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Fig. 6 Patterning of scxa-expressing head tendon progenitors is disrupted in cyp26b1 mutants. (A) At 60 hpf, bright puncta of condensing scxa-positive tenoblasts form in a line between the intermandibularis posterior and interhyal muscles (arrow), and other tenoblasts populate a mass of midline cells from the oral ectoderm to the mandibulohyoid junction. (A’) In the second pharyngeal arch, the ends of the hyohyal (arrowheads) and sternohyoideus (arrows) muscles are labeled by scxa-positive condensations. (B) In cyp26b1 mutants, the pattern of scxa expression suggests less condensation at the mandibulohyoid junction (arrow) and less separation of anterior and posterior tenoblasts. (C) At 4 dpf, the mandibulohyoid (arrow) and hyohyal (arrowhead) junctions express scxa strongly, and most muscle attachment points are scxa-positive. Notably, two spurs extend from the hyohyal junction (C’, outlines) and insert near the anterior edge of each interhyal muscle (E’, outlines). (E,E’) The sternohyoideus tendons insert in the midline, just dorsal to the hyohyal junction structures. (D) Four-day-old mutants display some scxa expression at most muscle attachment points, but expression associated with the intermandibularis muscles (arrow) and the hyohyal junction (arrowhead) is reduced. (D’) The spurs of the hyohyal junction are highly dysmorphic. (F’) The sternohyoideus tendons are rod-like but hypoplastic (arrows). (G,I) Cells expressing xirp2a are present at the tips of all cranial muscles at 60 hpf and 4 dpf. (H,J) Cells at muscle tips are also labeled with xirp2a in cyp26b1 mutants. All images ventral view, anterior to the left. Scale bars = 50 μm