<i>edn1</i> and <i>hand2</i> Interact in Early Regulation of Pharyngeal Arch Outgrowth during Zebrafish Development

<div><p>Endothelin-1 (Edn1) signaling provides a critical input to development of the embryonic pharygneal arches and their skeletal derivatives, particularly the articulating joints and the ventral skeleton including the lower jaw. Previous work in zebrafish has mostly focused on the role of Edn1 in dorsal-ventral (DV) patterning, but Edn1 signaling must also regulate tissue size, for with severe loss of the pathway the ventral skeleton is not only mispatterned, but is also prominently hypoplastic – reduced in size. Here we use mutational analyses to show that in the early pharyngeal arches, ventral-specific <i>edn1</i>-mediated proliferation of neural crest derived cells is required for DV expansion and outgrowth, and that this positive regulation is counterbalanced by a negative one exerted through a pivotal, ventrally expressed Edn1-target gene, <i>hand2</i>. We also describe a new morphogenetic cell movement in the ventral first arch, sweeping cells anterior in the arch to the region where the lower jaw forms. This movement is negatively regulated by <i>hand2</i> in an apparently <i>edn1</i>-independent fashion. These findings point to complexity of regulation by <i>edn1</i> and <i>hand2</i> at the earliest stages of pharyngeal arch development, in which control of growth and morphogenesis can be genetically separated.</p>