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Fig. 8. Model of phases of myogenesis in trunk and tail. (A) Slow muscle is first
formed in posterior somites (lower series, tail mode). Hh signalling from
ventral midline acts on medial somitic cells to promote XMyf5
expression (blue) and early slow myogenesis. These cells rapidly
differentiate, express XMyoD (purple) and move to the superficial
somite surface (orange arrows) where they elongate anteroposteriorly to make
superficial slow fibres (pink). Simultaneously, most somitic cells
differentiate into fast fibres, also elongating anteroposteriorly to form the
bulk of somitic muscle (yellow). Undifferentiated cells form a dermomyotome
(blue arrows). At later stages, a second population of slow muscle fibres
(orange) is generated from dermomyotome, probably at dorsomedial and
ventrolateral lips, independent of Hh signalling. In anterior somites (upper
series, trunk mode), despite early notochord-dependent XMyf5
expression (red arrow), a block on slow muscle formation prevents appearance
of the first wave of slow fibres. Fast fibre formation is abundant, and
precocious compared with zebrafish. However, some cells remain
undifferentiated to form the superficial dermomyotome. Dorsal and ventral
dermomyotomal lips continue to express XMyf5 and XMyoD,
reflecting their continued role as myogenic centres. Slow fibre formation is
initiated from dermomyotome independently of Hh signalling. Extra fast fibres
(green) probably also arise from dermomyotome at all anteroposterior levels.
At even later stages Hh signalling is again required for XMyoD
expression, somite growth and third wave slow fibre formation (dark red) at
dermomyotomal lips throughout the axis. (B) How first wave slow fibre
migration accompanied by terminal differentiation of fast fibres can appear
like somite rotation.
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