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.