evident. (B) When rcSF3 was added to the apical medium chamber, strand number increased as the junctions became deeper and more complex. Discontinuities were evident in the basal portions of the junctions. A similar distribution and density of gap junctions and microvilli were observed in E14 RPE in vivo (Fig. 1F). g, gap junctions; m, microvilli. Bar, 0.5 µm.