Joint defects are apparent in con/disp1 mutants. Ventral views (A-D) of wild-type (A,B) or con/disp1 mutants (C,D) labeled with RNA probe for bapx1 at 63 hpf (A,C) or gdf5 at 74 hpf (B,D). Alcian blue-stained cartilage at 96 hpf in wild-type (E-G) or con/disp1 mutant (H-K) embryos. (A,C) bapx1 prefigures bilateral and midline joints in wild-type embryos, but expression is reduced bilaterally and absent in the midline of con/disp1 mutants. Different color arrowheads used to denote individual cranial joints in A,C and E-G. Asterisks of the same color designate a reduced joint in con/disp1 mutants in H-K. (B,D) gdf5 is strongly expressed bilaterally in the first arch joint region (arrows) and in a group of cells in the midline that prefigures the basihyal cartilage element (arrow) (B). Expression of gdf5 is significantly reduced in both bilateral and midline domains in con/disp1 mutants (D). (E-G) In wild-type embryos, bilateral joints form between the Mc and pq (E, yellow arrowhead) and hs and ch (G, green arrowhead), as well as at the midline between the bilaterally formed Mc (F, red arrowhead). (H-K) In con/disp1 mutants, a poorly formed joint between the Mc, which fails to extend into a RAP element, and a severely reduced pq is visualized (H, yellow asterisk). In the second arch, we see either ectopic cartilage cells at the joint site between the hs and ch elements (J, green asterisk) or a failure of the hs and ch to join (K, green asterisk). Both phenotypes are occasionally present in the same larvae (left designates left side, right designates right side of single larvae). Further, the bilateral mc elements fuse at the midline to disrupt the joint (I, red asterisk).
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