Fig. 2

Fig. 2

Spatial distribution of Pax7⁺ cells in growing myotome. A. Somites at the trunk/tail border shown schematically to define myotomal regions. Schematics (left panels) and confocal optical sections (right panels) of 3 dpf Tg[mylz2:GFP] (green) fish showing slow myosin (red) and nuclei (blue) in lateral (left; anterior to left, dorsal top) and transverse (right; dorsal to top, medial left) views. Epaxial (dorsal) and hypaxial (ventral) somite was conceptually segmented first into superficial (s; dermomyotome and underlying slow fibre layer, judged as about one nuclear length (~6 µm) from the peri/epidermal surface) and deep (d; fast myotomal and myosepta) domains (separated by dots). Within each domain, a central portion (CP) was distinguished from myoseptal border regions lying within a nuclear length (~6 µm) of the somite surface (indicated by dashes). Vertical myoseptum (VM; at which fibre ends from adjacent somites meet), horizontal myoseptum (HM; at which muscle pioneer cells and the lateral line separate epaxial and hypaxial somite domains) and dorsal and ventral edges of the DM (DE/VE; at which the lateral and medial myotome surfaces meet and nascent fibres form (Barresi et al., 2001; Johnston et al., 2009) were distinguished. B,C. Single confocal slices from wholemount 4 dpf larvae taken in lateral view, orientated with dorsal to top and anterior to left. Wt (B) or Tg(−2.2mylz2:GFP)ⁱ¹³⁵ (C) larvae stained with anti-Pax7, Hoechst 33342 (detecting nuclei) and either A4.1025 (B, detecting sarcomeric MyHC) or anti-GFP (C). The superficial monolayer of slow muscle fibres aligned parallel to the horizontal myoseptum (HM) in somites 15–18 (B). Pax7⁺ nuclei surround the myotome (white arrowheads) at dorsal edge (DE), HM and vertical myoseptum (VM) and also occur in central portion (CP; yellow arrowheads) in both the epaxial and hypaxial domains. Pax7⁺ cells nestle amongst deeper fast fibres orientated oblique to HM in the CP region of the epaxial myotome (C). Bars 50 µm.