Fig. 2. Muscle fibre types in transverse myotomal sections from juvenile Arctic charr Salvelinus alpinus L. (A) A laboratory reared planktivorous morph stained with the S58 anti-slow muscle myosin antibody and counterstained with Haematoxylin–Eosin. Note the superficial layer of fibres darkly stained with S-58 (red) centred on the major horizontal septa (s-s). In contrast, the superficial fibres at the dorsal and ventral surfaces of the myotome are weakly stained or unstained with S-58 (s-u). Scale bar, 500 µm. (B) A wild-caught dwarf benthic morph, 4.5 cm FL, stained with S-58 antibody and counterstained with Haematoxylin. The majority of the slow fibres (s) stained intensely for S-58 whereas a small number of the slow fibres (arrowhead) and the fast fibres (f) were unstained. Scale bar, 100 µm. (C) A wild-caught dwarf benthic morph, 5.5 cm FL, stained with S-58 antibody and counterstained with Haematoxylin–Eosin. Note that at this body length all the slow fibres (s) stained with S58. There was no evidence for fibres showing intermediate expression for the antigen to S-58 between the slow (s) and fast (f) muscle layers. Scale bar, 200 µm. (D) A wild-caught dwarf benthic morph, 8.4 cm FL, stained for glycogen. The slow fibres (s) were more intensely stained (magenta) than the fast fibres (f). Scale bar, 100 µm. (E,F) A wild-caught dwarf benthic morph, 8.4 cm FL. (E) Epaxial region of the myotome stained for succinic dehydrogenase (SDHase) activity (blue staining). Slow fibres (s) stained intensely for SDHase whereas fast fibres (f) were weakly stained. (F) Region hypaxial to the major horizontal septum stained for myosin ATPase activity following 1 min preincubation at pH 4.3. The slow fibres (s) were lightly stained and the fast fibres (f) were darkly stained. Scale bars, 100 µm. (G) The fast muscle of a wild-caught dwarf benthic morph, 9.7 cm FL, stained for myosin ATPase activity following 90 s preincubation at pH 4.3. Note myosin ATPase had been inactivated in the immature small diameter fibres (a) but not in the larger diameter fibres (b). Shorter periods of preincubation resulted in uniform dark staining whereas longer periods inactivated the myosin ATPase activity in all diameters of fibres. Scale bar, 100 µm. hs, major horizontal septum; nt, notochord; ll, lateral line nerve; sc, spinal cord; sk, skin.