Fig. 9. Summary of interactions between the somitic lineages. Shortly after somite formation, expression of Myf5 and Myod1 appears in the muscle progenitors of the myotome. Following myotome specification, FGFs are activated at the center of the myotome. Myotomal FGFs, secreted from the myotome, signal to the underlying mesenchymal sclerotome, where they induce expression of Scx and tendon progenitor formation within the dorsolateral anterior and posterior sclerotome. The dorsolateral sclerotome also contains chondroprogenitors that are induced to express Sox9 in response to patterning signals, including Shh (and this same Shh signal negatively regulates Scx expression). The Sox9-expressing chondroprogenitors then activate expression of Sox5 and Sox6, which, in turn, are required for chondrocyte differentiation. The dorsolateral mesenchymal sclerotome can thus follow one of two differentiation pathways: axial tendon or cartilage. Sox5 and Sox6 inhibit expression of Scx such that those sclerotome cells undergoing differentiation into cartilage are blocked from adopting a tendon fate. However, in the absence of Sox5 and Sox6, when these same cells are prevented from differentiating into chondrocytes, they switch their fate to tendon and begin expressing markers of both tendon progenitors and differentiated tendons, suggesting that cartilage differentiation is required to actively repress tendon development.