Fig. 1. Freeze-fracture replicas show the assembly of tight junctional strands during normal development. Sheets of RPE and choroid were isolated from embryos of the indicated age. Microvilli (m) at the top of each panel indicate the apical end of the lateral membrane. (A-C) For E7, no tight junctional strands are evident in (A), but gap junctions (g) are evident. Single strands (short arrows), associated with gap junctions at their tips, are evident in (B) and a patch of strands with open-ended strands (long arrows) is evident in (C). (D,E) For E10, two images show a loose network of strands. A group of strands is separated from the rest of the junction by discontinuities (arrowheads), and the overlying junction consists of only two strands (D). A discontinuity is embedded in a more continuous network (E). (F) For E14, all replicas exhibit a continuous network of anastomosing strands. Gap junctions are bounded by a strand on at least one side. EF, E-face; PF, P-face; Bar, 0.25 µm.