Hypural fusions occur during caudal fin development. (A-D) Maximum intensity projection confocal images of Col2-immunostained whole-mount wild-type sibling (A,B) and double mutant (C,D) median fin folds (FFs). Actinotrichia (white dashed lines) proximodistally span the wild-type FFs. Col2 is aggregated at the base of double mutant FFs (white asterisks). Hypural elements are indicated and outlined (white dotted lines) in insets. Ph, parhypural; H, hypural. Scale bars: 200 µm. (E-H) Transmitted light differential interference (TD) images of A-D. Black dotted lines outline fin margins. Melanocytes (orange arrowheads) are deposited around the notochord (E,G) and migrate distally as the rays develop (F,H). Actinotrichia (dashed white lines) are seen in wild-type FFs (E,F). (I) Maximum intensity projection of B, showing only Col2 signal (also shown in Fig S8C WT1). (I′,I″) Magnified slices from a z-stack showing the region outlined in I at depths above (I′) and below (I″) the hypurals. Hypural 1 (H1) is labelled for reference. White dashed arrows indicate the actinotrichia. Hypurals reside among actinotrichia and seem to grow in the same plane (I′,I″). Scale bars: 200 µm. (J) Maximum intensity projection of And1-immunostained whole-mount 5.2 mm standard length (SL) wild-type sibling median FF (n=4 fish). (J′,J″) Magnified slices from a z-stack showing the region outlined in J at depths above (J′) and below (J″) the hypurals. Hypural 1 (H1) is labelled for reference. White dashed arrows indicate the actinotrichia. Hypurals are developing between two arrays of actinotrichia situated above and below the cartilages. n=4 fish in A,C,E,G,J; n=9 fish in B,F,I; n=7 fish in D,H. Scale bars: 200 µm.
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