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Fig. 5. Cyclopamine blocks early slow muscle formation. Xenopus embryos were de-vitellinized, treated with cyclopamine (100 µg/ml), or ethanol vehicle control, fixed at various stages and stained in whole mount for muscle (12/101) or slow (EB165) fibres. (A) Treatment at stage 22 leads to bent embryos with loss of posterior muscle, and severe loss of slow fibres by stage 36 (arrows). A separate group of ventral fast fibres is visible in posterior somites of cyclopamine-treated embryos (arrowheads). Posterior tissue is formed but fails to make muscle (brackets). Insets show the posterior somites at higher magnification. Note poor chevron formation. Slow muscle is greatly reduced or absent. (B) Embryos allowed to develop to stage 41 showing slow myogenesis in anterior somites of both control and cyclopamine-treated embryos, but continued posterior defects in treated embryo (arrowheads). Slow muscle initiates at dorsal and ventral extremities of the somite (arrows). (C,D) Cyclopamine treatment from stage 12 until stage 48 yields similar results. (C) Tail somites 15-30 of untreated embryos (left panels) are extensive and chevron shaped, with a ventral layer of slow fibres (asterisk), separated from a small group of slow fibres at the dorsomedial lip (arrows). Cyclopamine-treated embryos (right panels) have reduced differentiation, dorsoventral extent and less marked chevron shape. Note the initiation of fast myogenesis at dorsoventral extremity of somites in the absence of slow fibres (arrowheads) and lack of a separate row of dorsal slow fibres in the treated embryo (arrow). Slow myogenesis is reduced and commences more anteriorly than in controls (asterisk). (D) In trunk somites, cyclopamine causes reduction in dorsoventral somite extent (brackets), disorganised ventral body wall fast fibres (dot) and reduced somitic slow fibres (asterisk). Note the absence of a separate group of slow fibres at the dorsomedial lip (arrows).

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