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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D. B. Sibthorpe and I.A.J., unpublished results].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.
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