Fig. 3. Comparison of Rtnl1-GFP, PDI-GFP and Sec61-GFP in various embryonic tissues during development. (A-M') The labeling of Rtnl1, PDI and Sec61 by exon-trapping with GFP shows the endogenous expression patterns of all three proteins. All highlight similar ER structures in the early syncytial embryo before cellularization (A-C) and later within the embryonic epidermis (K-M, shown is the epidermis-amnioserosa interface), although levels differ in stage 14 embryos. Note the striking differences in expression within the CNS (D-F) where Rtnl1-GFP (D) is strongly concentrated in the axonal tracts but both PDI-GFP (E) and Sec61-GFP (F) are completely absent. Rtnl1-GFP is enriched in the SR (a specialization of smooth ER) of embryonic muscles (G), whereas PDI-GFP and Sec61-GFP strongly mark the perinuclear rough ER in muscles (H and I, respectively). G'-I' show higher magnifications of the areas indicated by red boxes in G-I. Arrows in G' and H' point to the SR, arrowheads in H' and I' point to the nuclear envelope. Note that the bright staining in G'-I' is the labeling within the embryonic epidermis and not the muscle. Bars, 10 µm.