Fig. 4.

Shh signaling correlates with branchial ray outgrowth in a shark, S. canicula, and a holocephalan, C. milii. (A) At st. 29 of S. canicula development, Shh is expressed in the hyoid arch and gill arches, and this expression pattern persists beyond (B) st. 31. Sections reveal that in S. canicula, Shh expression in the (C) hyoid and (D) gill arches is restricted to the distal pharyngeal epithelium, whereas Ptc2 expression in the (E) hyoid and (F) gill arches is restricted to pharyngeal mesenchyme. (G) At st. 29 of C. milii development, Shh is expressed in the hyoid arch and gill arches. However, by (H) st. 31 of C. milii development, Shh expression persists only in the developing hyoid arch operculum. Sections reveal that at st. 29 of C. milii development, Shh expression in the (I) hyoid and (J) gill arches is restricted to the distal pharyngeal arch epithelium, whereas expression of Ptc2 in the (K) hyoid and (L) gill arches is restricted to the pharyngeal mesenchyme. By st. 31, (M) Shh expression is maintained in the distal epithelium of the hyoid arch operculum, but is not detected in the (N) gill arch epithelium, and (O) Ptc2 expression is maintained in the mesenchyme of the hyoid arch but is not detected in the (P) gill arches. Sustained Shh signaling in the hyoid arch and gill arches of S. canicula correlates with branchial ray outgrowth from the hyoid arch and gill arches 1–4. Similarly, sustained Shh expression in the hyoid arch only of C. milii correlates with sustained branchial ray outgrowth exclusively within the operculum. Note the expression of Shh in the zone of polarizing activity (z) of the pectoral fin in (A) S. canicula and (G) C. milii. [Scale bars, 250 μm (A–H) and 100 μm (I–P).]