Figure 5. The fin folds of hmcn2/fbln1 double morphants display compromised cross fiber rearrangements and aberrant detachments of fin mesenchymal protrusions from collagenous actinotrichia.

All panels show transmission electron micrographs of transverse sections through the dorsal median fin at 55 hpf. Positions of sections along the apical-basal (distal-proximal) axis of the fin are indicated in upper right corners. See scale bars for magnifications. (A,C,E,G) wild type; (B,D,F,H) hmcn2/fbln1 double morphant. (A) In wild-type embryo the mesenchymal cell is located in a medial region of the fin and has long projections that align along the actinotrichia (inset with magnification), projecting apically. (B) In double morphant, the space between the two epidermal sheets is very narrow, and the mesenchymal cell body is located in basal regions of the fin. (C) In apical-most regions of the wild-type fin, numerous cross fibers (cf) span the entire dermal space in parallel bundles and at right angles to the actinotrichia (act), connecting to the apposed epithelial sheets. (E) In apical-medial regions, but still apical of the fin mesenchymal cells and their protrusions, cross fibers display a bouquet-like organization in regions close to the cell surfaces and actinotrichia (cf), while in central regions of the dermal space, they are decorated by electron-dense material (cf*). (G) In medial regions, protrusions of fin mesenchymal cells are attached to the inner surface of the actinotrichia, while weaving between the cross fiber bouquets. (D) Medial fin regions of hmcn2/fbln1 double morphants (D) display an organization of cross fibers similar to that of apical-most regions of wild-type fins (compare with C), with cross fibers spanning the entire dermal space. (F) The few protrusion of fin mesenchymal cells that make it into basal-medial positions are not attached to the actinotrichia as in wild type, although actinotrichia of are normal thickness and pattern (compare with E), but found in central regions of the dermal space, surrounded by cross fibers. (G) Aberrant tight attachments between fin mesenchymal cells and cross fibers are also observed in the fin base of hmcn2/fbln1 double morphants.

Abbreviations: act, actinotrichia; cf, cross fiber; cf*, decorated cross fiber; ep, epidermis; fm, fin mesenchymal cell; fmp, fin mesenchymal cell protrusion.