hmcn2/fbln1 double morphants display strongly compromised epidermal–dermal junction formation in the trunk, but not in the fin fold. (A, B) Transverse sections through trunk at 28 hpf, after anti-Laminin immunolabeling of BMs in green, Phalloidin-labeling of somitic muscle in red, and DAPI labeling of nuclei in blue. The double morphant (B) displays Laminin labeling at both sides of the blister, indicating that both epidermis and somites are properly attached to their respective basement membranes, and that blistering occurs within the dermal space in between. (C–K) Transmission electron micrographs of transverse sections through trunk (C–I) or dorsal tail fin (J, K) at stages indicated in upper right corners (for magnification, see scale bars). (C) Double morphant at low magnification, with most of trunk blister filled with amorphic material (indicated by arrows) at 55 hpf. (D, E) At 30 hpf, wild-type embryo (D) displays the first collagen fibers (arrow) underneath the yet indistinct BM, whereas much fewer fibers are present in the double morphant, with a fluid-filled blister underneath (E). Thinner and short fibers possibly integrated into and running perpendicular to the BM are visible in the double morphant (E; arrow), but more difficult to see in the wild-type (D, F), most likely due to the more compact organization of the tissue. It is tempting to speculate that they are the equivalent of the cross fibers of the fin fold (see Fig. 5), and involved in the organization of the ECM across the dermal space. (F, G) At 55 hpf, the collagenous network underneath the BM of the wild-type embryo consists of approximately ten layers organized in a plywood-like fashion (F; arrow), whereas the double morphant embryo displays a rupture within the second and third collagen layer (G; arrow). The shown specimen is one of the rare cases in which the blister seems to remain fluid-filled (see below). Arrowheads point to BMs. (H, I) In regions in which the architecture of the wild-type embryo requires a wider dermal space, such as at the base of the fins, it is filled with amorphic material underneath the sublaminal collagen fibers (H). In double morphants (I), most trunk blisters are filled with similar amorphic material, intermingled by disorganized and not properly anchored collagen fibers (arrows). (J, K) In the fin fold of wild-type embryos (J), collagenous actinotrichia are attached to the BM (indicated by arrowhead). In double morphant (K), actinotrichia and BM (indicated by arrowhead) appear unaffected, with rare and locally restricted detachments (indicated by arrow) in medio-basal regions of the fin. Abbreviations: act, actinotrichia; cf, cross fibers; dMO, double morphant; ep, epidermal cell; m, melanocyte.