Ednra1 is required for joint formation in the arches. Whole-mount RNA in situ hybridization (A-I), dorsolateral views (A-C), ventral views (D-F), lateral views (G-I) and Alcian-Blue-stained, flat-mounted cartilage (J-O). (A) Two-color in situ hybridizations show bapx1 in joint precursors dorsal to hand2 in the ventral mandibular NC. (B) bapx1 expression is reduced in Ednra1 morphants. (C) In Ednra1;Ednra2 double morphants, both bapx1 and hand2 expression are abolished. (D) bapx1 bilaterally marks large jaw joint regions (arrow) as well as two smaller, ventral midline domains in the mandibular and hyoid arches (arrowheads). (E) In Ednra1 morphants, the bilateral domains of expression remain, presumably in cells surrounding the joints and a single, large domain persists at the ventral midline (arrow). (F) In Ednra1;Ednra2 double morphants, the two joint domains are severely reduced in addition to the ventral midline domain. (G) sox9a marks prechondrogenic NC cells in D1 (PQ) and V1 (Ms). A small gap in expression marks the joint between PQ and Ms (arrowhead). (H) In Ednra1 morphants, this gap is eliminated (arrow). (I) In Ednra1;Ednra2 double morphants, sox9a is expressed only in the remaining PQ condensation. (J) At 96 hpf, PQ and Ms are separated by a joint region (arrowhead) in the mandibular arch. (K) In the hyoid arch HS and CH are separated by the interhyal cartilage (arrowhead). (L,M) In Ednra1 morphants, these individual elements of both arches fuse (arrows). (N) In Ednra1;Ednra2 double morphants, a small remnant of Ms (asterisk) remains attached to PQ in the mandibular arch. (O) In the hyoid arch, CH is completely lost (asterisk). CH, ceratohyal; HS, hyosymplectic; Ms, Meckel's; PQ, palatoquadrate.
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