Fig. 1

Pectoral fin development in Scyliorhinus canicula. Developmental stages (St.) at the top. a Scan electron microscopy (SEM) showing initial outgrowth of the lateral plate mesoderm (arrowheads), in the ventrolateral region between somites 6 and 16 (S6; S16), which will give rise to the pectoral fins. b-i SEM in upper panels (anterior to top) and Mallory’s trichrome stains of histological transversal sections in lower panels (dorsal to top). Dashed lines in upper panels indicate approximate plane of section shown in lower panels. b Initial pectoral fin buds (Pec) prior to the formation of the apical ectodermal ridge (AER). c Formation of the AER in the most distal ectoderm. d-e Formation of the apical ectodermal fold (AEF) by convergence of dorsal and ventral ectodermal cells at the distal fin tip. Note dermomyotome projections (Myo) starting to enter the pectoral fin buds. f Expansion of the abductor superficialis (Abd) and adductor superficialis (Add) muscles, ventrally and dorsally, respectively. g Pectoral fin with two identifiable domains: a proximal domain in which the endoskeleton elements differentiate (End) and a distal finfold (Ff), filled with mesenchymal cells and still capped with an AEF at this stage. Note: first chondrogenic condensations in the anterior part of the fins marked in blue (arrow) and abductor and adductor muscles in all presumptive End region. h Chondrogenic condensations in the End region, prominent finfold and AEF undetectable. Prominent abductor superficialis and adductor superficialis muscles covering the radials (Rad). i Shoulder girdle (Grd) forms parallel to the pharyngeal arches (Pha) that sustain the gills (Gl). Chondrogenesis of the radials (Rad) is close to completion as is development of the abductor superficialis and adductor superficialis muscles. j-p MyHC antibody stains throughout pectoral fin development. Dorsal views in J, K, L, M and P and ventral views in N and O. Note muscle projection expanding ventrally towards the fin field at stage 27 and entering the fin field by stage 28 (arrows). Adductor muscles are detected earlier (stage 29) than abductor muscles (stage 30). Adductor superficialis muscles are detected with MyHC stain in the entire endoskeleton domain between stages 31 and 32