Fig. 7. Fgf8-mediated promotion of lateral fast myogenesis in the zebrafish somite. Schematic representation of muscle differentiation in wild-type (upper) and Fgf8 loss-of-function (lower) 20 s embryos. Presomitic mesoderm expresses myf5 (blue) in response to Fgf8 (green) diffusing from tailbud (left). Fgf8 within the somite drives myod expression (red) in the lateral posterior stripe of immature caudal somites (middle) and is required for the later terminal differentiation of a full complement of fast muscle fibres (red cylinders) in maturing rostral somites (right). Hedgehog proteins from notochord (purple) induce myf5 and myod expression in adaxial cells, leading to slow muscle differentiation (yellow) and subsequent translation towards the lateral somite surface. In the absence of Fgf8, expression of myod in lateral somite is reduced and less fast muscle differentiates.