Fig. 9.

Model of FDS tendon formation and its unique features. Like the other autopod tendons, FDS tendons form first by induction of a short-range anlage attached to muscle, followed by the modular formation of independent distal and proximal tendon segments that depend on signals from cartilage and muscle, respectively. Unlike the other tendons, FDS tendon development is delayed (relative to all other tendons), the developmental boundary for modularity is the MP joint instead of the wrist, and proximal tendon elongation is regulated by a unique process of active muscle translocation from the paw into the arm (described by Huang et al., 2013). Overall, the development of FDS tendons is consistent with our general conceptual framework for limb tendon development, yet its unique features highlight these tendons as a useful model with which to test modularity and tendon elongation.