42 hpf Development
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By 42 hpf, pharyngeal arches 1 and 2 (pa1, pa2) have converged onto each other along the anterior-posterior axis and extended along the dorsal-ventral axis. Pharyngeal pouch 1, which is GFP-negative in fli1a:EGFP transgenic fish, appears to be regressing at this timepoint, although the neural crest cells maintain a tightly ordered arrangement in the dorsal region between pharyngeal arches 1 and 2. Pharyngeal arch 1 has increased in mediolateral thickness, and the oral ectoderm (oe), which is GFP-negative in fli1a:EGFP transgenic fish, is more constricted medially, from that seen at 36 hpf. The ventral region of pharyngeal arch 1 has further expanded medially, and the ventral region of pharyngeal arch 2 has begun to expand medially, compared to that seen at 36 hpf.