Fig. 3. The three phases of myogenesis in the fast myotomal muscle of the arctic charr Salvelinus alpinus: embryonic (blue arrow), stratified hyperplasia (orange arrow) and mosaic hyperplasia (mauve arrow) [based on (Johnston et al., 2004), and D. B. Sibthorpe and I.A.J., unpublished results]. Mosaic hyperplasia is quantitatively the most important phase of myogenesis. In teleosts maternal mRNA transcripts (maternal effects) drive development until the mid-blastula transition when zygotic transcription is initiated. The myoblasts that form the embryonic slow and fast muscle become committed to a myogenic progenitor cell population towards the end of gastrulation, which is much earlier than in amniotes. At least two further phases of myotube production can be distinguished in fast muscle, involving the production of muscle fibres within discrete germinal zones (stratified hyperplasia) and the widespread formation of fibres throughout the myotome (mosaic hyperplasia). (A) The rostral somites of an arctic charr embryo (large benthic morph) at the end of segmentation (751 h.p.f.) illustrating the embryonic phase of myogenesis. The arrows illustrate the intense staining for Pax 7 transcripts in the lateral margin of the myotome extending along the position of the major horizontal septum. The arrowhead shows intense staining in the dorsal region of the spinal cord. (B) Stratified hyperplasia (arrows) in the apical regions of the fast muscle layer of the myotome in an arctic charr juvenile, 4.5 cm fork length. (C) Past evidence of mosaic hyperplasia in the fast muscle of a piscivorous arctic charr morph 35.8 cm fork length. Mature fast fibres (f) are surrounded by daughter fibres at various stages of growth. The fibres labelled (a) and (b) are 14 and 18 µm diameter, respectively. Filled arrowheads represent myonuclei and unfilled arrowheads connective tissue nuclei. Abbreviations: nt, notochord; sc, spinal cord: sk, skin.